Introduction

Anadromous salmonids present a paradox; they are highly valued in commercial and recreational fisheries, as cultural icons for indigenous people, and for their ecosystem benefits, yet they are also often depleted and in jeopardy from human activities (Waldman and Quinn 2022). Consequently, there is a pressing need to better understand their behavior and ecology to mitigate impacts of anthropogenic activities and refine management and recovery strategies. Anadromous brown trout (Salmo trutta) and coastal cutthroat trout (Oncorhynchus clarkii clarkii) have received less attention from managers, relative to their often larger bodied and more abundant congeners that are the focus of commercial and recreational fisheries management (i.e., Atlantic salmon, Salmo salar, and Pacific salmon, Oncorhynchus spp., including steelhead O. mykiss). Consequently, management and conservation plans associated with these trout are less developed and often lack extensive and reliable data (Trotter et al. 1993; Walker et al. 2004) or evaluation of management strategies currently being utilized. This lack of information is concerning, given their recreational value and role in the ecology of freshwater and nearshore ecosystems (Knutsen et al. 2001; Jonsson and Jonsson 2011; Quinn 2018).

The anadromous forms of brown trout and coastal cutthroat trout evolved independently, in the northeastern sides of their respective oceans, yet they inhabit similar ecological/trophic/behavioral niches in their respective ecosystems, making them ideal candidates for a comparison of the ecology and behavior of two similar but allopatric species. Across their ranges, these species are referred to with a variety of names. Anadromous brown trout are most commonly known as “sea trout” (not to be confused with Cynoscion nebulosus, a non-salmonid estuarine fish commonly known as spotted seatrout or speckled seatrout, from southeastern North America) but also sewin in Wales, finnock (for young sea trout from a Scottish Gaelic word meaning white) in Scotland and Ireland, peal and mort in England, and white trout and slob trout in Ireland (the latter term referring to a specific migratory variant using estuaries). In marine waters, anglers refer to cutthroat trout as sea-runs, bluebacks (though this term is also applied, in some areas, to sockeye salmon, O. nerka, and coho salmon, O. kisutch), harvest trout, sea-run cutthroat (SRC) and cutties. Both anadromous brown trout and cutthroat are concentrated in areas with sheltered coastlines, making them accessible to anglers in small boats and on shore. Achieving smaller average sizes than their anadromous congeners (i.e., Atlantic salmon and Pacific salmon and steelhead), both species express considerable phenotypic plasticity as a result of their iteroparous life history (Hutchings 2004), flexible freshwater habitat use patterns, and little genetic separation between anadromous and resident life forms within streams (Hindar et al. 1991; Johnson et al. 2010; Lemopoulos et al. 2018). Specifically, brown and cutthroat trout can display resident or anadromous life history pathways and partial migration regardless of those expressed by their parents (Claiborne et al. 2020; Duval et al. 2021).

While research is limited on anadromous trout globally, recent work on anadromous brown trout has accelerated understanding and provided guidance for research on the less-studied coastal cutthroat trout (Fig. 1) (Birnie-Gauvin and Aarestrup 2019; Kristensen et al. 2019a; Nevoux et al. 2019; Rohtla et al. 2020; Kennedy et al. 2022). By comparing the behavior and ecology of these species, the function and adaptive value of behaviors can be more clearly determined, and recent advances can stimulate communication and growth of current research programs and prioritize data gaps for these ecologically similar species. Together, this information should lead to comprehensive management plans designed specifically for anadromous trout which are lacking in their respective ranges.

Fig. 1
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Source: Web of science search terms included “anadromous” or “sea run” and “cutthroat” and “sea trout” or “anadromous and brown trout”

Number of peer reviewed papers and reports published annually on anadromous brown (grey bars) and cutthroat trout (black bars) between 1950 and 2021.

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Here we review the literature on anadromous brown trout and anadromous cutthroat trout and introduce new data to better understand the behavior and ecology of trout inhabiting marine waters. Specifically, our objectives are to describe 1) key biological characteristics including size, maximum age and growth, 2) marine ecology and behavior including diet, parasites, and migration patterns 3) and management approaches for these two species. Finally, common threats and data gaps for anadromous brown trout and cutthroat trout revealed from this work are discussed to provide a foundation for further research and production of management plans where none exist.

Origin and phylogeny

The genera Oncorhynchus and Salmo diverged from a common ancestor several millions of years ago. A fossil discovered in 1964 assigned to O. rastrosus, the saber-toothed salmon, was estimated to originate from the late Miocene to Pleistocene period (Sepkoski 1992), indicating that the Salmo genus diverged from Oncorhynchus well before the Pliocene, and as early as the Miocene (20 MYA). While cutthroat and brown trout are direct descendants of unique ancestors, O. rastrosus and S. salar, respectively, (McKay et al. 1996; Bernatchez et al. 2004; Trotter et al. 2018), their origin is the product of similar circumstances (glaciation, dispersal limitations, geographic barriers, and other climate-driven factors) in the Pleistocene era. Specifically, brown trout diverged from Atlantic salmon into several isolated geographic lineages (Bernatchez 2001; Crete-Lafreniere et al. 2012) that remained separate from each other for roughly 2 million years, and all retained the capacity for anadromy (Sloat et al. 2014). The anadromous form is thus not phylogenetically distinct from freshwater forms (Ferguson et al. 2019). Within these lineages, some authorities recognize multiple subspecies (Page 2008; Whiteley et al. 2019; Schöffmann 2021) and this is an area of rapidly developing science (Tougard 2022), but for the purpose of this paper these putative subspecies will be referred to as brown trout in the species trutta, given that they all have access to the marine environment and commonly express anadromy.

Outside the native range of brown trout in the northeastern Atlantic, cutthroat trout evolved in the northeastern Pacific (Smith and Stearley 2018). They occur on both sides of the Rocky Mountains (a major faunal divide for North American fishes) including the headwaters of the Rio Grande, Colorado, and Missouri rivers, and interior basins that do not currently flow into marine waters. Although many subspecies of cutthroat trout have been described (e.g. Yellowstone, Westslope, Bonneville, etc.), coastal cutthroat trout represent the oldest existing lineage (Behnke 1992), the only anadromous one, and the focus of this review. Coastal cutthroat trout demonstrate considerable life history diversity between nearby watersheds (Guy et al. 2008; Zimmerman et al. 2015; Losee et al. 2017b) and across their range on the west coast of North America. This life history diversity occurs despite a cline in genetic diversity; Griswold et al. (1997) described low genetic diversity for cutthroat trout that recently colonized streams in Alaska at the northern end of their range and higher genetic diversity in long-established populations farther south. In contrast to their ages as species, anadromous brown trout and coastal cutthroat trout currently occupy environments that are relatively young in the current post-glacial period, thus the fish must have colonized and been extirpated repeatedly between glacial retreats and advances.

At the end of the last glacial period about 13,500 years ago (Pielou 2008), many of the modern-day fjords began to form, converting glacial canyons to marine basins. This process took place in the Baltic Sea and fjords of Norway, bays in Ireland and Great Britain, numerous waterways of coastal Alaska and British Columbia (McPhail and Lindsey 1986), and hundreds of other bays, channels and sounds across the Northern Hemisphere. The Baltic Sea in northern Europe and Salish Sea along the west coast of North America exemplify the results of these geological events, with habitats occupied by anadromous brown and cutthroat, respectively (Huitfeldt-Kaas 1918; Trotter et al. 2018). Specifically, over the last ~ 10,000 years the Baltic and Salish seas transitioned from freshwater lakes to inland marine waters connected to the Atlantic and Pacific oceans as glaciers receded and sea levels rose and flooded glaciated valleys throughout the Northern Hemisphere (Alt and Hyndman 1995).

Geologically, the Baltic Sea is very young, forming when the Scandinavian ice sheet retreated after the last glaciation in Europe approximately 12,000 years ago. However, since that time the Baltic Sea has experienced a variety of salinity changes and transitionary phases of marine/brackish water (Harff and Hoth 2011). The Baltic Sea as we know it today, with brackish water and narrow connection to the North Sea, has been in place for approximately 2,000 years (Emeis et al. 2003; Leppäranta and Myrberg 2009) with a surface salinity range from 0.3% to 0.9% (Viktorsson 2018). Similarly, the Salish Sea’s connection and estuary-like salinity stabilized in the last 4,000 years, providing a transitional environment between the rivers of the Pacific Northwest and the Pacific Ocean (Burns 1985; Alt and Hyndman 1995). These two habitats (the Baltic and Salish seas) and others like them promoted diversification and phenotypic plasticity of anadromous forms of brown and cutthroat trout (Campton and Utter 1987; Bernatchez 2001) under similar circumstances.

Distribution

Coastal cutthroat trout occur from northern California to south-central Alaska (Trotter 2008), though the other subspecies are more widely distributed. Brown trout is much more widely distributed than coastal cutthroat trout, having historically occupied much of Europe and a large area in Asia (Snoj et al. 2021) (Fig. 2). For both species, the greatest abundance of anadromous forms exists along temperate latitudes where sheltered coastlines are common. As a result of stocking by Europeans, beginning in the mid to late 1800’s, naturalized populations of brown trout (often anadromous) exist in every continent except Antarctica, and in many island groups in the southern hemisphere (MacCrimmon and Marshall 1968; MacCrimmon et al. 1970; Klemetsen et al. 2003).

Fig. 2
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Map showing native and introduced range of brown trout and coastal cutthroat trout, Revised from Muhlfeld et al. (2019), Pearcy et al. (2018), and MacCrimmon and Marshall (1968)

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Brown trout rarely go beyond the fjord of their home river (Jonsson and Jonsson 2011) but longer distance migrations have been documented (Bekkevold et al. 2021). Additionally, anadromous brown trout inhabit regions without large, fjord-like inlets in both their native ranges such as the coastlines of North Africa (Lobón-Cerviá et al. 2019) and France (Nevoux et al. 2019), and outside their native range such as Japan (Honda et al. 2012), South America (O'Neal and Stanford 2011; Minett et al. 2021), Newfoundland (Westley and Fleming 2011), and the Kerguelen Islands in the southern Indian ocean (Lecomte et al. 2013).

Off the coast of Iceland, brown trout spent the majority of time (72–93%) near the shore in water < 5 m deep. However, they have been detected > 70 m below the surface (Sturlaugsson 2017; Kristensen et al. 2019c), suggesting that offshore distributions to deeper water are possible. Similarly, spawning populations of anadromous cutthroat are present in numerous small independent tributaries of the Pacific Ocean in Oregon and California that lack protected fjord-like environments (Johnson et al. 1999). The depths occupied by cutthroat in the ocean have not been reported but they have been observed in offshore waters. For instance, Loch and Miller (1988) captured juvenile and adult cutthroat 31.5 km offshore near the Columbia River plume, and Pearcy et al. (1990) captured them up to 66 km offshore. In the latter study, an individual cutthroat was captured and released in Yaquina Bay, Oregon and recaptured 290 km to the south, near the mouth of the Umpqua River, thus demonstrating the potential for long-distance migrations, though this seems to be rare. Together, this information highlights the broad spatial distribution of anadromous brown and cutthroat trout within their respective ranges, but reliance on nearshore habitats at all marine life stages generally characterizes both species and distinguishes them from most other anadromous salmonids. Indeed, the failure to colonize the western shores of their respective oceans seems consistent with limited oceanic distributions rather than specialized habitat requirements in fresh water.

Self-sustaining populations of brown trout outside their native range threaten native species via predation and competition (Idyll 1942; Bisson et al. 1986; Sánchez-Hernández 2020) but also have considerable economic importance across the globe (Ahn et al. 2000; O’Reilly et al. 2006; O’Reilly 2015; Blicharska and Rönnbäck 2018). In contrast, spawning populations of coastal cutthroat trout have not been documented outside their native range, and efforts to transplant them have been limited (Crawford and Muir 2008) thus distribution patterns for coastal cutthroat have not changed significantly over the last 200 years (Metcalf et al. 2012). This difference in range expansion is likely due to the limited artificial breeding and stocking that has occurred for coastal cutthroat trout. This literature comparison with brown trout provides a glimpse into potential benefits and risks that could result from future efforts to artificially stock cutthroat outside their native range.

Life history and migratory patterns

Brown and cutthroat trout exhibit four basic life history pathways: lacustrine (lake dwelling), fluvial (riverine), adfluvial (river and lake), and anadromous (marine) (Trotter 1989; Lobón-Cerviá and Sanz 2017; Arostegui and Quinn 2019; Jonsson et al. 2019). The anadromous forms are the focus of this report; however, individuals entering marine water can express multiple life history patterns during their life span, either before or after their initial freshwater emigration (Saiget et al. 2007; Thorstad 2016). Moreover, offspring from anadromous parents may remain in freshwater habitats their entire life (Rohtla et al. 2017; Claiborne et al. 2020), and non-anadromous parents may produce anadromous offspring (Fig. 3). For instance, approximately 12% of anadromous brown trout sampled in the Baltic Sea near Estonia were produced by non-anadromous mothers, based on otolith microchemistry analysis (Rohtla et al. 2020). Such partial migration, where individuals from the same population exhibit differential migratory behavior, has been well documented for salmonids including brown and cutthroat trout (Saiget et al. 2007; O'Neal and Stanford 2011; Sloat 2013; Rohtla et al. 2020; Duval et al. 2021) and reviewed across various systems (Vélez-Espino et al. 2013; Sloat et al. 2014; Kendall et al. 2015). The factors determining whether individuals remain in fresh water or migrate to sea are not fully understood but involve a combination of genetic and environment controls; regardless, this flexibility tends to increase the resilience of populations (Jonsson and Jonsson 1993; Chapman et al. 2011, 2012; Hayes et al. 2012; Lemopoulos et al. 2018; Ferguson et al. 2019; Nevoux et al. 2019).

Fig. 3
figure 3

Generalized life cycle, examples of common patterns and some of the major threats of anadromous brown and anadromous cutthroat trout; adapted from Nevoux et al. (2019)

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The importance of the freshwater environment for both cutthroat and brown trout is evident in the extended and variable freshwater rearing period and the diversity of stream and lake habitats they use. Both species may rear for 1 to 6 years prior to ocean entry in spring but 2–4 years is typical (Sumner 1962; L'abée-Lund 1989; Trotter 1989). However latitudinal gradients in smolt age have been documented (Jonsson and L'Abee-Lund 1993) and recent studies have revealed the importance of rearing and migration patterns that were once assumed to be uncommon, including autumn downstream migration (Birnie-Gauvin and Aarestrup 2019; Jensen et al. 2022), use of non-natal streams (Taal et al. 2018; Källo et al. 2022), and dispersal of juvenile brown trout less than one year of age from natal waters into neighboring streams via the marine environment, reported by Taal et al. (2018). Degerman et al. (2012) hypothesized that this behavior could result from habitat limitations in the natal stream (e.g., density dependence, flow limitations etc.) and/or filling unused nearby rearing areas. However, the fate of individuals expressing such dispersal is unknown. No such pattern has been documented for cutthroat trout but they regularly spawn in small streams with limited flow (Pauley et al. 1989; Losee et al. 2016). These intermittent flows in the spring may support spawning, incubation, and emergence of fry but are not sufficient for larger juveniles in summer. In such cases, juveniles may seek suitable habitat in neighboring streams. The tools exist to clarify the role of these less understood life history patterns. Fisheries managers and scientists should prioritize this research, given the growing body of evidence that life history diversity is important in stabilizing and recovering anadromous fish populations (Moore et al. 2014; Schindler et al. 2015; Jonsson et al. 2019).

While a comprehensive understanding of movement patterns for anadromous brown trout and cutthroat is lacking, general behaviors have been described and provide an important area for comparison between the two species (Table 1). For instance, both species occupy similar ecological niches in their respective marine habitats and exhibit repeatability in migratory behaviors for individuals at the juvenile (Bohlin et al. 1996; Goetz et al. 2013; Zydlewski et al. 2014) and adult (Losee et al. 2017b; Halttunen et al. 2018; Birnie‐Gauvin et al. 2021) life stages. Specifically, they rarely migrate more than 150 km from land with typical migrations < 80 km from natal rivers (Thorstad 2016; Quinn 2018; Schöffmann 2021). For instance, Berg and Berg (1987) documented that 93% of Norwegian trout remained within 80 km of their natal river mouth, and Kallio-Nyberg et al. (2002) reported that > 90% of tagged sea trout were caught within 200 km. For cutthroat, results are sparse but the majority of cutthroat were caught within 30 km of their natal stream in south Puget Sound based on genetic stock identification (Losee et al. 2017a, b) and 77 km from their tagging location in the Pacific Ocean (Pearcy et al. 1990). While some longer distance migrations have been documented for both species (Kallio-Nyberg et al. 2002; Pearcy et al. 2018; Kristensen et al. 2019a) quantitative estimates of the proportions of fish offshore vs. nearshore exist are limited for brown trout and non-existent for cutthroat. As mentioned previously, these limited distances traveled are markedly different from their congeners. This difference was revealed by Moore et al. (2010), who compared migratory patterns between cutthroat trout and their close relative, steelhead, and their naturally occurring hybrids in Hood Canal, a fjord of Puget Sound. Moore et al. (2010) showed that the long, surface-oriented migrations to offshore waters characteristic of steelhead were not exhibited by coastal cutthroat trout which remained in nearshore waters within Hood Canal, and the hybrids showed intermediate patterns between those of the two pure species. Similarly, Klemetsen et al. (2003) highlighted the significantly shorter migratory distances traveled by brown trout compared to Atlantic salmon.

Table 1 Characteristics of anadromous brown trout Salmo trutta and cutthroat trout Oncorhynchus clarkii clarkii
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Given their flexible life history strategy, it is not a surprise that variations to general patterns of movement have been identified. For instance, Ferguson (2006) reviewed molecular methods used to describe two variations of anadromy for brown trout: the typical “sea trout” that migrates to marine water to feed prior to returning to natal rivers to spawn and the “slob trout” that is restricted to the estuary. Similarly, acoustic tracking of cutthroat trout in Hood Canal, revealed two movement “clusters” based on a variety of migratory metrics with some overlap between clusters. These clusters included individuals that exhibited an extended estuary residence time and limited movement and some with a larger range and less time spent in the estuary (Goetz et al. 2013). These categories were generally consistent with the results of Losee et al. (2017b) in three other fjords of south Puget Sound. Subsequently, Losee et al. (2018) observed high site fidelity of cutthroat, remaining within or returning to small sections of coastline for more than 12 months with short absences during the late winter, presumably moving into nearby streams to spawn.

This pattern of high site fidelity to marine beaches was observed for cutthroat trout as young as age 2, suggesting that individuals may disperse to locate favorable habitat (e.g., productive feeding areas, low predation risk, optimal temperature etc.) at a young age and then remain there for much of their lives. While site fidelity of trout in freshwater has been documented at various life stages (Bachman 1984; Heggenes and Slaveit 1990; Heggenes et al. 1991; Kuliskova et al. 2009; Slavík and Horký 2019) this behavior has not been described for anadromous brown trout in marine water. However, similar behavior was detected for anadromous bull trout in North Puget Sound, USA (Hayes et al. 2011), and anadromous grayling (Thymallus thymallus) in the Baltic Sea (Hellström et al. 2023), and it may also occur in anadromous brown trout in Europe. Identifying marine areas of high use by trout would provide managers an opportunity to protect vulnerable or threatened populations through refined fishery regulations or focused habitat improvement projects.

Considerable research has been focused on the factors affecting migratory patterns of anadromous trout. For instance, Jonsson and Jonsson (2014) reported that larger anadromous brown trout traveled farther from natal spawning locations than smaller individuals leaving streams of southern Norway, though this pattern has not been observed consistently (Thorstad 2016). For anadromous cutthroat, Goetz et al. (2013) showed that modes of behavior were not related to size or age, and Losee et al. (2017b) showed no effect of size on movement patterns across a broad length range. Thus, size and age are not consistently associated with migratory patterns, and small brown trout and cutthroat trout have been encountered far from their natal stream in their respective ranges (Pearcy et al. 1990; Thorstad 2016). Movement patterns of anadromous trout at various stages of the life cycle are likely determined by environmental and genetic tradeoffs between growth potential (e.g. temperature and food availability) affecting physiology (Archer et al. 2020), and mortality risk in differing environments (Zydlewski et al. 2008). Studies of movement, indicate that the most important determinants of migratory distance and behavior at sea include season (Moore et al. 1998; Losee et al. 2017b), fish condition (Haque 2008; Nevoux et al. 2019; Shry et al. 2019), predation (Dieperink et al. 2001), and ectoparasite intensity (Halttunen et al. 2018) but it is worth noting that few quantitative estimates of the proportion of anadromous trout found offshore vs. inshore exist, especially for cutthroat.

Following the marine phase of the brown and coastal cutthroat trout life cycle, the age and timing of the return to freshwater varies and can include trips to freshwater outside the spawning period (Taal et al. 2018; Claiborne et al. 2020). Furthermore, the iteroparous nature of both species adds complexity including individuals that return to freshwater for the first time but do not spawn, and individuals remaining at sea for more than one year, skipping a spawning event (Trotter 1989; Thorstad 2016). Typically, the initial return to freshwater occurs between total ages three and five when maturity is reached (Trotter 1989; Jonsson and Jonsson 2006; Claiborne et al. 2020), and is annual thereafter. The diversity of behaviors expressed by adult anadromous brown and cutthroat trout in freshwater have not been fully described but range from short intermittent visits, extended freshwater residence to overwinter and complex fluvial migrations.

The spawning season represents one of the greatest dissimilarities between these two species; coastal cutthroat trout typically spawn in the late winter and early spring (Losee et al. 2016) whereas brown trout spawn in the fall to early winter (Jepsen et al. 1998; Birnie‐Gauvin et al. 2021). Importantly, both species show considerable variation in the period between entry into fresh water and the spawning period, including so-called “premature migration” that may occur many months or even years prior to spawning, or migration immediately prior to spawning (Quinn et al. 2016; García-Vega et al. 2022). While the spawning season differs significantly between brown and cutthroat trout, both may spawn up to five times or more (Jones 1975; Peoples et al. 1988; Lobón-Cerviá and Sanz 2017) during their life. The flexible nature of both species has been revealed through observations of anadromous cutthroat trout spawning in the fall in the Elwha River system, WA, USA (McMillan et al. 2014) and spring spawning of anadromous brown trout in a tributary of the Black Sea (Latiu et al. 2020). This protracted or variable spawning period suggests that environmental influences and other co-occurring species are important determinants of the spawning period along with differences between the biology of brown and cutthroat trout.

The timing of freshwater entry is determined in large part by local stream conditions (Trotter 1989; Svendsen et al. 2004; García-Vega et al. 2022) but also biological/genetic controls given the repeatability in spawn timing across years of variable environmental conditions (Eldøy et al. 2019; Jensen et al. 2020). Specifically, adult anadromous brown and cutthroat trout can overwinter in freshwater (Pearcy et al. 1990; Birnie‐Gauvin et al. 2021), marine (Rikardsen et al. 2006; Jensen and Rikardsen 2012; Losee et al. 2018), and estuary habitats (Loch and Miller 1988; Etheridge et al. 2008; Jensen and Rikardsen 2008), and both display two main periods of freshwater entry: in the fall and winter (Trotter 1989; Dêbowski 2018) with summer river entry also common in some systems (Birnie‐Gauvin et al. 2021). While the conditions that separate the freshwater entry period from the spawning period are not completely known, bigger river systems and those with lakes may encourage an earlier entry timing as a result of safer and less energetically demanding habitat, as was shown for brown trout in streams in southern (Jonsson et al. 2018) and coastal Ireland (Kennedy et al. 2022), and Atlantic salmon in Ireland (Reed et al. 2017). For cutthroat, nutrient subsidies produced from sympatric semelparous salmon populations (e.g., eggs and flesh) provide a growth advantage for this fish entering earlier in the fall (Trotter 1989). In addition to the importance of stream size within a given region, the duration of feeding in marine waters also varies with latitude. For example, brown trout in northern Norway averaged only 68 d at sea (Berg and Jonsson 1989) with compressed migrations downstream and upstream, whereas those in some Irish rivers have very protracted migrations lasting more than 10 months (Quinn et al. 2016). Thus some anadromous brown trout and cutthroat trout are essentially freshwater fish that briefly enter marine waters to feed, whereas others are, after the smolt transformation, essentially marine fish that briefly return to streams to spawn, and then quickly return to marine waters (Quinn and Myers 2004; Quinn et al. 2016; Losee et al. 2018).

Given their small average size relative to other anadromous salmonids and physically variable spawning streams across the range, anadromous brown and cutthroat trout are limited by stream flow, sediment size, and temperature to successfully fertilize eggs and deposit them in redds (Quinn 2018; Ferguson et al. 2019; Nevoux et al. 2019). These size-based limitations and associated preferences for higher order spawning tributaries represent a key similarity between these species and may provide the benefit of reduction in hybridization by naturally segregating brown trout and cutthroat trout from their counterparts—Atlantic salmon and steelhead (Buehrens et al. 2013). However, conclusions regarding the selective pressures driving habitat preferences are speculative and need further investigation. Characteristics of typical spawning areas are similar between anadromous brown and cutthroat trout (Table 1). However, like other aspects of anadromous trout life history, variation from typical spawning locations has been described, including spawning in brackish water by brown trout (Limburg et al. 2001; Gabrielsen et al. 2021). Together, this information highlights the diverse movements of anadromous brown and cutthroat trout but also reveals predictable migratory and spawning patterns driven by biotic and abiotic factors and highlights key data gaps, particularly for cutthroat trout.

Physical characteristics

Anadromous brown and cutthroat trout share important physical similarities that differ from many non-anadromous trout species. Both anadromous brown and anadromous cutthroat trout have numerous irregularly shaped spots, as opposed to the round spots that are common among fish in the Oncorhynchus family (Fig. 4). Additionally, both lack spots on the ventral surface in the marine environment. The maxillary of anadromous brown and cutthroat trout extends beyond the eye, creating a relatively wide gape that supports an ontogenetic shift to a piscivorous diet at a small body size relative to other salmonids (Brodeur et al. 1987; Knutsen et al. 2001). In the marine environment, brown trout and cutthroat trout have a silvery coloration similar to other marine fish of similar size (Denton and Nicol 1966) but with heavier spotting and countershading, likely highlighting the importance of camouflage in the nearshore marine environment. Upon their return to freshwater as adults, yellowish or brown coloration is common. These changes in color are used by biologists and anglers to identify recent transitions from one environment to the other.

Fig. 4
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Illustration of anadromous brown trout (above) and cutthroat trout (below). By Joseph Tomelleri

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Both species display great variation in color patterns among populations and subspecies, but there are some consistent differences between them. Notably, cutthroat trout have red slashes under the jaw (especially prominent when they are in fresh water) that are the source of their common name, and spotting on both the dorsal and ventral lobes of the caudal fin. In contrast, brown trout often lack spots on the tail or carry light spotting on the upper lobe. When brown trout are in freshwater, red spotting is common with a white or blue “halo” around spots, but these are less common or absent in the anadromous form (Klemetsen et al. 2003). Sex specific traits observed in the field are common, and especially evident during the spawning phase. Male characteristics are affected by environmental conditions and population specific genetic controls and include longer upper and lower jaw, shorter dorsal and anal fins and a larger adipose fin relative to females (Gruchy and Vladykov 1968; Jonsson and Jonsson 2015). Females commonly have a round belly as a result of developing gonads and greater length, weight and reduced diversity in size relative to male brown trout (Jonsson and Jonsson 2006). This is consistent with other iteroparous salmonids (Beacham and Murray 1987; Young 2005; Tamate and Maekawa 2006; Englmaier et al. 2021). These patterns are likely consistent for cutthroat trout but sex-specific differences in size for anadromous cutthroat trout have not been documented. Variation in sex-specific traits across environments and life stages results in sex-specific chromosomes that allow for sex determination using genetic methods when field methods are not adequate (Li et al. 2011). A similar, non-lethal, approach to determine sex has not been developed for cutthroat trout but would be an important step to understanding behavioral differences and patterns of life-history expression between male and female cutthroat trout in the marine environment.

Anadromous brown trout and coastal cutthroat occupy a similar ecological niche and as a result their morphology and meristics are similar and were well described in Linnaeus (1758) for brown trout and in Behnke’s Native Trout of Western North America (1992) for cutthroat. Both have teeth developed on upper and lower jaws and two rows on the tongue. Coastal cutthroat trout have an additional set of teeth between the gill arches on the floor of the pharynx on the basibranchial (hyoid or basibranchial teeth) that are absent in brown trout, perhaps indicating slightly different diet or feeding behavior. Scale counts are similar, ranging from 118–130 for anadromous brown trout and 120–180 for anadromous cutthroat trout (Linnaeus 1758; Page and Burr 1991) (Table 1). Vertebrae counts range from 59 to 64 for coastal cutthroat and 56 to 61 for anadromous brown trout (Linnaeus 1758). Gill raker counts are low in both species, relative to other salmonids, ranging from 13–18 in anadromous brown trout and 15–21 in coastal cutthroat trout (Linnaeus 1758; Tåking 1950). Given that gill rakers restrict food from exiting the mouth, few gills rakers indicate a diet dominated by macroinvertebrates and fishes rather than plankton.

The broad distribution of both anadromous brown trout and cutthroat trout has resulted in diverse physical characteristics for adaptation to unique environmental variability. For instance, Vatandoust et al. (2014) reported significant differences between populations across a set of 31 morphological and 7 meristic characters for brown trout in a small geographic area in the southern Caspian Sea. Tåking (1950) demonstrated the importance of changes in temperature in regulating vertebrae count for brown trout suggesting phenotypic plasticity in some traits. Similarly, Zimmerman et al. (2015) documented four distinct meristic patterns among cutthroat trout in a tributary of the Nisqually River, Muck Creek, in Washington State, USA; differences in scale counts and dorsal and anal fin rays of cutthroat trout coincided with genetic differences between individuals from different study sites.

Adult anadromous brown trout and cutthroat exhibit similar body condition (weight at length) when sampled in marine waters (Fig. 5). At sea, both typically range in size from 20 to 56 cm (Jonsson and Jonsson 2007; Quinn 2018) but larger specimens are common in some areas. Moreover, the average size of trout sampled in marine water can vary considerably across seasons (L'Abée-Lund 1991), particularly for brown trout, which can exceed 80 cm, however sampling effort throughout the year in marine waters is limited (Fig. 6). Far fewer samples have been collected of anadromous cutthroat trout relative to anadromous brown trout, limiting a comparison of maximum size. Biologists measured cutthroat up to 60 cm in an upstream trap just above tide water in Puget Sound Washington (WDFW unpublished data) and one individual was reported measuring 75.6 cm in Victoria Harbor, B. C. (Roger Ptolemy pers. comm., Table 1). However, the average lengths of brown trout and cutthroat trout are less than those of their closest relatives occupying marine habitats (i.e., Atlantic and Pacific salmon and steelhead) (Brodeur et al. 1987; Jonsson and Jonsson 2011; Quinn 2018).

Fig. 5
figure 5

Fork length vs. weight of cutthroat (black) and brown trout (grey) captured in marine waters. Different shapes represent different data sets from Norway (J. Davidsen, Pers. Comm.), United Kingdom (B. Brown, Pers. comm.), Baltic Sea (SLU 2022), Denmark (Kristensen et al. 2019b), South Puget Sound (WDFW, Pers. comm.), North Puget Sound (T. Quinn, Pers. Comm.), Vancouver Island, B.C. (B. O’Connor Pers. Comm.) and Pacific Ocean from Central Oregon to Northern Washington (Pearcy et al. 2018)

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Fig. 6
figure 6

Mean monthly fork length of anadromous brown trout Salmo trutta (black) and coastal cutthroat trout Oncorhynchus clarkii clarkii (grey) sampled from Baltic Sea (SLU 2022), Norway (J. Davidsen, Pers. Comm.), Wales (B. Brown, Pers. comm.), Ireland (W. Roche, pers. Comm.), Puget Sound (WDFW), Vancouver Island, B.C. (B. O’Connor, Pers. Comm.) and coastal Oregon to northern Washington (Pearcy et al. 2018) and coastal California (W. Duffy Pers. Comm.)

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It is not clear why anadromous brown and cutthroat trout are so much smaller than other anadromous salmonids. Pearcy et al. (1990) hypothesized that the small size of adult anadromous cutthroat trout, relative to sympatric salmon and steelhead, allows them to spawn, and offspring to rear, in small streams with abundant marginal habitat that young of the year favor (Moore and Gregory 1988). Larger-bodied steelhead overlap in distribution but breed in larger streams and farther downstream than cutthroat trout (Jones 1975; Trotter 1989; Buehrens et al. 2013). This size-biased habitat segregation may be reinforced by the apparent survival deficit experienced by hybrids. That is, naturally occurring hybrids occur regularly, as inferred from sampling of juveniles from throughout their overlapping range in northern California (Baumsteiger et al. 2005), coastal Washington: (Buehrens et al. 2013), Puget Sound, Washington, (Campton and Utter 1985; Moore et al. 2010), and south-central Alaska: (Williams et al. 2007). In these and other studies, authors report that most hybrids are the product of F1 backcrosses whereby adult hybrids spawned with pure cutthroat or steelhead, indicating that some hybrids survive to adulthood. However, reports of adult hybrids are uncommon relative to hybrids at the juvenile stage, suggesting hybrids experience reduced fitness through factors that select against adults favoring similar spawning habitats and interbreeding (Ostberg et al. (2004).

Small size as adults relative to many sympatric salmonids also makes brown and cutthroat trout less conspicuous to predatory birds and mammals in the shallow nearshore environment that they inhabit (Pemberton 1976; Lyse et al. 1998). In this way, relatively small anadromous size of brown trout and cutthroat is likely a trait selected for that enables them to occupy a unique ecological niche that supports successful feeding and predator avoidance in the nearshore marine environment and as mentioned earlier, may reduce competition and hybridization with their larger congeners in fresh water. Recent work focused on trout physiology has strengthened understanding of anadromous brown trout and cutthroat (Boel et al. 2014; Desforges et al. 2021; Anlauf-Dunn et al. 2022), highlighting the role of temperature in determining rates of growth and life-history expression, particularly in freshwater. As this area of research grows, experimental studies on growth in controlled conditions (e.g., comparing brown and cutthroat trout) would be especially fruitful to understanding the eco-physiology of anadromous trout and the evolutionary significance of documented growth and size traits.

Feeding and growth

As visual, opportunistic predators, brown and cutthroat trout exhibit diverse feeding habits and diet studies reveal fine-scale differences in habitat utilization and prey availability (Keeley and Grant 2001). Diet analysis from anadromous trout suggests the prey consumed vary with the trout’s size, age, and season, but are typically dominated by prey from the marine littoral zone, reflecting movements largely restricted to the nearshore environment (discussed later). During the day, both species are observed feeding and can be caught by anglers, but brown trout also show some specialization for crepuscular or nocturnal feeding, based on diet differences and stomach fullness during daylight hours vs. night (Pemberton 1976), and reduced feeding intensity during daylight (Heggenes et al. 1993). Studies of this kind are limited for anadromous brown trout and absent in the literature for cutthroat, and this would be a good topic for future work to better understand behavioral adaptations, food web dynamics and habitat prioritization to aid in management of coastal trout.

Feeding studies are much more common for anadromous brown trout than cutthroat but both rely heavily on stomach content analysis. For anadromous brown trout, broad scale patterns of feeding have been described with a diet dominated by nearshore invertebrates and fish. Piscivory is consistently reported as important for anadromous brown and cutthroat trout, with increasing contribution of fish in the diet with increasing trout size, especially in the summer and fall (May–October) (Lyse et al. 1998; Keeley and Grant 2001; Knutsen et al. 2001; Sánchez-Hernández 2020). For instance, in coastal waters of western Scotland, Pemberton (1976) observed increased feeding on small fish among brown trout > 21 cm, especially in summer when clupeids such as Atlantic herring (Clupea harengus) and sand eels (Ammodytidae) were abundant. As mentioned previously, information on the marine diet and feeding behavior of anadromous cutthroat is limited relative to brown trout, however their diet is dominated by nearshore biota including small, schooling, planktivorous fishes, juvenile salmonids, and invertebrates (Pearcy et al. 2018). Stomach content analysis has revealed that food items with the greatest contribution by weight and number in the coastal ocean and Columbia River plume included herring, Pacific sand lance, anchovy and juvenile cabezon as well as crab megalope, euphausiids and mysiid shrimp (Brodeur et al. 1987; Loch and Miller 1988). Similarly, the diet of cutthroat captured in central Puget Sound was dominated by nearshore fishes throughout the year (Duffy and Beauchamp 2008). In the southern Puget Sound, where chum salmon are the most abundant anadromous fish species (Losee et al. 2019), Jauquet (2005) demonstrated the importance of their fry, flesh and eggs to the diet of cutthroat trout in marine waters. Similarly, Ellings et al. (2005) found that 90% of cutthroat trout sampled in tributaries of south Puget Sound during the chum salmon spawning period had diets dominated by chum salmon eggs. Similarly variable movement patterns to maximize seasonal prey resources was also described for brown trout, linking patterns of feeding and migration (Rikardsen et al. 2006; Boel et al. 2014; Bordeleau et al. 2018). Differences in diet by season and geographical area of both brown and cutthroat trout highlights the opportunistic nature of anadromous trout and are an important determinant of migration patterns. In this way, the presence and absence of anadromous trout may provide some insight into habitat quality and guide habitat improvement projects in the nearshore marine environment.

Maximum size may differ significantly between anadromous trout and other Pacific and Atlantic salmonids but maximum growth rates are often comparable between trout and salmon, and can exceed 1 mm per day for immature fish (Ricker 1981; Hartt and Dell 1986; Gonçalves et al. 2013). Specifically, Pearcy et al. (1990) documented growth rates as high as 2.6 mm a day for cutthroat trout, estimated by back-calculating ocean growth from scales. Similarly, Degerman et al. (2012) recorded growth rates up to 1.8 mm/day for brown trout marked and recaptured in Northern parts of the Baltic Sea. These high growth rates during the first summer at sea suggest that trout species, despite their small maximum size, enjoy the benefit of rapid growth during the early marine period documented for Pacific and Atlantic salmon (Hansen et al. 1998; Claiborne et al. 2011; Anderson et al. 2021). The rapid growth rate at a young age is likely an adaptation to avoid gape limited predators and improve body condition prior to over-wintering (Pearcy 1992; Beamish and Mahnken 2001; Thorstad 2016).

While growth rates of Pacific and Atlantic salmon and trout are rapid during the spring early marine period, a second phase of high growth rate and feeding occurs in the late fall for some trout populations (Knutsen et al. 2001; Rikardsen et al. 2006; Jonsson and Jonsson 2009a; Losee et al. 2018). For instance, Rikardsen et al. (2006) documented peak feeding rates, body condition and lipid content among brown trout captured in two fjords in Northern Norway (Ranafjord and Balsfjord) in August and September opposed to the typical spring period when anadromous salmon and trout first enter salt water. A similar pattern was documented among cutthroat sampled in southern Puget Sound over a 12 month period (Losee et al. 2018). This period of accelerated growth was associated with nutrient subsidies from spawning chum salmon for cutthroat trout, and increased abundance and access to clupeids, sand eels and crustaceans (e.g. shrimp and benthic amphipods) in brown trout (Haluch and Skora 1997; Rikardsen et al. 2006), again highlighting the highly opportunistic nature of anadromous trout and their ability to overwinter in marine water successfully.

Abundance and threats

Despite the scarcity of comprehensive population size assessments for anadromous trout, declines in abundance have been documented across the respective ranges of cutthroat (Giger 1972; Jones 1975; Hooton 1997; Johnson et al. 1999; Pearcy et al. 2018) and brown trout (ICES 2013; Pedersen et al. 2017). Despite these declines in abundance of for both species in the 1980’s and 1990’s, trends may have stabilized in recent decades. For instance, catches of anadromous brown trout on the west coast of Ireland (Connemara district) declined from approximately 10,000 fish annually prior to the 1990’s to less than 1,000 per year in the past 3 decades (Fig. 7). This unique data set reveals trends similar to those observed from of catch from other areas of Ireland and shows a consistent pattern across the country (Gargan et al. 2006). Additionally, in northern Wales, Davidson et al. (2007) documented a relatively stable anadromous brown trout run over more than 10 years based on counts of upstream migrating adults in the River Dee, beginning in the early 1990’s, but there were no estimates prior to the 1990’s. In the Baltic Sea, trends in anadromous brown trout abundance have declined relative to historic levels consistent with other areas (Pedersen et al. 2017; Dêbowski 2018), and many populations are vulnerable or have uncertain status (ICES 2021). However, HELCOM (2018) reported increasing abundance in four Swedish rivers (Kalixälven, Piteälven, Byskeälven and Vindelälven) following changes to fisheries practices in response to historic low levels in the 1990’s as was evident in River Umeälven where fish primarily from the tributary Vindelälven are counted (Fig. 7). In places where Baltic Sea populations remain at low levels such as the Gulf of Bothnia, Southern Baltic Sea and those originating in Germany, changes in fishing regulations that would result in reduced exploitation have been recommended (ICES 2021).

Fig. 7
figure 7

Trends in abundance for anadromous brown trout (upper tiles in grey) and anadromous cutthroat trout (lower tiles in black) from trapping locations across the range of distribution. Note that Y axis are not consistent between tiles

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Similarly, several populations of anadromous cutthroat trout declined in the 1980’s and 1990s but information on abundance is limited overall. Catch of anadromous cutthroat trout from traps in tributaries of Puget Sound (Minter Creek), the Columbia River (Kalama River) and the Oregon Coast (Umpqua River) demonstrated a significant decline followed by a recent period of stability at lower abundances (Fig. 7). Additionally, fish managers in Puget Sound, Washington have used redd surveys to assess adult abundance in small streams (Losee et al. 2017a). Redd counts ranged from 50 to 150 over the last 15 years but estimates of redd abundance prior to 2006 are not available. Together, the trends observed for anadromous brown and cutthroat trout highlight the importance of long term time series to avoid the masking of past declines (i.e., shifting baseline (Pauly 1995)) and the importance of the late 1980’s and 1990’s in altering baseline abundance levels for many populations of anadromous fish. Increased use of redd surveys may be an economical approach to estimating abundance of brown trout across a broader range as has been done for cutthroat trout.

Leading causes cited for the declines observed for both brown and cutthroat trout are consistent with other anadromous salmonids and include habitat degradation, anthropogenic barriers (e.g. dams, hydropower operation etc.), hatchery production and harvest (Trotter et al. 1993; Lichatowich et al. 2006; Skaala et al. 2014; HELCOM 2018). The mechanisms that lead to association between anadromous trout abundance and these constraints are described well in (Connolly 1996; Johnson et al. 1999; Lichatowich 1999; Costello 2008; ICES 2013; Williams et al. 2015). Other less understood factors include predation, parasite infections, hybridization, and climate related factors, which we discuss here. While predation can affect population size of anadromous trout to some extent, rates are typically low (Lyse et al. 1998; Lance et al. 2012). Specifically, aquatic mammals and birds feed on anadromous trout, especially at certain life stages and habitats (Collis et al. 2002; Hansson et al. 2018; Penaluna et al. 2021; Källo et al. 2023). For example, Hansson et al. (2018) indicates seals and cormorants have been the target of research and public scrutiny in the Baltic Sea especially given the increased abundance by both. However, recent evidence that cormorants eat few anadromous trout suggests additional information is needed to understand the role of predators in regulating trout abundance (Boström et al. 2009, 2012). Additionally, parasite induced mortality likely represents an important factor for anadromous trout, particularly for brown trout in Norway (Thorstad et al. 2015). The best documented source of parasite associated mortality is the parasitic copepod (Lepeophtheirus salmonis) commonly known as “sea louse,” with the highest infection levels typically associated with areas supporting commercial net pen operations. In a review of the effects of sea lice on anadromous brown trout, Thorstad et al. (2015) provided evidence for a direct relationship between net pen operation, associated infections of sea louse, and declining abundance of anadromous brown trout in European marine waters. Negative effects of copepod infestations include secondary infections (Johnson et al. 2004), changes in behavior (Gjelland et al. 2014; Halttunen et al. 2018), and reduced survival (Gargan et al. 2003). For coastal cutthroat trout, the effect of sea lice is much less understood but presence, abundance and intensity have been documented (Hoffman 1999; Losee et al. 2020). Of particular interest is the parasitic argulid, Argulis pugettensis that appears to specialize on coastal cutthroat trout. Argulids feed on the flesh of their host and have caused premature mortality of their host, therefore infections of A. pugettensis likely represents a threat to coastal cutthroat trout. While investigations of ectoparasites in cutthroat have increased in recent years including an online angler reporting tool produced by WDFW and the Coastal Cutthroat Coalition (www.coastalcutthroatcoalition.com) (Losee et al. 2022), to our knowledge, no study comparing movement patterns or survival of individuals with differing parasite loads has been conducted. This represents an important data gap, given the negative effects of these infections described on anadromous brown trout, the current/observed prevalence of ectoparasites on cutthroat hosts, and the increased interest in commercial aquaculture in habitats occupied by cutthroat trout that have been linked to increased ectoparasite loads (https://www.fisheries.noaa.gov/west-coast/aquaculture/aquaculture-west-coast).

Hybridization with closely related salmonids that overlap spatially and temporally may be a threat for both brown and cutthroat trout (Matthews et al. 2000; Al-Chokhachy et al. 2014) as briefly discussed earlier. The fossil record suggests strong geographic separation (ca. 8 mya) of cutthroat trout from their closest relative, rainbow trout (Smith and Stearley 2018). Despite this separation, hybridization between these species occurs across the range (Campton and Utter 1985; Ostberg et al. 2004; Baumsteiger et al. 2005; Williams et al. 2007), though documentation of hybrids surviving to maturity is limited. However, over the last two decades numerous studies have described patterns of hybridization and hybrid swarms where rainbow trout and cutthroat overlap geographically, particularly for the subspecies of cutthroat that did not co-evolve with rainbow trout (Rubidge and Taylor 2004; Metcalf et al. 2008). In Washington State, U.SA., Moore et al. (2010) showed that cutthroat/steelhead hybrids had an intermediate migration pattern in marine water relative to parent species and suggested that this conferred a survival disadvantage. Like cutthroat, brown trout hybridization is relatively common when sympatric with Atlantic salmon (Makhrov 2008; Adams et al. 2014) but has been selected against because hybrids rarely reach adulthood (Solomon and Child 1978; Verspoor and Hammart 1991) or are sterile (Palm et al. 2013). This reduced fitness may exert selective pressure on size and behavioral patterns associated with habitat use and spatial and temporal patterns of reproduction for both brown and cutthroat trout. The role and effect of hybrids for anadromous trout is an area ripe for future research. By assessing rates of hybridization and associated survival across all life stages, the population level effect of hybridizing with closely related species would be clearer. Until more is known, managers should preserve diverse habitat types and limit interaction with artificially propagated trout species, to limit hybridization and support natural mechanisms that optimize fitness for brown and cutthroat trout in their native range.

Climate related factors play a major role in regulating abundance (e.g., flows during incubation and rearing stages, temperatures in summer and winter, etc.) and natural fluctuations in the environment have been linked to variability in salmon returns for decades (Lichatowich 1999; Quinn 2018). Anthropogenic changes in climate are a major threat for fish across the globe and is expected to exacerbate many of the threats and challenges highlighted above, especially for mid-high latitude species that need multiple habitat types to complete their life cycle, like anadromous trout (Bernstein et al. 2008; Williams et al. 2015; Jonsson et al. 2016; Mottola et al. 2020). Specifically, in northern Europe and North America, increased frequency of extreme weather events is expected, coupled with wetter winters and dryer, hotter summers, and less productive marine conditions. Together, these forecasted changes should be expected to intensify current problems and reduce productivity of fish populations that are adapted to historic conditions (Bernstein et al. 2008; Jonsson and Jonsson 2009b, ICES 2013; Freeman In review.). In contrast, the flexibility in life history for anadromous trout (stream, lake, marine waters, iteroparity etc.) and resilience associated with a diverse life history may convey some benefits in a changing climate.

Management

Status and threats are similar for anadromous trout across much of their native range and recognition of their economic value associated with sport fisheries is shared among management agencies. However, approaches to their conservation and recovery vary and management plans specific to anadromous trout are rare, resulting in diverse regulations that are not linked to population performance. One example of a place where management plans do exist is for anadromous rainbow trout (steelhead). Across the range, many steelhead are monitored and regulations may be changed within the fishing season based on estimated abundance. In contrast, in the case of cutthroat trout, there are typically neither abundance estimates nor management plans contingent on them. In European countries, some level of commercial harvest of anadromous brown trout is permitted—primarily in marine waters using commercial traps and nets but also in some lake systems on a smaller scale. In the Scandinavian countries, commercial and recreational gill net fishing is extensive in some coastal areas, and brown trout are either directly targeted by the fishery or caught as bycatch (e.g. (Kuhn et al. 2022)). For anadromous cutthroat trout, low abundance and small size, relative to Pacific salmon species, precludes commercial fishing but some limited ceremonial and subsistence food fisheries occur among Indigenous peoples. Altering harvest regulations for commercial fisheries has shown promise in recovering wild trout populations in some areas of the United Kingdom and Baltic Sea as was observed in the early 2000’s (Fig. 7) (Jutila et al. 2006; ICES 2012). However, given the limited scale of commercial fisheries targeting wild anadromous trout today, recent focus from management agencies has been directed towards sport fisheries. For both anadromous brown and cutthroat trout, sport fishing regulations rely on a diversity of harvest control rules and gear restrictions as well as limitations on time and place to harvest. Broadly, regulations include minimum size limits and maximum bag limits for harvest, depth restrictions to avoid encounters of larger individuals and restriction of the use of bait to limit encounters and critical hooking locations such as the gills and throat (Pauley and Thomas 1993) however both commercial and sport regulations across the range of brown and cutthroat have been and are currently extremely diverse and often inconsistent within a management unit (Johnson and Bjornn 1975; Johnson et al. 1999; Guðbergsson 2014; ICES 2020; Government 2023). For instance, in marine waters of Washington State, USA, fishing regulations have changed considerably from the 1970’s when the daily harvest limit was 16 fish, to the current prohibition of harvest of cutthroat in marine waters as a response to declining abundance (Fig. 8). This change to “catch and release only” regulations in marine waters contrasts with diverse regulations in Washington fresh waters that range from conservative catch and release regulations to more liberal harvest regulations including a two fish daily harvest limit of fish over 8 inches (20 cm). Similarly, Along the northwestern coast of the Baltic Sea, a ban on gillnets shallower than 3 m has been enforced to protect declining stocks of anadromous brown trout (Degerman et al. 2015), however harvest is permitted in some nearby waters. Furthermore, formal evaluations of management strategies are uncommon, in part due to the lack of management plans that provide well-defined management objectives (ICES 2020), policy guidance and decision frameworks (i.e., regulations contingent on abundance estimates) for managers. This is concerning because the mixed stocks (Degerman et al. 2012; Losee et al. 2017b) and nearshore marine distribution make them vulnerable to overexploitation (Blyth and Rönnbäck 2022). Importantly, the regulations and management regimes have not adapted to the increasing scientific knowledge of life-history complexity and movement patterns and remain inconsistent and lacking formal management strategy evaluation or policy guidance.

Fig. 8
figure 8

Fishing regulations for cutthroat trout in marine waters of Washington State, USA between 1955 and 2022

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Both anadromous brown and cutthroat trout are targeted by fishers across their respective ranges. To support long term-sustainable fisheries, managers rely heavily on fishing regulations. To our knowledge a comprehensive understanding of fishing regulations and their effect on abundance of anadromous trout does not exist and represents an area for future research. However, some small scale studies with limited geographic scope have been conducted (Pauley and Thomas 1993; Solomon et al. 2007; Losee et al. 2017b; Skov et al. 2022) and suggest that conservative regulations (e.g. catch and release) in all environments occupied by resident and anadromous life histories can increase catch rates in marine waters, given the close association of resident and anadromous forms as described previously. Additionally, involving anglers in conservation and recovery work has been proven effective to achieve shared goals associated with fishing opportunity and protection of the resource and should be explored more broadly (Shephard et al. 2023). Overall, fishing regulations that consider life history diversity, such as those that protect juveniles, kelts and individuals migrating at the beginning and end of the typical run, combined with improved passage and habitat, are the most important tools to support the long-term recovery and sustainability of anadromous trout as has been adopted in the Shetland Islands and elsewhere (King et al. 2021). In the short term, catch and release regulations maximize fishing opportunity while minimizing impact to wild trout, given high rates of recapture, low rates of release mortality, high economic revenue associated with fishing, and anglers’ willingness to accept stricter regulations to protect the opportunity to fish (Elliot 1989; Degerman et al. 2012; Lothrop and Losee 2016; Liu et al. 2019).

Hatchery production has been applied broadly to management of anadromous brown trout to support fisheries and conservation goals. In the Shetland Islands of the United Kingdom, King et al. (2021) used genetic analysis to evaluate the contribution of hatchery produced brown trout to the total trout catch in the marine environment. They found that release of hatchery brown trout had little to no positive effect on the fishery, and highlighted environmental conditions and habitat quality, including fish passage, as the most important factors affecting wild trout population restoration and improved catch rates. Regardless, releases of hatchery produced brown trout continue across the range to improve catch rates in lakes, rivers and marine waters and in some cases to bolster declining wild populations. In contrast, the production of anadromous cutthroat has received limited attention from managers since the 1970’s when cutthroat trout programs were extensively evaluated (Jones 1977; Mercer and Johnston 1980; Mercer 1982; Rempel 1984; Tipping 1986). Those studies showed low survival rates among smolts released, and limited return to the fishery relative to wild cutthroat trout. Today, hatchery programs producing anadromous cutthroat trout are uncommon with stocking limited to British Columbia (Slaney et al. 1996; Costello 2008), and in Washington on the Columbia River (Tipping and Blankenship 1993; Tipping 2001). These programs are very small relative to salmon and steelhead hatchery programs. This contrast may be important when considering factors affecting wild cutthroat trout versus other species. Consequently, the scientific focus on genetic, ecological, and fishery conflicts between wild and hatchery-produced or farmed salmonids that is so prominent for Atlantic and Pacific salmon (Naish et al. 2007) is less relevant for anadromous brown and especially cutthroat trout.

Habitat protection and restoration represents an area of increasing importance for fisheries managers responsible for anadromous trout. Their use of small streams makes anadromous trout vulnerable to passage barriers such as poorly designed or maintained road culverts, and this is increasingly recognized as a priority area for salmonid conservation (Price et al. 2010; Januchowski‐Hartley et al. 2014), especially considering the effect freshwater conditions can have on marine survival and adult abundance of salmonids. For instance Flávio et al. (2020) demonstrated a high degree of mortality of Atlantic salmon in Northern Ireland, occurring in lower river areas that could have been mitigated through simple habitat-based management actions (e.g. improvement to riparian and in-river habitat complexity), potentially decreasing marine mortality, assuming compensatory mortality is low. Similarly, Losee et al. (2019) analyzed trends in survival rates across six species of hatchery and natural origin Pacific salmon in Washington State and reported the greatest declines occurring for those species that depend on extended rearing in freshwater (e.g. coho salmon and steelhead). There is a growing body of evidence from these and other studies that highlight the potential significant benefit from improved freshwater habitat conditions as a step toward improving marine survival rates and adult returns for anadromous salmon and trout. Analytical approaches that allow for forecasted recovery gains associated with freshwater habitat improvements such as Chasco et al. (2021) may provide managers and elected officials the incentive necessary to increase efforts to improve freshwater habitats.

In contrast to Atlantic salmon, Pacific salmon and steelhead, genetic baseline libraries for stock identification of anadromous brown trout and cutthroat trout have not yet been established and implemented in management and conservation work, however pioneering efforts have been conducted in the northern Baltic Sea and British North Sea coast (Bekkevold et al. 2021; Östergren et al. 2016) and have just begun in North America. Comprehensive genetic baselines for anadromous trout are challenging to establish given the often-numerous small populations occurring along the coasts. However, such work would provide a valuable tool for advancing current management work, especially in situations where mixed stock fisheries occur (as is generally the case for anadromous trout in the sea). For salmon, in some areas of North American and Europe, precise estimates of stock specific mortality are estimated for various stocks, aiming to protect small populations from overexploitation when intermingling with abundant populations in the presence of fisheries. Across the range of anadromous trout, population-specific management targets are seldom applied, although genetic conservation and an awareness of the genetic integrity of populations is being considered (Johnson et al. 1999; ICES 2020). As more sophisticated tools become available (genetics stock identification, stock specific forecasting of runs, etc.) and are applied to management of specific species of anadromous trout, information sharing by managers across the globe should lead to an acceleration of improved management techniques.

Finally, it should be mentioned that brown trout are among the most invasive species in the world (Budy et al. 2013). For example, the Rio Grande in Argentina supports one of the most abundant and diverse populations of anadromous brown trout in the world (O'Neal 2008). Where brown trout are not native, some eradication programs have been implemented to protect native fishes but have been largely unsuccessful. However, in other areas non-native populations of brown trout have been protected by management agencies and are highly prized for their economic value associated with fisheries, particularly for fluvial populations (Jones and Closs 2018). Interestingly, anadromous brown trout have been reported from watersheds also containing native coastal cutthroat (Idyll 1942; Bisson et al. 1986). Given the overlap in ecological niche and trophic position of the two species, sympatric existence would likely be to the detriment of native cutthroat, as has been seen in landlocked sympatric populations in the Yellowstone Lake system (Al‐Chokhachy and Sepulveda 2019). In their native range, introduced species have been identified as a risk to cutthroat and brown trout through predation, competition and genetic introgression and is expected to be exacerbated with an increasingly changing climate (Rahel and Olden 2008; Bae et al. 2018). Research on this topic has primarily been focused on brook trout, rainbow trout, round goby, and most recently invasive pink salmon for brown trout (Landergren 1999; Lehtonen 2002; Larranaga et al. 2019; Lutz et al. 2020; Staveley and Ahlbeck Bergendahl 2022). However, invasive pink salmon in western Europe provide some benefit through nutrient subsidies so more work is needed to understand the level of threat and appropriate response by managers (Dunlop et al. 2021).

Through improved stock assessment work, including genetic stock analysis and biologically relevant management objectives, a more refined approach to management of sport fisheries could be implemented across the range. However, given the complicated life history and numerous, small populations of brown and cutthroat trout, stock recruitment relationships may be applicable only on the stream where they were derived or very similar ones and so not broadly generalizable. Höjesjö et al. (2017) suggested that an approach consisting of many index areas across a broad range of habitat types and geographic distribution may provide managers with an understanding of changes in abundance across the range. This, combined with broad conservative regulations that protect both small, understudied populations and those that are large and receive monitoring efforts could provide a reliable management approach compared to stream-specific regulations that are not based on high-quality data. Together, this review highlights the need for fishery managers and recovery biologists to focus on improving conditions in freshwater for anadromous trout, combined with protective fishing regulations. Habitat improvement projects that promote diversity to buffer extreme weather events, maximize available spawning and rearing habitat, prioritize production of juvenile trout and encourage segregation between natural and hatchery origin fish, are likely to provide the greatest chance of restoring anadromous trout abundance to historic levels and support sustainable fisheries.

Data gaps

By applying a comparative approach to the review of anadromous brown and cutthroat trout, this work has revealed numerous data gaps for two similar species of anadromous trout from opposite sides of the globe. Areas for future research include increased sampling of trout in the marine water for diet and condition throughout the year, identification of genetic sex markers, improved understanding of the role of freshwater conditions in determining rates of marine survival, and investigation into site fidelity of brown trout in the marine environment. Additionally, this review has identified a number of data gaps that should be considered high priority to support long term sustainability of anadromous brown and cutthroat trout. Most conspicuous is the lack of good estimates of abundance patterns and trends across a range of rivers. With a greater understanding of stock abundance and status, more refined management approaches could be utilized to support the recovery of anadromous trout in some places and improved fishing opportunity in others. In some areas, traditional tools like redd counts, riverscapes, tagging (Floy, elastomer and PIT tag technology) and genetics, have been applied and tailored specifically for anadromous trout given their diverse life histories providing great promise for these two species and the management around them. Additionally, new tools being developed for other focal species will be available to apply to anadromous trout in the near future including SONAR (Gaudet 1990; Connolly et al. 2022), seismic monitoring (Dietze et al. 2020) and drones (Groves et al. 2016) to improve estimates of abundance.

Some of the most important knowledge gaps that exist, in addition to lack of abundance estimates, is an understanding of the underlying mechanisms driving life history variation and migration patterns of anadromous trout. This information would support more focused conservation efforts aimed at identifying limiting factors to abundance and resilience. If researchers look to the study designs and results that have emerged from the brown trout research across Europe, the state of knowledge for coastal cutthroat trout could accelerate, particularly with the utilization of otolith microchemistry and acoustic telemetry. In the same way, the propensity for brown trout to exhibit high site fidelity in the marine environment is unknown but likely occurring and would have important implications for fisheries managers in identifying important habitat. This is one recent advancement in the understanding of fine scale movements for anadromous cutthroat that could lead to a novel study design for anadromous brown trout, and to test hypotheses related to migration patterns, variation in life-history types, population abundance and vulnerability to fisheries in the marine environment among others. Together, an improved understanding of environmental and genetic factors explaining growth and performance would clarify the tradeoffs between various migratory patterns exhibited by anadromous trout.

Along with anadromous brown and cutthroat trout, numerous other closely related species exhibit similar behavior but suffer from the same lack of information needed to support sustainable fisheries management. Nearshore distributions of anadromous grayling Thymallus thymallus (Müller and Karlsson 1983; Hellström et al. 2023) and different species of char have been documented, including brook trout, Salvelinus fontinalis referred to as “coasters” and “salters”(Morinville and Rasmussen 2006), bull trout, S. confluentus (Hayes et al. 2011; Goetz et al. 2021), Dolly Varden, S. malma (Bond et al. 2013) and Arctic charr, S. alpinus (Mulder et al. 2020), though some of these species may also occupy offshore waters. Each of these facultatively anadromous species shows some similarities with brown and cutthroat trout as described here, hence future research may highlight important aspects of the behavior and biology of any one species that can help understand and refine management approaches of others.

Conclusion

This review aims to inform sustainable fisheries management of anadromous cutthroat and brown trout and aid in the progression of science relating to anadromous trout and char across the globe. Research is limited on anadromous trout, but these unique salmonids, with their use of many small, short coastal streams for spawning, restricted migrations at sea, and occupancy of primarily nearshore marine habitats, require a unique approach to management and conservation. Recent research in protected waters of the North Atlantic Ocean has increased understanding of abundance, maximum body size, maximum migration distance, feeding, ectoparasite infections, sexual dimorphism and potential for individuals to spawn in brackish marine waters. For anadromous cutthroat trout, these aspects remain poorly understood across their range and represent important data gaps, as are the major sources of mortality and especially the key predators. Recent developments in fish tracking technology, stable isotope analysis, and otolith chemistry have provided researchers with an improved toolbox to address these challenges, and researchers in Europe adopted these techniques and have been leading their application to anadromous trout. Recent studies have revealed spatially and temporally diverse migration patterns, thus the classic pattern “enter the marine environment in the spring and return to freshwater in the fall” is only one of many patterns and not always the most common one. As researchers use the current review as a roadmap for future work, managers should seek ways to incorporate less understood aspects of salmonid life history such as iteroparity, diverse river and marine entry timings and production of anadromous offspring by resident parents into management frameworks tailored specifically to anadromous trout and remain aware of new findings that separate anadromous trout from their semelparous relatives.

Brown trout and cutthroat trout that spawn near marine waters rely on diverse life history strategies spanning the freshwater, estuary and marine environments adding to their resilience in the face of a changing climate and increasing human population growth. Due to this diversity and broad geographic range the challenges anadromous trout face are multifaceted as highlighted in this review. It is evident that anadromous trout, with their opportunistic nature, have taken advantage of a unique niche resource to maximize growth, minimize predation and competition and adapt to variable environments in both the marine and freshwater ecosystems. With declining abundance, and the absence of anadromous trout specific management plans across most of their native range, managers should, in the short term, focus on improving connection between diverse habitat types given the benefit of removing anthropogenic barriers and estuary habitat improvement projects have on marine survival and ultimate adult abundance. Longer term goals should include changing to conservative fishing regulations in some areas that protect resident and anadromous forms and development of comprehensive anadromous trout management plans to provide a pathway to rebuilding anadromous trout populations to historic abundance and diversity while protecting those that are performing well.