Reviews in Fish Biology and Fisheries

, Volume 19, Issue 3, pp 329–347

Biological patterns and ecological indicators for Mediterranean fish and crustaceans below 1,000 m: a review

Authors

    • Institut de Ciències del Mar (CSIC)
  • J. B. Company
    • Institut de Ciències del Mar (CSIC)
  • G. Rotllant
    • Institut de Recerca i Tecnologies Agroalimentaries (IRTA)
  • M. Coll
    • Institut de Ciències del Mar (CSIC)
    • Department of Biology
Article

DOI: 10.1007/s11160-009-9105-6

Cite this article as:
Sardà, F., Company, J.B., Rotllant, G. et al. Rev Fish Biol Fisheries (2009) 19: 329. doi:10.1007/s11160-009-9105-6

Abstract

The Mediterranean Sea is a relatively deep, closed sea with high rates of fisheries exploitation. In recent years fishing activity has tended to shift towards deeper depths. At the same time, the Mediterranean displays some rather special hydrographic and biogeographic conditions. The present paper reviews the present state of knowledge of the fisheries, biology, and ecology of the deep-sea fish and crustacean species in the Mediterranean dwelling below 1,000 m with potential economic interest, placing special emphasis on the western basin, for which more data are available, as a basis for future studies of the ecology, biodiversity, and effects of climate change and exploitation in this zone. This review reveals that mediterranean deep-sea fishes and crustaceans employ highly conservative ecological strategies, and hence the low fecundity and low metabolic rates in a stable environment like the deep-sea make these populations highly vulnerable. Moreover, ripe females of the main species mentioned here concentrate in the deepest portions of their distribution ranges. Deep-sea fish and crustaceans have high trophic levels and low to medium omnivory index values. The ecological indices discussed here, in combination with the limited knowledge of deep-sea ecosystems, clearly call for an approach based on the Precautionary Principle.

Keywords

Biological patternsMediterraneanDeep-seaBiodiversityFishesCrustaceansEcological indicatorsTrophic levelPrecautionary Principle

Introduction

The Mediterranean Sea is the largest and deepest enclosed sea on our planet. The mean depth is around 2,000 m, going down to a maximum of 5,050 m at the trench in the Ionian Sea. These features condition the sea’s morphology and constrain its natural connection to the Atlantic Ocean via the Straits of Gibraltar and its man-made connection to the Red Sea and the Indian Ocean via the Suez Canal (Hopkins 1985; Bas 2002; Fig. 1).
https://static-content.springer.com/image/art%3A10.1007%2Fs11160-009-9105-6/MediaObjects/11160_2009_9105_Fig1_HTML.gif
Fig. 1

Map of the main basins in the Mediterranean Sea

The main hydrographic features of western Mediterranean deep-sea waters are a temperature of 12.7–12.8°C and a salinity of 38.44–38.46 psu. Values are higher in the eastern Mediterranean (T: 13.8°C and S: 38.74 psu below 700 m) (Miller et al. 1970). Still warmer temperatures of around 14°C (Politou et al. 2004) have been recorded at depths greater than 2,500 m. The eastern Mediterranean is more oligotrophic than the western Mediterranean (Estrada 1996), but as a rule the large basins can be regarded as having sufficient oxygen, with values of between 4 and 5 ml/l of oxygen at depths greater than 3,000 m. General oceanographic conditions in the Mediterranean have been described in Miller et al. (1970), Hopkins (1985), Bas (2002) and Sardà et al. (2004a).

The Mediterranean’s relatively narrow continental shelf in conjunction with low primary production, increasing demand for marine resources, and the resulting high levels of fishing intensity have placed the marine resources in the Mediterranean under considerable pressure (Lleonart 1993; Abelló et al. 2002; Coll et al. 2006, 2007; Libralato et al. 2008). Most demersal stocks in coastal areas and on the continental shelves are either fully exploited or overexploited (Farrugio et al. 1993; Papaconstantinou and Farrugio 2000; Bas et al. 2003; Lleonart and Maynou 2003).

Traditional trawl and long-line fisheries capable of reaching depths down to nearly 900 m have been operating in the Mediterranean Sea for many years, mainly in the western basin (Bas et al. 1985; Bas 2002; Sardà et al. 1998a, 2004a, b). In the western Mediterranean, the deep-sea red shrimp (Aristeus antennatus) targed fishery had the need to reach deeper grounds to be profitable, going from 300 m in the 1960s to nearly 900 m at present (Sardà et al. 2003a, b). Thus the “fishing down the deep on marine food webs” (Morato et al. 2006) has also taken place in the Mediterranean Sea in tandem with increases in the size and fishing capacity and power of the fishing fleet, all of which has combined to make fishing activity more dependent on deep-sea resources and especially on deep-sea shrimp on account of the high market prices attained by this target species. There is increasing concern about growth overfishing for some demersal resources, because for many fish species the size at first catch is nearly the same as the size at recruitment (Lleonart 1993; Lloret and Lleonart 2002).

The biomass of the by catch deep-sea shrimps is now composed by dwellers as Alepocephalidae (Alepocephalus rostratus), Macruridae (Trachyrynchus scabrus), Moridae (Mora moro), and Geryonidae (Geryon longipes) and it is relatively high at depths around 1,000 m (Rucabado et al. 1991; Stefanescu and Cartes 1992; Stefanescu et al. 1992a, b; Morales-Nin et al. 1996a, b; Sardà et al. 2004b). As a consequence, the still relatively high biomass levels of Mediterranean deep-sea species could be attractive targets for future exploitation. Nevertheless, turnover in these deep-sea species and the resilience of the ecosystem that supports them are not well known, although important collapses of exploited deep-sea marine resources outside the Mediterranean have highlighted the vulnerability of deep-sea species (e.g., Koslow et al. 2000; Herring 2002; Roberts 2002). Based on our current scant state of knowledge of deep-sea species, it appears that deep-sea bottoms are inhabited by generally slow-growing and longevity species which tend to be specialized feeders and present low fecundity (Carrassón 1994; Gage and Tyler 1991; Merrett and Haedrich 1997; Herring 2002).

Here we review the available information on growth rates, reproductive patterns, fecundity, oxygen consumption, daily ration and diet, individual densities, and species diversity pointing up their large gaps, which need to be filled if we wish to make progress towards a full understanding of how Mediterranean deep-sea ecosystems are structured and function. Gaining an understanding of how these ecosystems work is an urgent need in the context of growing anthropogenic impacts on marine environments and climate change (Koslow et al. 2000; Roberts 2002). The Mediterranean Sea is marked by certain exceptional features, namely, the relatively high, constant temperature (12–14°C) at the sea bed throughout the sea, the much more highly oligotrophic nature of the eastern basin compared to the western basin, the high oxygen concentrations in the deepest regions, and the fact that the Mediterranean is a closed sea in respect of other deep-sea regions. All these special features make the Mediterranean a natural laboratory for comparisons and for checking the effects of the environment on the metabolism, distribution, and abundance of deep-sea dwelling organisms.

Materials and methods

This review considers only those studies dealing with surveys carried out on bottoms at depths below 1,000 m in the Mediterranean Sea, though data for depths shallower than 1,000 m have been included for purposes of comparison. These comparative data have mainly been compiled by experimental fishing carried out in the past 20 years, especially in the Catalan and Balearic seas in the western Mediterranean, where there is a long-standing tradition of deep-sea fisheries and research. More recent data for the Ionian Sea and the eastern Mediterranean have also been included, though the body of data is smaller (Klausewitz 1989; Galil and Goren 1994; Galil and Zibrowius 1989; Goren and Galil 1997). The species considered here have been selected because of their potential interest to fisheries on the basis of their relative abundance or biomass or on the basis of available scientific information.

Various primary sampling systems have been used below 1,000 m in the deep Mediterranean. Frame dredge nets like the Agassiz system were commonly used in the twentieth century (Klausewitz 1989; Galil and Goren 1994; Galil and Zibrowius 1998; Galil 2004), and more recently sampling has been carried out using traps in the eastern Mediterranean (Christiansen 1989; Jones et al. 2003) or underwater photography (Gilat and Gelman 1984). Sampling has also been carried out using bottom trawls equipped with doors, for instance, commercial otter trawls used in demersal fisheries. Specifically, the OTSB trawl gear (Sulak 1982; Rucabado et al. 1991) was used to collect samples down to 2,256 m in the Catalan and Balearic sub-basin in the 1980s. The OTMS system (Sardà et al. 1998b), comprising a commercial gear with wings and bridles adapted so that it can operate at great depth, came into use in the early 1990s, reaching depths of 4,000 m in the basin of the Ionian Sea (Sardà et al. 2004b). This gear is towed by a single warp, uses a backstrop in front of the doors, and has a 27.5 m headrope, yielding a 12.5 m horizontal opening and a 1.4–1.6 m vertical opening. The OTMS trawl achieves a higher vertical opening than OTSB14 and is more hydrodynamic in shape; hence the gear is also able to catch nektobenthic species. Thus, the Agassiz and OTSB gears can be said to underestimate nektobenthic species, while the OTMS gear can be said to underestimate strictly benthic species, since the gear does not worked right against the substrate. Scanmar remote trawl control was used in OTMS.

Most work in the deep-sea region to date has been carried out below 1,000 m and mostly relates to exploratory surveys, listing species, abundance levels, and geographic and depth distributions (Bauchot 1963; Rannu and Gaborit-Rezzouk 1976; Allué 1983; Allué et al. 1984; Golani 1987; Bouchet and Taviani 1992; Goren and Galil 1997; Maynou and Cartes 2000; Cartes et al. 2004) along with diversity indices (Shannon–Weaver index). Important bulk of work is also reporting on the ecological implications of certain biological aspects, e.g., age at maturity and fecundity of individual species (Abelló and Cartes 1992; Stefanescu and Cartes 1992a; Stefanescu et al. 1992a, b; Sardà et al. 1994; Company and Sardà 1997, 2000; Company et al. 2001; Sardà et al. 1998a, b). However, the data mainly concern deep-sea fishes and decapod crustaceans, with information on other deep-sea invertebrates being scarcer (Ramírez Llodra et al. 2008).

The body of trophic information on deep-sea organisms collected in both the western and eastern basins in the Mediterranean has been growing since the 1990s. The quantitative data relating to the trophic ecology of crustaceans and fishes in the western and eastern Mediterranean compiled here have been based on studies of stomach content expressed as percentage wet weight of prey species or groups or percentage volume (Cartes and Sardà 1989; Carrassón and Matallanas 1990, 1998, 2001, 2002; Carrassón et al. 1997, Carrassón et al. 1992; Cartes and Abelló 1992; Cartes 1993, 1994, 1998; Cartes and Maynou 1998; Cartes and Carrassón 2004). These data have been analysed according to three depth strata, namely, depths above 1,000 m, depths between 1,000 and 1,400 m, and depths below 1,400 m.

A species’ trophic level (TL) identifies its position in the food chain (Lindeman 1942; Odum and Heald 1975). By convention, primary producers (e.g., phytoplankton and macro algae) and detritus have a TL = 1, while values for other groups are determined using mass-balance models, gut content analysis, or isotope data (Stergiou and Karpouzi 2002). TL can be formulated as:
$$ {\text{TL}}i = 1 + \sum\limits_{j = 1}^{n} {{\text{DC}}ij\cdot{\text{TL}}j} $$
where i is the predator of prey item j, DCij is the fraction of prey item j in the diet of predator i, and TLj is the trophic level of prey item j.

Data on Mediterranean deep-sea species from the literature and the TrophLab routine (University of British Columbia, available on-line) were used here to calculate the TLs of various deep-sea fish and crustacean species. The square root of the omnivory index (OI) (Pauly et al. 1993) is described as the standard error of the TL and thus is a measure of the uncertainty attaching to the TL value (Christensen et al. 2005). The omnivory index was calculated as the variance of the TL for the prey groups consumed by a predator. An OI value of zero indicates that a consumer is specialized and feeds on a single TL, and a large OI indicates that a consumer feeds on many trophic levels.

Biological patterns

Life cycles

Little is known about the life cycles of most of the deep-dwelling species in the Mediterranean, and hence these traits cannot be used as indicators of adaptation to a given habitat. On the whole, fish species can be said to follow the same metabolic patterns as Atlantic species (Gage and Tyler 1991; Childress 1995; Merrett and Haedrich 1997; Herring 2002). As a rule deep-sea species have slower growth rates, lower fecundity, and lower metabolic rates compared with the more coastal-dwelling species of the upper shelf (Childress and Nygaard 1973; Morales-Nin 1990; Morales-Nin et al. 1996a, b; Company and Sardà 1998). At the same time, because of the high temperature in the Mediterranean, organic matter is highly degraded by the time it reaches the bottom. Organisms dwelling in the nutrient-poor deep-sea environment have low levels of body energy and thus tend to be relatively slow moving, to have high water contents in their tissues, and to support extended periods between occasional encounters with prey items (Childress and Nygaard 1973; Roberts 2002).

Reproductive periods are likewise adapted to fluxes of organic matter from the photic zone during seasons of high primary production (Gage and Tyler 1991; Herring 2002; Company et al. 2003). This has been well documented in crustacean species of the genus Plesionika, in which juveniles and ripe females aggregate at depths coinciding with nepheloid layers (Puig et al. 2001).

Several genera of the gadiform family dwell in Atlantic deep waters and the spawning period of this family vary according to the species and its area of distribution (Bergstad et al. 1999; Koslow et al. 1995; Albertelli et al. 1992). In the Mediterranean, the most coastal-dwelling gadiform species, Phycis blennoides (down to depths of 800 m), spawns all at once, in a single spawning period, while the deeper-dwelling species Mora moro and Lepidion lepidion, which inhabit more stable and oligotrophic habitats, spawn quasi-continuously, the females being batch-spawners. Accordingly, these species have adapted to the environmental conditions of the depths by lengthening their spawning periods, in as much as the impact of seasonal fluxes is lessened at these depths. Since it is no longer necessary to synchronize gonadal development with seasonal energy inputs, spawning can take place quasi-continuously, or a single resting period in the summer suffices (Rotllant et al. 2002).

Species of the family Macruridae like Hymenocephalus italicus, Nezumia sclerorhynchus, and Coelorhynchus coelorhynchus (D’Onghia et al. 2000) and the family Alepocephalidae like A. rostratus (Morales-Nin et al. 1996b) spawn quasi-continuously. In all these species the sex ratio is significantly skewed towards females, which tend to have higher proportions of ripe individuals towards the lower boundaries of their depth ranges.

Reproduction in deep-sea invertebrates has been described for actinians, peracarid crustaceans, brachiopods, bivalves, and echinoderms (Harrison 1988; Tyler 1988; Bishop and Shalla 1994; Campos-Creasy et al. 1994; Ramírez Llodra et al. 2000; Sumida et al. 2000). Deep-sea species have adopted both seasonal and continuous reproductive patterns, even when they coexist in the same area, but continuous reproduction is still the most common in deep-sea habitats below 1,000 m (Tyler 1988; Tyler et al. 1994). Decapod crustaceans have not usually been included in comparative reviews, even though they are one of the most widespread taxa in subtropical waters like the Mediterranean Sea. Decapod crustaceans are the largest megafaunal invertebrate group and have adopted a wide variety of habits and life strategies (Company and Sardà 1997, 2000; Company et al. 2008) all along the continental margins throughout the Mediterranean. At the same time, there is strong evidence of the convergence of traits among species with different life habits and/or among different genera, families, and even infraorders. Some fish species, as macrurids (Stefanescu et al. 1994) or A. antennatus (Sardà et al. 1994), distribute their reproductive population to a specific depth range while in others, like Phasiphaea multidentata, reproductive patterns is not affected by canyon morphology (Ramírez Llodra et al. 2008).

The seasonality of reproduction of coastal and shelf-dwelling species is highly variable, with many species spawning throughout the year and many others being distinctly seasonal (Giangrande et al. 1994). Similar variability has been described for the duration of spawning in deep-sea species dwelling below 1,000 m (Blake 1993; Tyler et al. 1994). Variability within the two separate environments would therefore appear to be linked to a broader range of factors. Seasonality, or the lack of it, can be linked directly to phylogenetic constraints and/or opportunism/adaptation to specific habitat characteristics in both shallow and deep-sea waters, for example, rocky habitats at shallow depths or hydrothermal vents at great depth [sea Ramírez Llodra et al. (2000) for details on the reproduction of three hydrothermal vent-dwelling caridean shrimps]. But, for the intermediate depths considered here, environmental factors seem to play a proximate role in determining spawning period duration.

Population patterns and trends in body size and growth

Basically speaking, Gage and Tyler (1991), Merrett and Haedrich (1997), and Morales-Nin (1990) deemed growth to be relatively slower in deep-sea fish than in shelf-dwelling fish on account of the low levels of energy reaching the bottom.

Between 600 and 1,200 m the trend for fish species is “bigger–deeper” (Stefanescu et al. 1992b; Moranta et al. 2004). This is a significant pattern for most Mediterranean species. Below this latter depth Stefanescu et al. (1992b), Morales-Nin et al. (1996b), and Moranta et al. (2004) report decreasing mean lengths along with decreasing biomass levels for the various fish species. This is a consequence of the replacement of dominant species that are relatively moderate to large in size (A. rostratus, M. moro, T. scabrus, P. blennoides) by smaller species (Coryphaenoides guentheri, Challinura mediterranea, Bathypterois mediterraneus). This indicates that below this depth larger species are unable to meet their energy requirements. Below 1,400 m both density and mean size fall off, the so-called “smaller–deeper trend”. This trend has also been documented in crustaceans (Sardà and Cartes 1993).

Nevertheless, these findings are not absolute. Moranta (2007) reported that for 23 species below 1,000 m, 13 followed the bigger–deeper trend, 2 the smaller–deeper trend, and 8 others no trend. As mentioned above, the 1,000–1,200 m depth interval contains larger species whose largest individuals are concentrated in the deeper part of the interval.

To date been there have few studies focusing on the growth of deep-dwelling species in the Mediterranean. Otolith-based studies have recorded growth bands with translucent and opaque increments on the margins but have not been able to validate the periodicity of deposition. For instance, the growth of certain deep-water species like B. mediterraneus has been studied, seeking to establish growth patterns for deep-sea species in the Mediterranean (Morales-Nin 1990; Morales-Nin et al. 1996a). Otolith studies have yielded relatively long life spans for these species. A. rostratus has been estimated to reach over 20 years of age and B. mediterraneus to reach around 15 years of age. Thus, B. mediterraneus has been reported to have a relatively slow growth rate and to be well adapted to the nutrient-poor deep-sea environment (Morales-Nin et al. 1996b). D’Onghia et al. (2004a, b) recorded this species down to 3,300 m, with the highest abundance levels between 1,500 and 2,000 m. No significant trends linking size to depth were found, most likely because this species already dwells primarily in the deepest, most energy-poor, and most stable habitats. Ripe individuals were mostly at the larger end of the size range and the species was revealed to be a simultaneous hermaphrodite.

In the case of chondrichthyes species, Sion et al. (2004) recorded five species in the Balearic Sea, four in the western Ionian Sea, and six in the eastern Ionian Sea below 1,000 m. The abundance of all these species declined with depth. Galeus melastomus followed the smaller-deeper trend while Etmopterus spinax followed the bigger–deeper trend, reversing the trend for bony fishes.

Growth has only been studied in one crustacean species, the deep-sea red shrimp A. antennatus. This species has a life span of around 5 years and a mean growth rate of 10–12 mm carapace length per year. No differences have been observed over its depth range, suggesting that growth does not differ according to depth (Sardà et al. 2003b, 2004b).

Analysis of changes in growth with depth of maximum abundance for various decapod crustacean species revealed no significant decreases or increases in absolute growth rates as a function of depth (Company and Sardà 2000). However, an intrafamily comparison showed that the deepest-dwelling species had higher growth rates than the shallower-dwelling species. The growth-performance index for A. antennatus, Polycheles typholps, and G. longipes was higher than expected, having regard to their deep depth distributions, since deep-sea species are generally considered to have lower growth rates than species dwelling at shallower depths. These three species are commonly found at depths below 600 m, and their depth ranges can extend down to 2,000 m.

Biomass

Rucabado et al. (1991) and Stefanescu et al. (1993) observed a large biomass increase in the 1,100–1,300 m depth interval. This biomass peak was subsequently pinpointed more exactly at around 1,200 m by Morales-Nin et al. (1996b, 2004), Moranta et al. (2004, 1998). The higher biomass levels recorded between 1,000 and 1,300 m were attributable to the high abundance and sizes of A. rostratus, T. scabrus, M. moro, L. lepidion, and G. longipes. Tables 2 and 3 summarize the dominant fish and crustacean species in the different depth intervals.

The above biomass peak was recorded both in the Balearic Sea (Rucabado et al. 1991; Stefanescu et al. 1993) between 1,000 and 1,200 m and in the Algerian sub-basin (Moranta et al. 2004). The biomass consists of moderately large to large fish attaining peak abundance levels at these depths, e.g., A. rostratus (Morales-Nin et al. 1996b) and other species that follow the bigger–deeper pattern, e.g., T. scabrus and P. blennoides (Massutí et al. 1995, 1996).

These biomass peaks were subsequently linked to suprabenthos abundance between 800 and 1,200 m (Stefanescu and Cartes 1992; Cartes et al. 2001; D’Onghia et al. 2004c). Stefanescu et al. (1993) regarded the 1,200 m isobath as the lower boundary for the mesopelagic fauna associated with a substantial decrease in food resources.

Massutí et al. (2004) found the biomass and abundance of the different fish species to be lower in the Mediterranean than at the same depths in the Atlantic Ocean because the Mediterranean is a more oligotrophic sea (Table 1). At the same time, crustacean abundance relative to fishes is higher in the Mediterranean than in the Atlantic. The explanation postulated for this finding is that the more degraded organic matter is more beneficial to crustaceans than to fish species, with their higher energy requirements. These same trends were reported by Company et al. (2004), who observed that the relative predominance of crustaceans over fish increases with longitude from the western to the eastern Mediterranean as well as with depth. In other words, the increasing oligotrophy associated with geographical longitude from the western to the eastern Mediterranean exerts the same effect as depth, whose effect is likewise attributed to the greater paucity of nutrients at depths below 1,500 m compared with shallower regions. The low levels of fish biomass and the decrease in diversity observed at depths below 1,400 m can thus also be explained in terms of the paucity of food resources. Carrassón et al. (1992, 1997), and Carrassón and Matallanas (1998), working on such species as B. mediterraneus, L. lepidion, A. rostratus, and Centroscymnus coelolepis, suggested that depth resulted in heavy selection pressure among species based on their feeding strategies.
Table 1

Comparison of fish biomass per depth interval (k.km−2) between w. Mediterranean (Balearic Sea) and Atlantic (Rockall Trough)

400–800 m

800–1,400 m

1,400–1,700 m

Mediterranean

    160 ± 10

420 ± 50

120 ± 10

Atlantic

    4,160 ± 134

4,360 ± 510

3,020 ± 810

Data adapted from Massutí et al. (2004)

Moranta (2007) reported seasonal variations in the size structure and relative roles of the dominant species in deep-sea fish assemblages. Seasonal differences in species richness and density with depth in the Balearic sub-basin are related to temporal variability in the amount of organic matter reaching the bottom in late spring. Accordingly, Helicolenus dactylopterus and T. scabrus were more abundant in the Balearic sub-basin, while Hymenocephalus italicus, G. melastomus, and C. coelolepis were more abundant in the Algerian sub-basin. The biomass of H. dactylopterus in the submarine canyons in the Balearic sub-basin doubles from December to April. In contrast, the relative abundance of P. blennoides on the upper slope increases in December. T. scabrus is more abundant on the middle slope in spring. No such seasonal variations have been observed in the Algerian sub-basin. These differences can be explained by higher primary production in the surface layers, river runoff, and the role of the large submarine canyons (not present in the Algerian sub-basin) in the lateral transport of particulate matter.

Ecological indicators

Main boundary and assemblages

Various researchers have reported boundaries affecting community structures. The first observations for the deep-sea zone in the Mediterranean (down to 2,200 m) were published by Stefanescu et al. (1993), who defined four fish assemblages centered on four isobaths, i.e., at 1,097 ± 111 m, 1,364 ± 61 m, 1,566 ± 115 m, and 1,847 ± 214 m. In the Algerian sub-basin south of the Balearic Islands, Moranta (2007) reported two stable fish assemblages in two depth zones, one between 800 and 1,400 m and the other between 1,400 and 1,700 m. Based on abundance, he found that A. rostratus predominated in the shallower interval and B. mediterraneus in the deeper interval. Grouping samples down to 4,000 m in the western Mediterranean and in the central Mediterranean (western and eastern Ionian Sea), D’Onghia et al. (2004c) observed a different depth zonation pattern with intervals defined as 600–800, 800–1,300 m, and below 1,300 m, more akin to the pattern in the Atlantic (Haedrich et al. 1980; Haedrich and Merrett 1988) and later described in the Mediterranean (Massutí et al. 1995; Moranta et al. 2004).

Cartes and Sardà (1992, 1993) defined three depth intervals for decapod crustaceans, with boundaries located at around 600, 1,200–1,300, and 1,900–2,000 m, separating the upper, middle, and lower slopes, respectively. These distribution ranges are consistent with more recent results for fishes published by Cartes et al. (2004) or D’Onghia et al. (2004c). Finally, Sardà et al. (2004a) observed changes in the abundance and biomass of the eurybathic deep-sea red shrimp for the 800–1,000, 1,000–1,400, and >1,400-m depth intervals. Peak abundance of this species occurs at around 800 m and then gradually declines down to 1,400 m. All ecological indicators (biomass, abundance, diversity, species richness, mean weight) fall off sharply below this latter depth. The different zonation patterns reported by the different researchers could represent a spectrum produced by the small number of samples from the deep-sea region available to date, different numbers of hauls in the depth intervals being compared, the time of year when sampling was carried out, or the definitions of the depth intervals sampled on each survey. Nevertheless, there are certain basic similarities which can be interpreted jointly for crustaceans and fishes and generalized to yield well-defined boundaries at around 800 and 1,400 m, with differing species compositions in each zone and distinct drops in abundance. Table 2 summarize the contribution of more representative species on the different depth intervals.
Table 2

Biomass dominance and abundance (> 30% of total weight and number) of dominant fish and crustacean species by depth interval in the western Mediterranean Sea

Depth intervals (m)

Biomass

Abundance

Fishes

    <1,000

G. melastomus

T. scabrus

    1,000–1,200

A. rostratus

A. rostratus

    1,200–1,500

A. rostratus

B. mediterraneus

    1,500–2,500

C. coelolepis

B. mediterraneus

    >2,500

Ch. mediterranea

Ch. mediterranea

Crustaceans

    <1,000

A. antennatus

M. tenuimana

    1,000–1,200

G. longipes

M. tenuimana

    1,200–1,500

A. eximia

A. eximia

    1,500–2,500

A. eximia

A. eximia

    >2,500

A. eximia

A. eximia

Data adapted from Massutí et al. (2004) for fishes and Cartes and Sardà (1992) for decapod crustaceans

Diversity and species richness

Diversity and species richness decrease with depth. This phenomenon is typical of all seas except in certain habitats like the upwelling zones around oceanic ridges in the Atlantic. In the Mediterranean Sea these decreases are distinctly discernible below 800 m and are particularly pronounced below 1,500 m, where diversity and abundance drop off sharply to their respective minimums (Cartes and Sardà 1992; Company et al. 2004; Moranta et al. 2004; Sardà et al. 2004a).

The density of the Levantine deep-water fauna has long been presumed to be the lowest in the Mediterranean (Fredj 1974). However, data are admittedly scanty, perhaps due to sparse research efforts in different parts of the Mediterranean (Fredj and Laubier 1986; Company et al. 2004; Galil 2004).

Fish and crustacean communities adapt to the poorer, environmentally more stable deep-sea bottoms by reducing diversity, abundance, and biomass and increasing evenness (Fig. 2) (Sardà et al. 2004a; Moranta 2007). Populations at depths below 800–900 m consist of species that are more eurybathic than more coastal species simply because environmental conditions are more stable. Nevertheless, populations use two main strategies to adapt to these conditions because of the general decline in food availability. Either species mutually exclude certain stages of their life cycles across their depth ranges to avoid competition (Puig et al. 2001) or they modify their population structure over their depth range (Sardà et al. 2004a). By way of an example of the former, juveniles and ripe females of certain species of the genus Plesionika are concentrated at depths where there are maximum levels of suspended particulate matter called as nepheloid layers (Company and Sardà 1997). This is a feature of species with distribution ranges between 400 and 800 m. As an example of the latter, the deep-sea red shrimp A. antennatus modifies its population structure, concentrating ripe females and large individuals in the upper portion of its range where there is greater resource availability and relegating individuals with a lower mean size to very low densities down to below 2,000 m in depth (Sardà et al. 2004b). Juveniles are found mainly at depths around 1,200 m, where nepheloid layer is found, food availability is higher and predation is presumably lowest (Sardà and Cartes 1997).
https://static-content.springer.com/image/art%3A10.1007%2Fs11160-009-9105-6/MediaObjects/11160_2009_9105_Fig2_HTML.gif
Fig. 2

Biomass, diversity, and species richness for a fishes and b decapod crustaceans at various depths in the Mediterranean Sea set up from Tables 2 and 3 and re-calculated from Sardà et al. (2004a, b) data

The above-mentioned trend towards decreasing fish abundance and increasing crustacean abundance with depth from west to east in the Mediterranean has been ascribed to lower food availability and the ability of crustaceans to adapt to lower levels of energy input. The relative increase undergone by crustaceans in the shallow waters of the eastern Mediterranean basin is as well indicative of their greater adaptability to more energy-poor regions (Danovaro et al. 1999; Psarra et al. 2000; Tselepides et al. 2000; Company et al. 2004). These differences are observable not only with longitude but also with depth, i.e., with crustacean abundance exceeding fish abundance in number of individuals at 4,000 m in the Ionian trench (Table 3).
Table 3

Total fish and decapod abundance (no.·km−2) by depth (m) and basin in the Mediterranean Sea (data compiled from Company et al. 2004 and D’Onghia et al. 2004c)

Western

Central

Eastern

Abyssal

Depth (m)

Fish

Crust

Depth (m)

Fish

Crust

Depth (m)

Fish

Crust

Depth (m)

Fish

Crust

600

1,300

2,200

600

1,850

1,800

590

3,500

5,800

3,300

211

422

800

2,900

1,800

800

1,700

900

610

1,800

6,200

4,000

122

511

1,000

1,950

800

1,000

1,350

600

805

800

1,100

   

1,200

6,800

800

1,200

2,900

2,300

810

1,000

850

   

1,500

1,250

1,300

1,500

3,050

1,000

1,100

1,250

150

   

2,500

900

1,100

1,700

950

350

1,300

800

200

   

2,800

950

1,100

2,000

250

950

1,700

1,000

1,650

   
      

2,200

250

1,000

   
      

2,600

250

1,900

   

The biogeographic separation between basins by shallower, narrow channels gives rise to biogeographic separation of the deep-sea water masses as well as the deep-sea species that inhabit them (Company et al. 2004; D’Onghia et al. 2004c). The species A. rostratus, N. aequalis, and C. coelolepis are found only in the western basin and have not been recorded in the central or eastern basins. Such other species as T. Fus, L. lepidion, M. moro, and Cataetyx laticeps are much more abundant in the western basin. Thus, the biomass peaks observable in the western Mediterranean do not appear to occur in the other Mediterranean basins.

In addition, certain crustacean species are found only in the western Mediterranean and in the western region of the Ionian basin but not in the easternmost region. These include Paromola cuvieri, Pontophilus norvegicus, and P. multidentata, while Stereomastis sculpta is present only in the western Mediterranean. This last-mentioned species is taken at depths below 1,500 m. Relative species abundance also changes with longitude, with Aristaeomorpha foliacea being most abundant above 1,000 m in the Ionian Sea, while A. antennatus is much more common in the Balearic sub-basin (western Mediterranean). These differences on geographic distribution also occur in other very deep-dwelling species, but more surveys are needed to compile records at depths below 1,500 m.

Lastly, genetic studies carried out on deep-sea and coastal populations of A. antennatus have revealed high gene flows between these “subpopulations”, specifically in individuals taken at 800 m and at 1,500 m. This proves that populations of this species are continuous across both the geographic distribution area and depth in the Mediterranean (Sardà et al. 1998a).

Feeding habitats, trophic structure and ecological role

The main dietary and trophic data available are summarized by predator species and depth in Table 4 for fish and in Table 5 for crustaceans (on an annual or seasonal basin). Information on deep-sea organisms is more abundant for crustaceans (Cartes and Sardà 1989; Cartes and Abelló 1992; Cartes 1993, 1994, 1998; Cartes and Maynou 1998; Cartes et al. 2001) than for fishes and other invertebrates (Carrassón and Matallanas 1990, 1998, 2001, 2002; Carrassón et al. 1992; Cartes and Carrassón 2004), and most of the information has been collected in the western basin. Generally speaking, the diets of deep-sea fish and crustacean species are selective and based mainly on invertebrate prey items (principally natantian and reptantian decapods and suprabenthic species). Among fishes, E. spinax has the highest proportion of other fish species in its diet, followed by C. laticeps and A. rostratus. Cephalopod species also make up a sizable portion of the diets in E. spinax and A. rostratus. Deep-sea crustaceans base their diets on suprabenthic organisms and other benthic invertebrates like euphausiids (Tables 5, 6). Available trophic data by season reveal substantial variation in food availability during the year.
Table 4

Review of dietary and trophic data on various Mediterranean deep-sea fish species by depth range

<1000 m

1000–1400 m

>1400 m

References

Specie

TL (Sd)

OI

Main preys (%)

TL (Sd)

OI

Main preys (%)

TL (Sd)

OI

Main preys (%)

Alepocephalus rostratus

sp: 3.41 (0.48); su: 3.48 (0.57)

sp: 0.23 su: 0.33

sp: tun (76), nat (16,4); su: dec (81), eup (9)

3.78 (0.61)

0.37

nat (40), ceph (20), ost (20)

Carrassón 1994, Carrassón and Matallanas 1998

Bathypterois mediterraneus

3.19 (0.38)

0.14

bcop (42), mys (30)

3.06 (0.36)

0.13

mys (50), bcop (22)

Carrassón 1994, Carrassón and Matallanas 2001

Cataetyx laticeps

3.27 (0.60)

0.36

ost (22), rep (15), sup (10), nat (7)

Carrassón 1994

Centroscymnus coelolepis

4.36 (0.42)

0.18

cep (87), ost (6)

Carrassón et al. 1992, Carrassón 1994

Chalinura mediterranea

3.19 (0.49)

0.24

nat (44), mys (21), amp (12)

Carrassón 1994

Coryphaenoides guentheri

3.27 (0.44)

0.19

amp (26), pol (25), nat (10)

Carrassón 1994

Etmopterus spinax

4.22 (0.69)

0.48

ost (54), ceph (23)

 

Macpherson 1980, 1981

Galeus melastomus

3.79 (0.6)

0.36

nat (49), ceph (17), ost (7)

4.17 (0.64)

0.41

ceph (37), ost (33), nat (26)

https://static-content.springer.com/image/art%3A10.1007%2Fs11160-009-9105-6/MediaObjects/11160_2009_9105_Figa_HTML.gif

Macpherson 1980; Carrassón et al. 1992, Carrassón 1994

Lepidion lepidion

3.67 (0.57)

0.32

nat (36), ost (17), rep (11)

https://static-content.springer.com/image/art%3A10.1007%2Fs11160-009-9105-6/MediaObjects/11160_2009_9105_Figa_HTML.gif

Carrassón and Matallanas 1990, Carrassón 1994, Carrassón et al. 1997

Mora moro

3.56 (0.61)

0.37

rep (80), nat (14) ost (5)

Carrassón 1994

Nezumia aequalis

3.23 (0.52)

0.27

rep (31), amp (29), pol (10)

Macpherson 1981; Carrassón and Matallanas 1990

Phycis blennoides

3.52 (0.52)

0.27

nat (36), rep (31)

3.45 (0.57)

0.32

rep, nat, amp, iso, mys (*)

Macpherson 1981; Carrassón 1994

Trachyrhynchus scabrus

3.62 (0.63)

0.40

rep (88), ost (12)

3.39 (0.54)

0.29

rep (60), nat (10)

Macpherson 1981; Carrassón 1994

Information has been summarized by season and main prey items identified. Trophic level (TL), standard deviation (Sd), and omnivory index (OI) are given.

ost osteichthyes, ceph cephalopoda, nat natantia, rep reptantia, dec decapoda, amp amphipods, so isopods, sup suprabenthos, bcop benthic copepods, pol polychaetes, tun tunicata, eup euphausiids, mys mysids, sp spring, su summer. Arrow indicates the feeding depth range. (*) Qualitative data

Table 5

Review of dietary and trophic data on various Mediterranean deep-sea decapod crustacean species by depth range

<1000 m

1000–1400 m

>1400 m

References

Specie

TL (Sd) >1000

OI

Main preys (%)

TL (Sd)

OI

Main preys (%)

TL (Sd)

OI

Main preys (%)

Aristeus antennatus

a: 3.10 (0.47); w-su: 3.27 (0.47); w-c: 3.03 (0.43); a-c: 3.04 (0.36)

a: 0.23; w-su: 0.23; w-c: 0.19; a-c: 0.13

a: iso (23), pinv (20), nat (14); w-su: nat (20), iso (15), pol (10); w-c: pol (40), nat (13); a-c: pol (33), cuc (30)

3.03 (0.43)

0.18

pol (20), biv (10), amp (7)

3.07 (0.43)

0.18

biv (15), pinv (15), pol (12)

Cartes (1994)

Acanthephyra eximia

3.85 (0.67)

0.45

ost (39), nat (35)

3.63 (0.62)

0.38

nat (50), ost (25)

3.76 (0.66)

0.44

nat (40), ost (24)

Cartes (1991)

Chaceon mediterraneus

 

 

3.25 (0.45)

0.20

pol, gas, iso.(*)

Cartes (1991)

Geryon longipes

su: 3.32 (0.55); a: 3.11 (0.48)

su: 0.30; a: 0.23

su: rep (27), biv (21), ost (16), iso (10); a: rep (32), ech (20), pinv (12)

su: 3.14 (0.49)

0.24

pol (21), pinv (13), ost (12)

3.42 (0.41)

0.17

pinvp (26), polp (23), mol (12)

Cartes (1991)

Munida tenuimana

su: 3.75 (0.57); a: 3.33 (0.50)

su: 0.33; a: 0.25

su: ost (32), nat (23), mys (11); a: eup (33), nat (16)

su: 3.61(0.55); a: 3.43 (0.59)

su: 0.30; a: 0.35

su: amp (30), jel (22), ost (15); a: nat (28), ostb(23), eup (12)

3.53 (0.6)

0.36

nat (37), ost (23)

Cartes (1991)

Paromola cuvieri

3.67 (0.63)

0.40

rep (31), ost (30), pinv (18)

https://static-content.springer.com/image/art%3A10.1007%2Fs11160-009-9105-6/MediaObjects/11160_2009_9105_Figa_HTML.gif

   

Cartes (1991)

Pasiphaea multidentata

su: 3.23 (0.54); a: 3.32 (0.51); w-c: 4.06 (0.70); a-c: 4.05 (0.71)

su: 0.29; a:0.26; w-c: 0.49; a-c: 0.50

su: ost (23), iso (18), eup (12), nat (12); a: eup (48), ost (37), amp (16); w-c: ost (61), rep (21), eup (18); a-w: ost (55), nat (24)

3.36 (0.52)

0.27

eup (36), nat (28), ost (20)

https://static-content.springer.com/image/art%3A10.1007%2Fs11160-009-9105-6/MediaObjects/11160_2009_9105_Figb_HTML.gif

Cartes (1991)

Plesionika acanthonotus

su: 3.28 (0.54); a: 3.25 (0.58)

su: 0.29; a: 0.34

su: ost (44), iso (13), nat (10); a: eup (48), ost (26)

sp: 3.34 (0.49); a: 3.18 (0.53)

sp: 0.24; a: 0.28

sp: amp (24), ost (17), jel (16), nat (13); a: ost (40), eup (15)

   

Cartes (1993)

Polycheles typhlops

w-sp: 3.29 (0.54); a: 3.27 (0.51)

w-sp: 0.29; a: 0.26

w-sp: ost (28), nat (20), mys (18); a: nat (24), ost (16), eup (15), amp (10)

3.38 (0.49)

0.2401

ost (17), nat (18), pol (14), iso (13), mys (12), amp (10)

3.33 (0.52)

0.27

amp, iso, pol, ost.(*)

Cartes (1991), Cartes and Abelló (1992)

Information has been summarized by season and main prey items identified. Trophic level (TL), standard deviation (Sd), and omnivory index (OI) are given

ost osteichthyes, nat natantia, rep reptantia, amp amphipods, iso isopods, pol polychaetes, ech echinoderms, cuc sea cucumbers, biv bivalves, gas gastropods, mol mollusks, jel jellyfish/hydroids, eup euphausiids, mys mysids, pinv planktonic invertebrates. sp spring, su summer, a autumm, w winter, c canyon. Arrow indicates the feeding information range. (*) Qualitative data

Table 6

Summary of the biological indicators of the main deep-sea megafauna species according different references cited in the text

Species

Range distribution (maximum. abundance)

Maximum size (cm)

Abundance (ind. km−2)

Biomass (k.km−2) depth intervals (m)

Reproduction

Females growth (K.year−1)

Observations

<1,000

1,000–1,400

>1,400

Osteichthyes

Alepocephalus rostratus

500–2,100 (1,400)

48 (TL)

5,922

109

1,332

 

Year around

0.08

b–d

Trachyrhynchus scabrus

450–1,200 (950)

10 (AL)

697

80

  

su–a

0.14

b–d

Nezumia aequalis

450–1,500 (750)

7 (AL)

174

17

40

 

Year around

0.17

b–d

Coryphaenoides guentheri

1,409–2,800 (1,900)

70 (AL)

295

  

8

 

0.2

 

Bathypterois mediterraneus

1,308–2,800 (1,800)

18 (SL)

641

 

5

 

s

0.46

H

Chauliodus sloani

1,200–2,800 (1,900)

10 (TL)

517

 

12

14

   

Lepidion lepidion

400–2,250 (1,300)

37 (TL)

1,160

 

53

 

a–w–sp

0.45

b–d

Mora moro

800–1,326 (1,150)

52 (TL)

765

471

  

a–w–sp

  

Cataetyx laticeps

1,700–2,800 (2,000)

 

99

  

33

   

Phycis blennoides

60–1,200 (400)

62 (TL)

532

56

  

a

 

b–d

Selaceans

Etmopterus spinax

800–1,600 (1,200)

 

151

 

40

   

b–d

Centroscymnus coelolepis

1,200–2,800 (1,900)

 

517

 

12

248

   

Galeus melastomus

650–1,500 (1,300)

70 (TL)

1,003

389

  

Year around

 

s–d

Crustaceans

Aristeus antennatus

170–3,300 (800)

 

1,044

13

5

1

sp–su

0.28

F–M

Acanthephyra eximia

400–2,800 (2,100)

 

420

 

1

1

sp–su

 

F

Acanthephyra pelagica

1,000–2,800 (2,500)

 

18

 

100

1

   

Plesionika acanthonotus

421–1,750 (650)

 

239

1

  

sp–su

0.55

=

Geryon longipes

335–1,950 (650)

 

219

18

10

 

su

0.54

M

Chaceon mediterraneus

2,250–2,800 (2,500)

 

68

  

8

  

M

Paromola cuvieri

600–1,500 (650)

 

11

1

1

 

Year around

 

F

Pasiphaea multidentata

600–1,500 (650)

 

22

1

108

 

a

0.8

F

Polycheles typhlops

600–1,400 (750)

 

22

1

101

 

Year around

0.45

F

Munida tenuimana

600–1,400 (750)

 

65

61

79

 

s–a

0.4

=

TL total length; ST standard length; AL pre-annal length; H hermaphrodite; b-d, “bigger–deeper” trend; s–d “small–deeper” trend; F, female dominance; M, male dominance;= sex-ratio 1:1; su summer; a autumn; w winter; sp spring

Among the fish species, B. mediterraneus had the lowest trophic level (TL) and C. coelolepis the highest, both for the <1,400 m depth range. Among the crustaceans, A. antennatus above 1,000 m in autumn and winter had the lowest TL and P. multidentata likewise above 1,000 m in autumn and winter the highest. The TLs for A. antennatus, A. eximia, and G. longipes all decrease from >1,000 m to the 1,000–1,400 m depth interval, indicating changes in feeding to lower trophic levels at greater depth, but they then rise again towards the <1,400 m interval. The TLs for P. multidentata and P. typhlops appear to increase with depth, indicating changes in feeding to higher trophic levels with depth. The TL increases for G. melastomus and A. rostratus with depth and decreases for B. mediterraneus, P. blennoides, and T. scabrus. By depth interval, Acanthephyra eximia has the highest TL among crustaceans in the deep sea above 1,000 m, from 1,000 to 1,400 m, and below 1,400 m, followed by Munida tenuimana also in all three intervals. Among fish species, E. spinax clearly has the highest TL both above 1,000 m and in the 1,000–1,400 m interval, while the chondrichthian C. coelolepis has the highest TL for the interval below 1,400 m.

On the whole, fish species have slightly higher TLs than crustaceans in the three depth intervals considered (Fig. 3). The TLs for fish species, moreover, decrease with depth, while the data for crustaceans indicate that TL values decrease from 1,000 to, 1,400 m and then a non significant slightly increase at <1,400 m. Nevertheless, these differences are not significant and highly depends on species particularities.
https://static-content.springer.com/image/art%3A10.1007%2Fs11160-009-9105-6/MediaObjects/11160_2009_9105_Fig3_HTML.gif
Fig. 3

Mean trophic level (TL) values for the fish and crustacean species set out in Tables 5 and 6. TLs for all species were weighted equally

TL results obtained from studies carried out at shallow and deep depths suggest that there are certain differences between shallower-dwelling species and deep-sea species. The TLs for crustaceans from the deep-sea region are higher than the values calculated using ecological models for the shelf and upper slope in the southern Catalan Sea, the northern and central Adriatic Sea and the coastal area in the Bay of Calvi (Corsica, NW Mediterranean) (Pinnegar 2002; Coll et al. 2006, 2007). The TLs for deep-sea fishes are also on the high side compared with the results obtained in the shallower areas mentioned above. Higher trophic levels have been described as corresponding to species with low fecundity and low metabolic rates (e.g., Chondrichthyes). This could also highlight relative bigger organisms from deep sea in comparison with the continental shelf, or a higher proportion of predators due to a lower fishing pressure following the paradigm of fishing down the food webs (Pauly et al. 1998). The results for trophic levels calculated from stomach content analysis are consistent with the results obtained using stable nitrogen and carbon isotope analysis (δ15N and δ13C). This is illustrated in a study performed off the SW Balearic Islands (NW Mediterranean Sea) (Polunin et al. 2001). This study based on stable isotopes yielded strong positive correlations between the δ15N and δ13C data for plankton, fishes, and crustaceans, probably indicating a single primary source material for Mediterranean deep-sea communities attributed to marine snow.

In contrast, the omnivory index for crustaceans and fishes is medium to low (Tables 5, 6) with the highest values presented by the chondrichthyes species of E. spinax and G. melastomus, and by T. scabrus. This evidences a highly specialized trophic behavior of the organisms inhabiting the Mediterranean deep-sea environment. This implies that they show lower levels of omnivorism in relation with those described in organisms from coastal areas and the continental shelves. Higher omnivory has been related to lower variability of the food-web and the facilitation of estability of shallowest and coastal species (McCann et al. 1998; Bascompte et al. 2005).

General patterns and final remarks

This review of available information on Mediterranean deep-sea organisms has provided for the first time a list of helpful ecosystem indicators describing the main biological and ecological patterns of the deep-sea ecosystem. Major gaps in the available information have been identified and should be addressed by research conducted in Mediterranean deep-sea environments in the near future if a comprehensive picture of Mediterranean deep-sea ecosystems is to be drawn. As a general rule, systems that are, a priori, simpler and less variable, like deep-sea systems, represent an excellent opportunity to test hypotheses and put forward approaches that could be extrapolatable to more highly variable regions like the littoral zone.

A summary of the patterns observed are shown in Table 6. Here follows a list of patterns useful as indicators of the current status of deep-sea populations in the Mediterranean Sea.
  • A main boundary has been detected at 1,400–1,500 m (zonation). Below this depth all ecological index values (biomass, abundance, diversity, species richness, mean weight) fall off sharply.

  • The main assemblages in the Algerian Bassin inhabit the <800, 800–1,400, and 1,400–1,700-m depth intervals.

  • Biomass increases between 1,000 and 1,400 m resulting from the high abundance and size of A. rostratus.

  • Between 600 and 1,400 m fish species generally follow a “bigger–deeper” trend.

  • Spawning female fish occupy the deeper portion of a species’ distribution range.

  • Spawning female fish outnumber males in deep-sea waters.

  • Deep-sea fishes display low fecundity compared with shallower-dwelling fishes.

  • Below 1,400 m large fish species are replaced by smaller species.

  • Below 1,000 m crustaceans generally follow a “smaller–deeper” trend.

  • Crustacean diversity and species richness also decrease dramatically below 1,000 m.

  • Deep-sea fish and crustacean species have high trophic levels and low to medium omnivory index values, evidencing specialized feeding strategies. Low omnivory in the food-web can suggest low tolerance of external disturbance such as fishing.

Accordingly, this review reveals that deep-sea fishes and crustaceans employ highly conservative ecological strategies, and hence the low fecundity and low metabolic rates in a stable environment like the deep-sea make these populations highly vulnerable. Moreover, ripe fish females of the main species mentioned here concentrate in the lowest portions of their total (shallow and deep-sea) distribution ranges. Deep-sea fishes and decapod crustaceans have high trophic levels and low to medium omnivory index values. The ecological indices discussed here, in combination with the limited knowledge of deep-sea ecosystems, clearly call for an approach based on the Precautionary Principle. For all the above reasons, the authors recommend not undertaking any commercial exploitation below depths of 800–1,000 m following on the trawling band established by the Council of Mediterranean Fisheries (GCMF) (WWF/IUCN 2004) and expanding it to other fishing fleets, even though some high fish biomass can be found in this region of the Mediterranean Sea.

Acknowledgments

The authors would like to thank the Captains and crews of R/V “García del Cid” (CSIC) for their technical support, and Mr. J. Rucabado and Dr. D. Lloris as the pioneers in using the otter bottom trawl as a sampling scientific method in the Mediterranean deep-sea. M. Coll has been founded by a postdoctoral fellowship from the Ministerio de Ciencias e Innovación from Spain.

Copyright information

© Springer Science+Business Media B.V. 2009