Diet of the Otter Lutra lutra inhabiting a forest stream in SW Poland

Diet of the otter inhabiting a forest stream in SW Poland was studied in order to show the impact of the otter on �sh and other aquatic animals. The diet was examined by means of spraint analysis. A total of 157 spraints were collected from 14 sites, and 605 prey items were extracted. Fish comprised the staple diet. It was supplemented by frogs, birds, small mammals and invertebrates. Fish were represented by at least 23 species belonging to seven families. Two families Cyrinidae and Percidae dominated the �sh component of the diet (together 70.1%). The most numerous �sh species consumed were Perca �uviatilis (24.1%) and Rutilus rutilus (12.1%). The otter diet was most diversi�ed in the coldest months of the year (November-April; B = 10.0-10.5), and least in the warmest months of the year (July-August; B = 5.7). The niche breath was wider in 2006 (B = 12.2), compared with 2007 (B = 20.4). However, only the proportion of G. gobio and Esox lucius were statistically different in those two years compared. This study con�rms previous �ndings that the otter is a generalist piscivorous predator hunting opportunistically on locally and seasonally most common �sh species, mainly of low economic importance.


Introduction
Once rare and declining, the Eurasian otter Lutra lutra begin to expand in Poland.Its increase in numbers coincides with the occupation of small river courses and sh ponds, regarded as its suboptimal habitats (Romanowski 2006).As a result of this expansion, the species is today widespread and relatively common throughout Silesia, SW Poland (Kopij 2016).It inhabits not only Oder, the largest river in this region, but also all its tributaries (rivers of the II order), and some rivers of the III orders (e.g.Ścinawa Niemodlińska, Biała, Czerna).Opportunistic foraging habits enable it to colonize also man-modi ed habitats, such as channels, larger sh-ponds (e.g.near Niemodlin), and water reservoirs (e.g.Nyski and Otmuchowski).
The otter is recognized as one of the piscivorous top predators in freshwater ecosystems, and thus has the potential to play an important role in the functioning of these ecosystems.The diet varies in different localities, different habitats and different seasons of the year (Jędrzejewska et al. 2001; Krawczyk et al.

2016).
Although the otter diet has been thoroughly studied in Europe (Jędrzejewska et al. 2001;Clavero et al. 2003; Krawczyk et al. 2016), it is not well known from the suboptimal habitats of small rivers colonized recently.From Poland, such data are available only from small water courses in agricultural landscape (Krawczyk et al. 2011).The aim of the present study is to determine the diet composition and its changes on the monthly and annual basis of otters inhabiting forest streams.

Material And Methods
Spraints were collected from 14 sites located on the Czerna Wielka River.It is III order rivulet, a tributary of Bóbr River (II order), which in turn is a tributary of the Odra River (I order).The river is 72 km long and its drainage area is 949 km 2 .Its spring is located in Izerskie Upland at 283 m a. s. l. (51.38.02N/15.18.21E) and its con uence with Bóbr River is located in Żagań town at 92 m a. s. l. (51.11.06N/15.13.30E).The river ows mainly through an extensive forests dominated by Scots pine Pinus sylvestris.
The diet composition was determined through spraint content analysis.The spraints (n = 158) were collected throughout all seasons from 16 May 2005 to 2 July 2007 in 14 sites on the Czerna River between Węgliniec and Żagań (Table 1).Analysis followed the standard procedure (Andrzejewska et al.Krawczyk et al. 2011).In a laboratory the spraints were individually soaked and washed through 0.5 mm mesh sieve to retrieve clear prey items, which were subsequently dried and sorted.Identi cation of prey items was aided with a stereomicroscope (10 x).Prey items were identi ed by fragments of vertebrate bones (operculum, pharyngial teeth, vertebrae, mandibles, ileum, and frontoparietale), sh scales, mammal hairs and teeth, avian feathers and arthropod exoskeletons.Fish were identi ed down to species level, amphibians and crustaceans down to the genus level, while all other prey to the class or order level.Prey items were identi ed by comparing them with a reference collection and using the following keys: Brylińska 1991; Kołodziejczyk and Koperski 2000.Characteristic bones and scales were used to determine the number of individuals of a given prey taxon within a spraint.Duplicate bones or differences in size of particular bones/scales were su cient to distinguish the presence of more than one individual within a given prey taxon.The frequency of prey occurrence in sites (FO) was calculated as the percentage of sites, where the given prey taxon was present.The frequency of prey occurrence in spraints (FS) -as the percentage of spraints, where a given taxon of prey was present.The frequency of occurrence of prey items (FP) refers to the number of prey items of a given prey taxon in relation to the total number of all prey items extracted from all spraints, and was also expressed in percentages.

2001;
The Levins's index (Levins 1968) was used to calculated the niche breath: B = 1/∑p i 2 where B is the Levins's measure of niche breadth and p is the proportion of each food category consumed by otters.Species in the case of sh, and classes in the case of all other prey were used as food categories.Spearman's rank correlation test was used to check for relationships between the monthly and year-to-year occurrence of particular sh families while x 2 -test was used to compare otter diet composition in the year 2006 and 2007.

Results
Fish comprised the staple food of the otter.It was supplemented by frogs, birds, small mammals and invertebrates.Fish were represented by at least 23 species belonging to seven families.Two families Cyrinidae and Percidae dominated the sh component of the diet (together 70.1%).The most numerous sh species consumed were Perca uviatilis (24.1%) and Rutilus rutilus (12.1%).Relatively numerous (each with > 5%) were also Cottus gobio (7.6%), Gasterosteus aculeatus (6.9%), Gymnocephalus cernus (5.8%), Gobio gobio (5.3%) and Esox lucius (5.0%).Amphibians were represented exclusively by frogs (Ranidae), while invertebrates by mussels (Mollusca: Bivalvia), cray shes (Crustacea: Decapoda: Astacus spp.) and insects (Table 2).The otter diet was most diversi ed in the coldest months of the year (November-April; B = 10.0-10.5),and least diversi ed in the warmest months of the year (July-August; B = 5.7).Its average values were found in spring (May/June; B = 7.5) and autumn (September/October; B = 7.9) (Fig. 1).The contribution of two main sh families: Cyprinidae and Percidae in the diet of the otter was high (70.1% of all prey items).The proportion of percids was higher than cyprinids in the warmer months of the year (May-July), while the proportion of cyprinds was higher than percids in colder months of the years (September-April).The proportion of G. gobio was not correlated with the proportion of P. uviatilis (R 2 = 0.044), R. rutilus (R 2 = 0.001) and E. lucius (R 2 = 0.085), while the proportion of P. uviatilis was correlated with the proportion of R. rutilus (R 2 = 0.696), and not correlated with the proportion of E. lucius (R 2 = 0.169); proportion of E.lucius was correlated with the proportion of R. rutilus (R 2 = 0.352).
The niche breath was wider in 2006 (B = 12.2), compared with 2007 (B = 20.4).However, only the proportion of G. gobio and E. lucius were statistically different in those two years (Fig. 2).

Discussion
The major nding of this study is that the diet of the otter inhabiting a small river in an extensive woodland was strongly dominated (> 90%) by sh.In terms of biomass, the diet was almost exclusively (> 99%) composed of sh.In Poland and other countries in Eastern Central Europe, the proportion of sh in the otter diet ranged from 35 to 96% (Table 3).A similarly high proportion of sh in the otter diet (c.90%) was recorded only in sh ponds (  2016) calculated that sh comprised on average 72% of all prey items and 75% of biomass.There were more sh in the diet in standing than in owing water; and more in wetlands than in farmlands and woodlands.However, our results do not support this generalization.The proportion of two main sh families in the otter diet, Cyprinida to Percidae, was 1 : 0.6 in this study, and its overall contribution to the otter's diet (79%) was the highest ever reported from Poland.In other sites in Poland, the proportion ranged from 1: 02 to 1 : 3.5, but the overall contribution to the diet was nowhere higher than 67% (Table 4).In the Netherlands, it was 1 : 0.  In our study, most sh preyed by otter were small-sized species, i.e.Barbatula berbatula, Cobittis taenia, Cottus gobio, Gasterosteus aculeatus, Gobio gobio, Leucaspius delineates, Misgurnus fossilis, Rhodeus sericeus.They comprised 32.7% of all sh consumed.They have no economic value.Also Perca uviatilis (25.8%) is of low economic value, often regarded by freshwater shery as a strong competitor and predator of sh species of high economic importance (Brylińska 1991).Fish of economic value preyed upon by the otter were in most cases of small sizes (5-15 cm).In other places in Poland, smallsized sh constitute a bulk of the otter's diet.Only in the upper Biebrza River relatively high predation on Lota lota, and large proportion of Salmo trutta in some streams in the Carpathians Mts. have been reported (Harna 1993 2021).In our study, as in other aquatic habitats in Europe, the size of sh preyed by the otter is positively correlated with the size of sh dominating in these habitats (Libois et al. 1991;Brzeziński et al. 2006Brzeziński et al. , 2013;;Krawczyk et al. 2011).
In our study, seasonal variations recorded in the diet of otter re ect seasonal variation in the abundance and availability of prey.Esox lucius, Carassisius carassius, Cyprinus carpio and Gobio gobio are more available in winter/spring moth, while Barbatula barbatula, Cottus gobio, Gymnocephalus cernus, Perca uviatilis, Rutilus rutilus are more common in summer months.Frogs are more available in winter and spring as they overwinter in the river mud, and in spring they spawn.Cray sh and insects are most common in summer.Results obtained from other parts in Poland show that, in habitats with a small number of sh species, like this in our study, the diet of the otter is less diverse than in rich habitats.The diet composition is, however, more stable (Brzeziński et al. 2006).
It has been shown that the otter diet is more diverse in less stable environment, where non-sh prey are more often preyed upon (Ruiz-Olmo and Jimenez 2009; Dettori et al. 2022).In our study, as in many other European habitats, alternative vertebrate prey other than amphibians, played a little role.In most places, reptiles, birds, mammals did not contribute separately more than 10% of otter diet (Jędrzejewska et al. 2001).Amphibians (especially Ranidae and Bufo bufo) are often preyed upon by the otter throughout the species' European range (Fairley 1984 Very low contribution of frogs in our study indicates that amphibians are rare in small rivers owing through woodlands dominated by pine plantations.In another small river, yet in almost pristine stage in the Białowieża Forests, Jędrzejewska et al. (2001) reported frogs as comprising 58% of all prey items.Also in small arti cial channels, but in an extensively managed farmland, frogs comprised an important prey (16%; Krawczyk et al. 2011).
In Europe, the proportion of invertebrates and other non-sh prey in the otter diet shows a longitudinal trend, i.e. the proportion increases southwards (Clavero et al., 2003) Most studies in Poland do not report mussels (Mollusca: Bivalvia) in the otter diet (Harna 1993;Kłoskowski 1999;Jędrzejewska et al. 2001;Brzeziński et al. 2006Brzeziński et al. , 2013;;Wiśniowska 2006;Pagacz and Witczuk 2010).None were recorded even in rivers well known for the abundance of unionid mussels (Brzeziński et al. 2006).Kłoskowski et al. (2013) recorded only one unionid specimen out of 1465 prey items retrieved from 478 spraints in river-channel systems in E Poland.Also in this study only single mussel was reported.However, predation by the otter on the mussels has been reported by Kopij (2011) and Krawczyk et al. (2011).Through direct eld observations Kopij (2011) reported relatively high predation on the threatened Anodonta cygnea in a sh-pond in SW Poland, while Zając (2014) using telemetry recorded such case in a small water reservoir in S Poland.It appears that in places when mussels are abundant, like in some sh-ponds, water reservoirs and larger rivers they may constitute an important component in the otter diet (Georgiev 2006;Kopij 2011;Zając 2014), but their remnants are poorly represented in spraints.This study con rms previous ndings (Lanszki et al., 2007(Lanszki et al., , 2009;;Jędrzejewska et al., 2001) that the otter is a generalist piscivorous predator hunting opportunistically on locally and seasonally most common sh species, mainly of low economic importance.

Declarations ETHICAL STATEMENT:
Funding: none   Month-to-month variation in the main prey categories in the diet of the otter in Czerna River.

Figures
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Table 1
Number of spraints collected at particular sites.

Table 2
Diet of the otter in Czerna River, SW Poland.FO -frequency of distribution in 14 sites, FS-frequency of distribution in 158 spraints, FP -frequency of distribution in 605 prey items.

Table 3
Proportion of main prey groups in the diet in Poland and other East European countries.

Table 4
Proportion of Cyprinidae and Percidae in the diet of the otter in Poland.