Errors associated with otter Lutra lutra faecal analysis. II. Estimating prey size distribution from bones recovered in spraints

New methods of estimating the size of salmonids Salmo spp. and eels Anguilla anguilla consumed by otters Lutra lutra are presented, based on feeding trials involving captive, tame otters. These methods involve modelling the size-related differential recovery of key fish bones recovered in faeces (spraints) and avoid two sources of errors that may have occurred using previous techniques which relied solely on a series of correlations between fish length and the length of individual vertebrae. Sources of errors avoided are: (a) that vertebrae recovered in spraints could not always be assigned to the correct correlation equation for mean, maximum, or minimumsized bones; and (b) that the number of fish represented in a spraint was assumed to be one, in the case of similarly-sized vertebrae, and two in the case of bones varying considerably in size. We tested the use of salmonid atlas vertebrae to determine the largest minimal numbers estimate, the length frequency distribution of salmonids consumed, and to estimate the proportions of trout and salmon in the diet. Eels do not contain an atlas which is resistant to digestion and so equivalent estimates of the minimum numbers of fish represented in spraints are not possible. A model was therefore developed to estimate the distribution of lengths of fish consumed from the lengths of thoracic vertebrae recovered in the spraints using a series of equations. For each type of prey (salmonids or eels), the proportions of identifiable bones recorded in spraints were related to the original size of ingested fish. Such models greatly enhance the value of otter spraint analyses, particularly for size-selection studies.     

FEEDING ECOLOGY OF THE MARINE OTTER (LUTRA FELINA) IN A ROCKY SEASHORE OF THE SOUTH OF CHILE

The marine otter ( Lutra felina ) lives exclusively along exposed rocky shorelines on the South American Pacific coast from Peru (6°S), to Cape Horn, Chile (56°S), and Isla de los Estados, Argentina. L. felina diet and its relationship to prey availability and energy content was assessed by spraint and prey remains analysis, direct observation, and the use of crab pots and fish traps, at four sites on the Valdivian coast in the south of Chile, between June 1999 and June 2000. Based on spraints analysis, the diet was composed of 25 species; 52% (13/25) of the species identified were crustaceans, 40% (10/25) were fish, and 8% (2/25) were mollusks. Crustaceans were found in 78% of 475 spraints, 100% of 929 prey remains, and 90.8% of prey determined by direct observation, fish in 20% of spraints and 9.0% of prey determined by direct observation, and mollusks in 2% of spraints and 0.2% of prey determined by direct observation. Observed seasonal variation in prey availability was reflected in the otter diet. Fourteen prey species were trapped; 43% (6/14) were crustaceans and 57% (8/14) fish, crustaceans were 93% of 566 trapped individuals, fish 7%. L. felina showed opportunistic feeding behavior, selecting prey seasonally according to their availability rather than to their energy input.     

Analysis of otter (Lutra lutra) spraints: Part 1: Comparison of methods to estimate prey proportions; Part 2: Estimation of the size of prey fish

The accuracy of five different methods used in spraint analysis for extimating prey proportions is tested through a feeding experiment with three otters in captivity. Estimated prey proportions are copared to the actual diet of the otters. The results of analysing 362 spraints and the time budget indicated a score-bulk estimate to be the most appropriate method of estimating prey diet and it is concluded that dietary studies using different methods can be compared in broad outlines. The relatins between the methods are confirmed in the analysis of 978 spriaints, collected in the field.     

Diet of otters (Lutra lutra) inhabiting small rivers in the Bialowiea National Park, eastern Poland

The diet of otters Lutra lutra L. inhabiting small rivers in the primeval forests of the Bialowieza National Park was studied in 1988–91 by analysis of 135 spraints. Anurans, mainly Rana temporaria, and fish, mainly Cyprinidae, were two staple prey of otters. In autumn and winter (1 October–31 March), 99% of spraints contained anuran remains and 45% fish remains. This corresponded to 66 and 34% of biomass consumed by otters, respectively. In spring and summer, anurans dropped to 55% occurrence in spraints (38% of biomass eaten by otters) and that of fish increased to 69% occurrence (50% of biomass). Fifty per cent of fish caught by otters were very small specimens (body length 6.5–10 cm) and 35% were small (> 10–15 cm). Pike were the only big fish captured by otters (mean body length 31 cm). Water beetles Dytiscidae were commonly eaten by otters in warm seasons (72% of occurrence in spraints, 9% of biomass taken by otters in spring and summer). Birds, snails, mammals and crayfish were minor supplements to the otters' diet.     

Food habits of the otter (Lutra lutra) in two habitats of the Doñana National Park, SW Spain

Food habits of otters Lutra lutra were studied in two localities of the Doñana National Park (SW Spain) 16 km apart. These were the lowest stretch of Arroyo Rocina, a eutrophic stream where American red-swamp-crayfish were introduced in 1976, and at Lucio Bolín, an artificial pond 150 m in diameter, situated on the border of the Guadalquivir marshes. In all, 598 spraints were analysed between 1979 and 1984. Crayfish, occurring in 80% of the spraints, was the dominant food item in Arroyo Rocina, where other common foods were fish (63%), insects (55%) and amphibians (18%). Fish, occurring in 94% of the spraints, were by far the most abundant food item in Lucio Bolin, followed by insects (32%) and amphibians (28%). Seasonal differences in diet were small but interesting: reptiles were eaten more often in the dry season (April-September) and insects and crayfish in the rainy season (October-March). Among fish, eels were more commonly eaten in the rainy period and the remaining species in the dry period. This pattern of seasonal variation is opposite to the usual in temperate Europe. Mean size of the captured eels was statistically higher in Lucio Bolin than in Arroyo Rocina. A large amount of very small fish such as Gambusia was eaten. A comparison of prey consumed with estimates of prey abundance reveals a strong preference for eels. The diet of Mediterranean European otters seems to be more diverse than that of northern European ones, including more insects, amphibians and reptiles.     

Predation by marine-feeding otters (Lutra lutra) in south-west Portugal in relation to fluctuating food resources

Patterns of predation by otters ( Lutra lutra ) inhabiting the Portuguese south-west rocky coast, were inferred from the analysis of 757 spraints collected along a 5-km stretch of shore, from November 1990 to October 1993. Undigested remains were identified, counted and measured; the length and weight of ingested prey was estimated from regression equations. Monthly fluctuations in diet were compared to changes in the abundance of the primary prey species along the coast. The most commonly taken fish species had median lengths between 6.7cm and 15.0cm, and median weights between 3.5 g and 34.2g. Corkwing wrasse dominated the diet in terms of biomass (51.6%), and was the most important prey in terms of numbers (22.6%). Gobies (19.4%), Valenciennes' blennies (15.0%), shannies (1 1.4%), crayfish (7.2%), and shore rocklings (4.1%) were other numerically important prey. The contribution in biomass of gobies (14.5%), shore rocklings (6.7%) and conger eels (5.5%) was also significant. The dominant prey species and sizes in the diet were consistent over the years. From late winter to early spring the diet was numerically dominated by corkwing wrasse and gobies, whereas for the rest of the year shannies, Valenciennes' blennies and crayfish were also important. The contribution of these last species in biomass was small at all times of the year, and the corkwing wrasse was always the dominant prey. Prey was twice as heavy in winter as in spring. There was close correspondence between seasonal variation in diet and fluctuations in the abundance of the primary prey along the coast, but most dietary variation could be accounted for simply by fluctuations in the abundance of corkwing wrasse. In south-west Portugal, the most important food resource for otters seems to be the corkwing wrasse, probably because they are large and easy to catch. Corkwing wrasse are also exploited by local fishermen, and the possibility of competition between otters and humans should be investigated.     

Seasonal and spatial differences in food selection by otters (Lutra lutra) in Shetland

Otters Lutra lutra in Shetland feed almost exclusively in the sea. Their diet was assessed from direct observations of 13, 313 dives, of which 27% were successful, and 2028 prey could be identified. The results are compared with published data on the availability offish for different seasons, time of day, state of tide, and different types of coast. The most frequently taken prey was eelpout Zoarces viviparus (34%), but they were relatively small, and rocklings Ciliata mustela dominated the diet in terms of biomass (18%, against eelpout 15%). Small eel-shaped, bottom-living species dominated in the diet at all times, but of those, otters took the larger fishes in the populations (median weight of prey 28 g). The size and species composition of prey of males was the same as that of females with cubs, but females without cubs took smaller fish.
In spring, prey was smaller than at other times, and otters were least successful when diving. Diving success and mean prey weight was highest in winter, although prey availability was highest in summer.
Eelpout were caught mostly along sheltered coasts, rocklings in more exposed areas. It is argued that for optimal exploitation of the seasonal availability of different species of prey, otters need to use different types of coast, and the size of their ranges would be related to the spacing of coastal types.
Most fishes were caught in daytime, and around high tide otters fed less than at other times; hunting therefore took place during periods of inactivity of prey. For the main prey species, Ciliata mustela , it was estimated that otters took a substantial part of the total population.     

Spatial distribution and diet of the Neotropical otter<Emphasis Type="Italic">Lontra longicaudis</Emphasis> in the Ibera Lake (northern Argentina)

The seasonal spatial distribution and diet of the Neotropical otterLontra longicaudis (Olfers, 1818) were studied on Ibera Lake within the Esteros del Ibera, an important wetland of NE Argentina (Ramsar site no. 1162). Twelve lake perimeter sites were regularly checked for otter tracks and signs. The amount of signs was compared to environmental parameters to test their influence on otter presence and movements. A positive correlation was found between the amount of signs and the physical structure of the littoral areas, as well as vegetation structure. Lakeside otter presence decreased during the summer sampling throughout all sites. In order to determine otter diet, 205 spraints were examined. Otters fed mainly on fish (mostly Cichlidae), but also on crustaceans and molluscs. Other fish consumed were Characidae, Synbranchidae, Loricariidae and Erythrinidae. Seasonal variation was observed in diet composition: in summer, diet crustaceans and vertebrates other than fish increased. A higher percentage of benthic fish species was also observed in summer, while pelagic and benthopelagic species increased in winter. Such dietary changes may be explained by the different habitat use of otters in different seasons, from the lake coast (winter) towards more internal marshy areas of the wetland (summer).     

Factors affecting the predation of otter (Lutra lutra) on European pond turtle (Emys orbicularis)

In this case study, the ecological background of an unusual hunting behaviour was investigated, when otters Lutra lutra preyed upon European pond turtles Emys orbicularis in a Hungarian fish pond system during an 18-month period. Predation on turtle was found only during cold periods (established by spraint analysis and also by the collection of 182 turtle carcasses in 2003). The relationship was not close between fish availability and turtle consumption ( r _P =−0.325, P =0.19). The crude protein content of the turtle head and leg was higher than that of fish, frog and turtle body, whereas the energy content of the samples was similar. The mean body weight of the killed turtles (460 g) fell within the range of the optimal prey size of the otter. Turtles were used as cache foods by otters during extreme environmental conditions (as in the long winter), but occurred only rarely as buffer foods during moderate winter. In fish ponds, the conservation of the coexistent otter and turtle depends on pond management. The maintenance of a higher fish availability in ponds during winter makes it possible to avoid the need to acquire a proper hunting technique on turtle, indicated by the scarcity of primary fish food.     

Diet of otters (Lutra lutra) in closely associated freshwater, brackish and marine habitats in south-west Portugal

The diet of otters was studied in closely associated freshwater, brackish and marine habitats, from spraints collected on the Portuguese south-west coast over an 18-month period. In areas where marine prey was the only available resource, diet was dominated by blennies, wrasses and gobies, but other prey was taken in areas near coastal lagoons, marshes and estuaries. Eels and amphibians were considered the typical prey of freshwater habitats, and grey mullet the typical prey of brackish water habitats. If the inland habitats near the coast were large enough, otters preferred to forage there rather than in the sea. In one area where otters alternatively used marine and inland habitats, the former were used most extensively in autumn and winter, and the latter in spring and summer. It is suggested that otters prefer to forage inland rather than in the sea.     

Do non‐native fish as prey favour the conservation of the threatened indigenous Eurasian otter?

1. Biological invasions are considered a major threat to biodiversity. Most research has focused on the distribution, biology and impacts of non‐native species on native fauna and flora. However, few studies have explored their role as prey for native predators of conservation concern. 2. To assess the incidence and intensity of predation by the Eurasian otter Lutra lutra on established non‐native fish species, data were collated from the published literature. To be selected, studies had to cover at least 1 year, analyse more than 100 spraints and report the study period and percentage relative frequency (%RF) of all prey fish species. 3. To permit reliable, time‐related comparisons with %RF of non‐native fishes in otter diet, we also reviewed available information about both the distribution of non‐native fishes and history of their introductions to European countries, revealing a decrease with longitude in the number of naturalised non‐native fishes taken (ranging between 5 and 34) and their percentage in each fish assemblage. 4. Our selective criteria were met by 30 dietary studies from 44 study areas in 15 European countries during 1970–2010. The extent to which otters rely on non‐native fishes was almost negligible (mean %RF = 4.8), with the number of non‐native fishes preyed upon by otters decreasing with both latitude and longitude. 5. The %RF of non‐native fish in the diet increased slightly with time, with otters preying significantly more on non‐native fish in study areas where alterations of the fish assemblage had been highlighted in the reference papers. No relationship was found between otter diet breadth and the occurrence of non‐native fishes in their diet. 6. The current role of non‐native species in otter diet suggests that effective otter conservation management plans should focus on the maintenance and/or enhancement of native fish assemblages.     

Latitudinal variation in diet and patterns of human interaction in the marine otter

The marine otter (Lontra felina) inhabits patches of rocky coastline from central Peru to southern Chile and is classified as Endangered by the IUCN. Given the limited information available about the species, we set out to assess marine otter diet with a view to detecting latitudinal differences, and to assess marine otter activity budgets and interspecific interactions (including anthropogenic) at Peruvian fishing villages and to compare results with similar Chilean studies. Nine study sites from central Chile to southern Peru were sampled for otter spraints to assess relative frequency of prey types and two fishing ports in southern Peru were monitored through focal and scan observations to assess activity patterns, interspecific interactions, habitat use patterns, and dive durations. Results indicate that toward the northern part of its range, crustaceans become less important and fish more important in the diet. Interactions were observed between marine otters and other species, including stray dogs and cats. The strong dependence of marine otters on the availability of safe rocky shelters, and the species’ apparent tolerance to living alongside humans raise conservation concerns about vulnerability to anthropogenic threats. These factors, if not correctly managed, could turn some of these rocky seashore patches into population sinks.     

Trophic diversity of the otter (Lutra lutra L.) in temperate and Mediterranean freshwater habitats

Aim To analyse the geographical patterns in the composition and diversity of otter's (Lutra lutra L.) diet and their relationship with climatic characteristics. Location European freshwater habitats under Mediterranean and temperate climatic regimes. Methods Thirty‐seven otter diet studies were reviewed, twenty‐one from temperate and sixteen from Mediterranean areas. All studies were based on spraint analysis and their results expressed as relative frequency of occurrence of seven main prey categories. Principal Component Analysis was performed to extract the main gradients of diet composition. Pearson's correlation and t‐tests were used to assess the relationship between diet characteristics (composition, diversity and taxonomic richness) and geographical and climatic variables. Results A clear latitudinal gradient in diet composition was observed. Otter diet was more diverse and featured more prey classes in southern localities, while the species was more piscivorous towards the north, where it predated upon a higher number of fish families. This pattern was similar when temperate and Mediterranean localities of Europe were compared. Mediterranean otters behaved as more generalist predators than temperate ones, relying less on fish, and more on aquatic invertebrates and reptiles. Main conclusions Geographical differences in otter feeding ecology in Europe seem to be related with the two contrasted climatic conditions affecting prey populations. The otter can act as a highly specialized piscivorous predator in temperate freshwater ecosystems, which do not suffer a dry season and have a comparatively stable water regime compared to Mediterranean ones. However, the unpredictable prey availability in Mediterranean areas, affected by strong spatial and temporal water shortages, favours a diversification of the otter's diet.     

Comparison of the feeding habits of Eurasian otters on a fast flowing river and its backwater habitats

The diet and feeding habits of Eurasian otters Lutra lutra were studied by spraint analysis over a 2- year period, on three sections of the Drava River and three backwaters, in south-west Hungary. The primary food was fish (mean: 89.8% and 87.5% for riverine and backwater habitats, respectively); otters living in riverine habitats compared to backwaters, consumed more birds (3.9% and 0.7%, respectively), less mammals (0.5% and 0.9%, respectively), less reptiles and amphibians (5.6% and 10.2%, respectively) and less invertebrates (0.1% and 0.6%, respectively). In riverine habitats otters preyed more frequently on larger fish than in backwaters, but the main fish prey was small-sized (below 100 g in weight, 85.6% and 91.7%, respectively). On river sections they preyed more frequently on reophil (flow preferring fish, 18.9% and 3.3%, respectively), and less on stagnophil fish (stagnant waters preferring, 9.8% and 24.5%, respectively) than in backwaters. The main fish prey was eurytopic (tolerant of rivers and stagnant waters, 71.3% and 72.2%, respectively). Preference (by Ivlev's electivity index, E_i) in the Drava River for various fish guilds differed, as the otters preferred eurytopic (E_i= 0.30) and stagnophil fish (E_i= 0.24), and avoided reophil fish (E_i=−0.58). Otters did not threaten the rare, flow preferring fish species and the main diet consisted of economically unimportant species.     

Determinants of Eurasian otter (Lutra lutra) diet in a seasonally changing reservoir

Otter diet in reservoirs is known to experience seasonal changes. We selected a reservoir with a large population of exclusively wintering great cormorants and seasonal changes in stored water volume to test the relative influence of abiotic and biotic factors on otter foraging ecology. DNA metabarcoding of otter spraints revealed a dietary change from autumn to winter. Otters had a diet dominated by the exotic goldfish in autumn, but predated intensively on the native northern straight-mouth nase in winter. This change was likely caused by predation of cormorants on goldfish and to fish biology. Secondly, macroscopic analysis of spraints revealed that otters shifted from a diet dominated by fish (in terms of biomass) to a diet dominated by red swamp crayfish during spring–summer, when the latter became overabundant. As revealed by modelling, this second shift was most likely influenced by the sudden increase in stored water volume in spring, but also by the cumulative effect of cormorant predation on fish during autumn–winter. Macroscopic analyses of otter spraints collected in a second reservoir with no cormorants revealed a lack of seasonality. Hence, the combined influence of both biotic and abiotic factors explained otter diet seasonality in a lentic-water novel ecosystem.

     

Effects of season and body-size on the distribution and diet of the Iberian chub Leuciscus pyrenaicus in a lowland catchment

Seasonal and ontogenetic variations in space and food use by Leuciscus pyrenaicus were analysed in a Portuguese lowland catchment. Large fish occurred mostly in deep permanent-flowing sites whereas small fish occurred mostly in the shallowest intermittent-flowing site. No seasonal or size-related changes in feeding intensity were found, but the diet changed both across seasons and throughout ontogeny. The diet was dominated by aquatic prey over all seasons, but during winter and summer more plant material and terrestrial prey, respectively, were eaten. Throughout ontogeny fish shifted from soft-bodied to hard-shelled prey and decreased animal prey breadth. Mean prey size increased with fish size but the prey size spectrum was more variable for medium-sized fish than for either small or large fish. It is suggested that: (i) large fish avoid shallow drying areas owing to the risk of mortality, either by thermal and respiratory stresses or increased predation by mammals and birds; (ii) seasonal changes in diet are a response to differences in prey availability; and (iii) morphological constraints, prey handling costs and habitat partitioning are responsible for size-related changes in diet.     

Similar estimates of population genetic composition and sex ratio derived from carcasses and faeces of Eurasian otter Lutra lutra

Collecting faeces is viewed as a potentially efficient way to sample elusive animals. Nonetheless, any biases in estimates of population composition associated with such sampling remain uncharacterized. The goal of this study was to compare estimates of genetic composition and sex ratio derived from Eurasian otter Lutra lutra spraints (faeces) with estimates derived from carcasses. Twenty per cent of 426 wild-collected spraints from SW England yielded composite genotypes for 7-9 microsatellites and the SRY gene. The expected number of incorrect spraint genotypes was negligible, given the proportions of allele dropout and false allele detection estimated using paired blood and spraint samples of three captive otters. Fifty-two different spraint genotypes were detected and compared with genotypes of 70 otter carcasses from the same area. Carcass and spraint genotypes did not differ significantly in mean number of alleles, mean unbiased heterozygosity or sex ratio, although statistical power to detect all but large differences in sex ratio was low. The genetic compositions of carcass and spraint genotypes were very similar according to confidence intervals of theta and two methods for assigning composite genotypes to groups. A distinct group of approximately 11 carcass and spraint genotypes was detected using the latter methods. The results suggest that spraints can yield unbiased estimates of population genetic composition and sex ratio.     

The diet of the feral mink (Mustela vison) in southwest Britain

Mink diet in three study areas in southwest Britain was studied by faeces (scat) analysis. In these areas mink ate a wide range of prey, mainly fish, birds and mammals. Proportions of prey in the diet were determined largely by availability, but were also affected by prey behaviour, the physical nature of the habitat and overall prey abundance. Seasonal variation in predation was recorded on moorhens and fish. Changes in rod and net fishing success on the river Teign could not be correlated with the arrival of the mink. Changes in fish populations in Slapton Ley appeared to have causes other than mink. It is concluded that, in the areas studied, domestic stock make up a very small proportion of the mink's diet, and that the wild prey species are not suffering any obvious population depletion.     

A comparison of the feeding biology of Mink Mustela vison and otter Lutra lutra

The feeding habits and prey selectivity of Mink Mustela vison and otters Lutra lutra were compared in two localities in Devon: a eutrophic lake and a moorland river, in which both species occurred and had access to the same prey populations. The effects of prey availability on the predators' diets were assessed by comparing prey consumed, as revealed by scat analysis, with estimates of prey abundance and size range. Otters specialized in fish at all times of year but showed seasonal variation in species taken. Selection for slow-moving fish and seasonal changes in behaviour of some fish species were the probable causes of this variation. Otters diversified more into non-fish food in summer, when fish availability was reduced. The main alternative prey in the lacustrine habitat was waterfowl, but in the riverine habitat, rabbits. Mink were more generalized carnivores, taking a variety of fish, waterside and terrestrial prey in all seasons. These three prey categories were taken to an almost equal extent in the lake but terrestrial prey dominated in the riverine habitat. Fish were taken most frequently in winter and birds and mammals in summer. Neither predator showed selection in respect of prey size. In each area, about one third of the otter and Mink diets was common to both species. Fish was the principal group of the shared component, and dietary overlap in respect of them was greatest in autumn and winter. In view of the dietary preferences of each predator, the existence of alternative prey items and limited degree of dietary overlap, it is considered unlikely that the two species competed for food to any extent. Other factors must therefore be responsible for the spread of feral Mink and the decline in otter populations in many parts of Britain.     

Competition between Eurasian otter Lutra lutra and American mink Mustela vison probed by niche shift

Interspecific competition is one of several constraints that might prevent an individual from maximising its energy intake. When an interspecific competitor is introduced, an individual is often forced to shift its diet according to the intensity of the competitive pressure. In this paper, we explore whether the introduced American mink ( Mustela vison Schreber) shifts its diet when the density of its potential competitor, the Eurasian otter ( Lutra lutra L.), is increased. We compared the diets of otter and mink at the same location but at two moments in time when the relative densities of these two species were different while controlling for the abundance of aquatic prey. Mink and otters are semi-aquatic mammals belonging to the same guild of mustelids and otters are expected to be the dominant competitor because they are larger and better at hunting underwater. The diets of otters and mink overlap to a great extent but while otters specialise mainly on aquatic prey, mink are able to exploit both aquatic and terrestrial prey. These observations prompted the hypothesis investigated in this work that at higher otter densities the diet of mink should change to include a higher proportion of terrestrial items. This hypothesis was supported by the data and at higher otter densities mink diet was observed to consist of a higher proportion of mammals and birds while fewer fish were present, although this pattern was present only in winter while no changes were observed in spring. Meanwhile the diet of otters remained basically unchanged. In the second part of the study, we investigated whether niche breadth and niche overlap between otter and mink changed at different otter densities. We found that niche overlap declined as the density of otters increased, in agreement with the prediction of habitat selection theory.