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.     

Seasonal and area-related variations in the diet of otters Lutra lutra on Mull

The diet of otters Lurra lutrn living in a sea loch on the Isle of Mull was described by means of spraint analysis. Seasonal and spatial differences in the spraint composition were explored in the context of fluctuations in prey availability, as revealed by trapping. Butterfish Pholis gunnellus and the cottids Myoxocephalus scorpius and Taurulus bubalis were the most commonly occurring prey in spraints. In the winter, the proportion of rocklings ( Ciliata spp. and Gaidropsarus spp.) and non-rockling gadoids (Gadidae) in the diet increased. Their relatively large size makes these latter species an important constituent of the winter diet. Seasonal and spatial variation in fish consumption largely reflected fluctuations in availability. The mean size of principal prey fishes in spraints was similar to that observed in the traps. These observations suggest that otters were not strongly selective with regard to fish species and size. Shore crabs, in contrast, were not taken in proportion to availability. The findings are discussed in the light of other studies of the diet of coastal otters. It is suggested that the lack of selectivity demonstrated in this study, compared with previous studies, may be explained by area-related variations in prey availability.     

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.     

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 use of fish vertebrae in scats for estimating prey size of otters and mink

A method of using fish vertebrae in scats of otters and mink in order to estimate prey size is described. Significant positive correlations between fish fork length and length of abdominal and caudal vertebrae respectively were demonstrated and described for a series of reference fish of different species. Comparison of direct length measurements of fish fed to captive mink with length estimates obtained from vertebrae recovered in then scats and derived using the appropriate regression lines determined from the reference fish revealed a close measure of agreement and indicated that the method achieves a high level of accuracy. Use of vertebrae from scats for estimating fish length is considered to have several advantages over use of scales, and the method greatly extends the range, accuracy and value of the information that can be obtained from scat analysis.     

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.     

Consumption, abundance and habitat use of Anguilla anguilla in a mesotrophic reservoir

The role of the eel Anguilla anguilla as a piscivorous species was investigated in a biomanipulation experiment in the mesotrophic Saidenbach Reservoir. The distribution and abundance of the eels were investigated by point abundance sampling, snorkelling and scuba diving. Also, the total yearly consumption of the eel population was compared with the standing stock of prey fishes and the production of roach Rutilus rutilus eggs. A restricted availability of shelter habitats at low water levels had no influence on the distribution of the eels during the nocturnal activity period. Fishes were consumed in low numbers but in high proportions of biomass by large eels. The maximum estimate of the annual consumption of the prey fish standing stock by the eels was 19%. Fish eggs were consumed during a small time period by a large proportion of the eels but the total consumption was <10% of the total annual production of roach eggs. By consuming small individuals of planktivorous fishes, the eels contribute to the biomanipulation programme in the Saidenbach Reservoir.     

Assessment of errors associated with harbour seal (Phoca vitulina) faecal sampling

Six harbour seals, ages 4–8 years, were held as pairs in a 10 times 20 times 2 m tank filled with sea water, and on 60 occasions were fed a meal of a specific species of fish or cephalopod of known size. The tank was drained periodically, and harbour seal faeces were collected on a 0.5 mm sieve. Number and size of otoliths and beaks found in faeces were determined. Fifty-eight percent of 670 fish and 37% of 36 cephalopods fed to harbour seals were represented by their otoliths or beaks in faeces. Estimated number of prey consumed was determined from the greatest number of left or right otoliths or upper or lower beaks collected in faeces. Estimated length ofprey was determined from measurements of otoliths and beaks recovered in the tank and relationships of otolith and beak measurements to prey length. Estimated number of fish eaten was not significantly different among pairs of harbour seals, but was different among species of fishes. Only 24–35% of fish species with small otoliths were represented in faeces, whereas more robust otoliths from other species were less apt to be completely dissolved. Estimated length of fishes was significantly less than lengths of fishes fed to harbour seals in 39 (76.5%) of 51 trials. Cephalopod beaks were not affected by passage through the harbour seal digestive tract. Amount of otolith dissolution was not related to species of fish; estimated fish length was underestimated by an average 27.5%. Although some (7.4%) of the otoliths were collected within 100 h after the fish were ingested, more than 90% were recovered within 24 h after the fish was eaten. Correction factors were developed which will allow researchers to estimate more reliably number and size of fish and cephalopod prey eaten by harbour seals.     

Prey selection in coastal Eurasian otters Lutra Iutra

Prey preferences and dietary differences between sex and age categories of Eurasian otters were studied in coastal Norwegian habitats Relative to their trapping frequency potential prey species with hard, spiny exoskeletons (crabs and sea urchins) or otherwise tough, spiny integuments (Labridae) were much less frequently found in spraints than fish species with soft integuments Spines did not protect fish with otherwise soft integuments from otter predation The number of non-fish taxa per otter stomach did not vary significantly between otter age categories despite presumed differences in hunting abilities (small cubs large cubs and subadults, adults) Relative frequency of occurrence of crabs and sea urchins was < 5% in the stomachs in each of these otter categories Anadromous, katadromous and freshwater fish species were infrequently eaten The coastal otter population during the study period probably had access to an adequate, and preferred, supply of marine fish prey
At the otter population level no prey size selection was conclusively demonstrated within the range of fish sizes sampled However, fish sizes eaten differed significantly between otter sex and age categories The fish sizes per stomach were on average larger in males than in females, regardless of age Adult males tended to eat the largest fishes Among the self provisioning age categories (subadult and adult otters) fish lengths differed significantly between otter males and females, but not between the otter age categories, and did not covary significantly with otter body length Fish eaten by females with old placental scars (potential mothers of fisheating cubs) were significantly smaller than those eaten by small cubs, provisioned by their mothers     

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.     

Characterizing habitat preference of Eurasian river otter (Lutra lutra) in streams using a self-organizing map

We studied the habitat preferences of Eurasian river otters (Lutra lutra) using the distribution patterns of the numbers of spraints and sprainting spots of otters, as well as related environmental variables (habitat zone, river management, bank type, vegetation coverage, width, depth, etc.) in two streams. The numbers of otter spraints and sprainting spots were sampled monthly in two streams on Geoje Island, Republic of Korea, from January to December 2004. Additional environmental variables were measured at the sampling sites. A self-organizing map (SOM), which is an unsupervised artificial neural network, was used to characterize the habitat preferences of otters. In our results, the SOM classified three different groups of study sites based on their habitat conditions, and the habitat differences were effectively visualized on the trained SOM map. Otters showed spatial and temporal dynamics in the numbers of spraints and sprainting spots, and revealed habitat preferences for shallow, narrow areas of streams and edges of water that were not far from reservoirs but covered with trees and shrubs. Additionally, otters preferred an environment in which weirs reduced the drift of water and gathered fishes and had a natural type of stream bank; these findings are relevant for river management. Otters adapted to places close to roads, residential areas, and agricultural areas with some tolerance of human interference.     

Relative spraint density and genetic structure of otter (Lutra lutra) along the Drava River in Hungary

In this study, we used genetic-based approaches to estimate population size and structure of Eurasian otter along the Drava River in Hungary, and compared these results to traditional survey-based methods. The relative spraint density of otter was estimated based on the number of fresh (D_f) and total number (D_t) of spraints collected on standard routes over a 2-year period. Nine microsatellite loci were screened, generating 17 individual otter genotypes composed of 45 different alleles. The expected heterozygosity ranged from 0.53 to 0.89 and observed heterozygosity from 0.25 to 0.92. The mean density (D_g) estimated over six different sites was 0.17 individuals per km of shoreline. A close correlation was found between the number of genotypes and spraint counts along a standard route (fresh spraints: r_P=0.85, P<0.01; total spraints r_P=0.76, P<0.05). All genotypes found within the 50 km-long study area were closely related (D_m ranged between 0.08 and 0.21).     

Non-Invasive Genetic Mark-Recapture as a Means to Study Population Sizes and Marking Behaviour of the Elusive Eurasian Otter (Lutra lutra)

Quantifying population status is a key objective in many ecological studies, but is often difficult to achieve for cryptic or elusive species. Here, non-invasive genetic capture-mark-recapture (CMR) methods have become a very important tool to estimate population parameters, such as population size and sex ratio. The Eurasian otter (Lutra lutra) is such an elusive species of management concern and is increasingly studied using faecal-based genetic sampling. For unbiased sex ratios or population size estimates, the marking behaviour of otters has to be taken into account. Using 2132 otter faeces of a wild otter population in Upper Lusatia (Saxony, Germany) collected over six years (2006–2012), we studied the marking behaviour and applied closed population CMR models accounting for genetic misidentification to estimate population sizes and sex ratios. We detected a sex difference in the marking behaviour of otters with jelly samples being more often defecated by males and placed actively exposed on frequently used marking sites. Since jelly samples are of higher DNA quality, it is important to not only concentrate on this kind of samples or marking sites and to invest in sufficiently high numbers of repetitions of non-jelly samples to ensure an unbiased sex ratio. Furthermore, otters seemed to increase marking intensity due to the handling of their spraints, hence accounting for this behavioural response could be important. We provided the first precise population size estimate with confidence intervals for Upper Lusatia (for 2012: N^ = 20 ± 2.1, 95% CI = 16–25) and showed that spraint densities are not a reliable index for abundances. We further demonstrated that when minks live in sympatry with otters and have comparably high densities, a non-negligible number of supposed otter samples are actually of mink origin. This could severely bias results of otter monitoring if samples are not genetically identified.     

Patterns of growth in the presacral vertebral column of the leopard gecko (Eublepharis macularius)

Postnatal growth patterns within the vertebral column may be informative about body proportions and regionalization. We measured femur length, lengths of all pre‐sacral vertebrae, and lengths of intervertebral spaces, from radiographs of a series of 21 Eublepharis macularius, raised under standard conditions and covering most of the ontogenetic body size range. Vertebrae were grouped into cervical, sternal, and dorsal compartments, and lengths of adjacent pairs of vertebrae were summed before analysis. Femur length was included as an index of body size. Principal component analysis of the variance‐covariance matrix of these data was used to investigate scaling among them. PC1 explained 94.19% of total variance, interpreted as the variance due to body size. PC1 differed significantly from the hypothetical isometric vector, indicating overall allometry. The atlas and axis vertebrae displayed strong negative allometry; the remainder of the vertebral pairs exhibited weak negative allometry, isometry or positive allometry. PC1 explained a markedly smaller amount of variance for the vertebral pairs of the cervical compartment than for the remainder of the vertebral pairs, with the exception of the final pair. The relative standard deviations of the eigenvalues from the PCAs of the three vertebral compartments indicated that the vertebrae of the cervical compartment were less strongly integrated by scaling than were the sternal or dorsal vertebrae, which did not differ greatly between themselves in their strong integration, suggesting that the growth of the cervical vertebrae is constrained by the mechanical requirements of the head. Regionalization of the remainder of the vertebral column is less clearly defined but may be associated with wave form propagation incident upon locomotion, and by locomotory changes occasioned by tail autotomy and regeneration. Femur length exhibits negative allometry relative to individual vertebral pairs and to vertebral column length, suggesting a change in locomotor requirements over the ontogenetic size range.     

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.     

Nesting biology of the Black-necked Grebe

The fish diet of young Kingfishers Alcedo atthis on the River Roding near Abridge, Essex was assessed by analysing regurgitated bones collected from the nest tunnel. More than 3000 bones recovered from two nest sites in 1979, 1981 and 1982 revealed that nine fish species were eaten. Despite variation between samples, cyprinids consistently represented the main prey, with Minnow Phoxinus phoxinus the single most important species. Stickleback Gasterosteus aculeatus was also a prominent prey item, particularly in 1981. Ignoring Stoneloach Noemacheilusbarbatulus, the proportion of fish species found in riffles within the study area was similar to that gleaned from the nest tunnel samples. Strong correlation of live fork-length and wet weight to pharyngeal bone length enabled the size of Minnow prey to be determined accurately: analysis of 2276 pharyngeal bones produced a mean size of 54 mm fork-length and 1.5 g wet weight. The mean length was comparable to that of Bullheads Cottus gobio eaten by chicks more than 10 days old on the River Lesse in Belgium.     

The use of otoliths as indicators of Little Penguin Eudyptula minor diet

The validity of using otoliths from stomach contents quantitatively to determine the number and size of fish consumed was tested on Little Penguins. They were fed different meal sizes of known number and size of fish and the stomach contents were recovered after various time intervals. There were no differences in estimates of original fish size when calculated from otolith length or weight. Rate of digestion of otoliths tended to decrease with increased meal size but increased with time after ingestion. Digestion of otoliths proceeds rapidly and, if ignored, estimates of numbers of fish consumed and of original fish size can be significantly underestimated. This problem can be partially solved by inspection of otolith condition and restricting calculations of fish size to otoliths unaffected by digestion. Many factors introduce variations into rate of otolith degradation and further species-specific studies are required before appropriate correction factors can be applied.     

The use of otoliths as indicators of Little Penguin Eudyptula minor diet

The validity of using otoliths from stomach contents quantitatively to determine the number and size of fish consumed was tested on Little Penguins. They were fed different meal sizes of known number and size of fish and the stomach contents were recovered after various time intervals. There were no differences in estimates of original fish size when calculated from otolith length or weight. Rate of digestion of otoliths tended to decrease with increased meal size but increased with time after ingestion. Digestion of otoliths proceeds rapidly and, if ignored, estimates of numbers of fish consumed and of original fish size can be significantly underestimated. This problem can be partially solved by inspection of otolith condition and restricting calculations of fish size to otoliths unaffected by digestion. Many factors introduce variations into rate of otolith degradation and further species-specific studies are required before appropriate correction factors can be applied.     

On the food of the freshwater eels and their feeding relationship with the salmonids

Following the results of the cursory investigations made by Jones & Evans (1960,1962) into the predatory habits of eels we decided to enlarge on this work. Consequently we looked at the stomach contents of about 5000 eels. In addition and at the same time two Welsh streams carrying salmonids were selected and sampled specifically to try to determine the relationship in the feeding habits of eels and salmonids.
The food of eels consisted mainly of trichopteran and dipteran larvae and plecopteran and ephemeropteran nymphs, though at times Gastropoda, Annelida, Crustacea and fish were common in the stomachs. Fish was not an important dietary item but, when eaten, the fish most commonly taken were elvers and eels.
In the River Dwyfach the salmonids ate more or less the same type of food as the eels, but the trout ate more surface food than the young salmon.
The similarity in feeding between the eels and salmonids resulted in apparent competition during the warmer months. In winter when the eels ate little the competition was thus almost negligible. Even though we speak of competition for the food available in the summer months we feel that before this can be accepted as fact we must know the relative abundance of each food organism eaten at this time.