Otter Diet Research Articles

Why do Eurasian otters eat so few invasive blue crabs?

Invasive species can become key trophic resources for native predators, and thus understanding predator-invasive prey interactions is relevant to assess invasion impacts. The Atlantic blue crab (Callinectes sapidus) has recently invaded coasts and lower stretches of rivers across the Western Palearctic, often establishing very abundant populations. The blue crab is an important prey for a large number of predators within its native range, but predation upon invasive populations is still virtually unknown. The Eurasian otter (Lutra lutra) is a flexible predator, known to incorporate invasive prey into its diet when they become available. We studied otter diet in a small estuary of southern Spain hosting a very abundant blue crab population. The role of the blue crab as otter prey was only anecdotal since, out of 223 spraints (otter faeces) collected between 2021 and 2023, its remains were found in a single one. The avoidance of the blue crab by otters, and also by other predators, seems to be a generalised phenomenon in the invaded range of this crustacean. Predation on invasive blue crabs may require a long-lasting learning process to overcome the crab’s anti-predatory structures and behaviours. Trophic interactions between invasive blue crabs and native predators deserves attention for three main aspects: (i) its implications for the functioning and structure of invaded food webs, (ii) its potential role in limiting the invasion, and (iii) the interest in the generation of new predatory traits among animal populations.

Fecal DNA metabarcoding reveals the winter diet of Eurasian otter (Lutra lutra) in Northeast China

As the top predator of freshwater ecosystems, the Eurasian otter (Lutra lutra) plays a vital role in maintaining food web structure and ecosystem stability. Understanding the dietary composition and dynamics of the Eurasian otter is crucial for comprehending its ecosystem function and interspecies interactions, providing vital information for determining conservation and management strategies. Currently, DNA metabarcoding technology has been extensively applied in dietary studies as a more effective and accurate method for identifying prey species. However, there remains an insufficient amount of research on otter diet using this technology, especially in Northeast China. In this study, we analyzed the vertebrate dietary composition and spatial variation of otters during the winter in Northeast China using DNA metabarcoding technology. We confirm 106 fecal samples belonging to Eurasian otter DNA from 118 putative otters fecal samples, and then performed polymerase chain reaction (PCR) amplifications using vertebrate universal primers with the otter blocking primer (OBS1) and the human blocking primer (HomoB) to the sequence. Subsequently, high-throughput sequencing was performed to analyze the dietary composition of Eurasian otter. We identified a total of 34 prey taxa, including 31 fish, 2 frog and 1bird. Our results showed that the winter diet of the Eurasian otter was dominated by fish (60.11 %), (e.g., Cottidae, Cyprinidae), followed by Ranidae (39.88 %). Furthermore, geographical variations in dietary preferences were observed. This study confirms that Eurasian otter is a generalist piscivorous predator, and that its diet could be related to the availability of resources in the habitat. This study enriches the information on the diet of Eurasian otter, and can be used to provide a reference for the development of effective conservation strategies. At the same time, it confirms the reliability of the fecal DNA metabarcoding technique for carnivore predation analysis, and provides a new perspective for the investigation and research of otters and other mammals.

Assessing river ecological status through Eurasian otter diet analysis

AbstractTo identify and mitigate biodiversity decline, it is necessary to develop and implement appropriate ecological assessment methods. This particularly relevant in ecosystems under strong human pressure, such as riverine environments, in which ecological assessment requires several standardized methods useful in a wide range of riverine characteristics and habitats. In this work, we related otter diet composition with a set of environmental variables in representative river reaches of two contiguous river basins with a high diversity of habitats and varying degrees of human alteration. Our goal was to determine to what extent otter prey communities were sensitive to habitat characteristics and human pressures. Our results suggest that otter prey community in heavily anthropized rivers vary significantly across three gradients: habitat productivity, habitat structure and habitat stability. We also compared the results obtained from the otter diet with those obtained from electrofishing. Overall, the otter diet changed spatially in a similar way to the fish community sampled by electrofishing, but both methods showed disagreements that suggest different biases in some habitats and for some species. We conclude that otter diet analysis is a potential supplementary method for assessing river ecological status and that human alterations related to habitat structural simplification and water extraction can prevent freshwater biodiversity recovery in human‐dominated landscapes despite improvements in biochemical water quality.

Physiological adaptations to semi‐aquatic habits: the thermogenic macronutrient intake of the Eurasian otter

Abstract Maintaining thermal homeostasis in water is a major challenge for semi‐aquatic mammals, which entails morphological, behavioural and physiological adaptations. Foraging strategies and dietary‐induced thermogenesis (DIT) contribute to regulating body temperature in cold environments. As DIT differs among macronutrients, semi‐aquatic mammals may be expected to regulate their macronutrient ratios to enhance their capability of thermoregulating. We estimated the nutritional intake target of a widespread semi‐aquatic mustelid, Eurasian otter Lutra lutra by reviewing the available literature on otters' diet throughout Europe and assessing the nutritional composition of non‐commercial fish species forming the bulk of otter diet through laboratory analyses. The macronutrient intake target for the otter was assessed as 61% protein and 39% lipid energy ratios. The mean percent protein energy was higher than that reported for typical hypercarnivores and tended to increase with decreasing water temperatures. The additional source of heat provided by the thermogenic effect induced by protein digestion may contribute to balance heat loss, compensating for the costs associated with protein overconsumption. If, as suggested by fossil records, semi‐aquatic habits appeared after the development of otter's ‘fish specialist’ dentition, the thermogenic effect of its fish‐based diet may have promoted the tight relationship occurring between otters and water habitats.

A DNA‐based evaluation of North American river otter diet with respect to fishes: Implications for the conservation of aquatic predators and their prey

Abstract The North American river otter Lontra canadensis was once widespread in North America but became extinct in the State of Indiana owing to habitat losses and unregulated trapping. However, following reintroduction, the population has expanded markedly since 1995, representing a notable conservation success story. The recent population growth has led to conflict with recreational angling groups who perceive otters as competitors. To investigate the extent of this competition, a DNA‐based study was conducted to determine whether material from the stomachs of trapper‐harvested otters could be used to identify fish prey species they had eaten. Representative specimens from 33 wild fish species were used as positive controls for DNA assays of otter prey. Mitochondrial DNA barcoding sequences generated from each fish species were used to develop a piscine barcoding assay to identify otter prey species. Fourteen otter carcasses were dissected, and multiple independent DNA extractions and mitochondrial sequencing reactions were performed on the prey bolus from each stomach. The sequences were analysed bioinformatically for taxonomic assignment to prey species. DNA evidence of nearly two dozen fish species, including species of recreational (e.g. bass) or conservation interest (darters) was found in the stomach material. This approach could help to determine the presence of prey species in otter diets, including those fish of conservation concern. That, in turn, would help to determine the best management options for conserving both otters and fish.

Diet of the European otter (Lutra lutra) in the Massif central (Auvergne, France): spatial and seasonal variations between two study sites

In order to evaluate predation strategy and variations during an active population dynamic, diet of European otter (Lutra lutra) was studied during a year-round cycle in two different aquatic systems in Auvergne (France). First were tributaries of Allier River in Haute-Loire and second were marshes, creeks, rivers and ponds in Puy-de-Dôme. Preys were identified and quantified (occurrences, abundances and biomasses) from otter spraints sampling. Diet was mainly consisted by little- and medium-sized fish (74% to 89% of consumed biomass, size generally less than 17.5 cm). Dominant prey in otter’s diet were the most numerous species in studied rivers and marshes, which corresponds to the most efficient predation strategy. Crayfish, amphibians, reptiles and small mammals constituted most of time secondary prey, but sometimes dominated diet. Spatial and temporal differences in prey diversity, abundances and biomasses were recorded. Data illustrating species diversity in rivers and marshes recently recolonized by European otter were not modified by its predation impact. The study of a top-predator diet can therefore be considered as an additional way in habitats studying and biodiversity and management conservation strategies, in natural or anthropogenic alteration contexts.

Diet of the North American river otter (Lontra canadensis) in North Carolina using 2 methods

AbstractRiver otters (Lontra canadensis) are key predators in North Carolina's aquatic systems, but they are often seen as competitors by anglers and fish hatcheries. River otter diets typically consist of fish and crayfish, but also include occasional herpetofauna, mammals, and birds. While standard diet studies focus on identification of prey through manual examination of stomach contents and feces, metabarcoding DNA analysis has become more popular to determine the presence or frequency of species that are often missed, misidentified, or underestimated. We collected river otter carcasses from licensed trappers and fur dealers across North Carolina from the 2009–10 trapping season through the 2015–16 season. We conducted necropsies and analyzed the stomach contents using standard observational methods and metabarcoding DNA analysis. We manually examined 522 river otter stomachs, of which 377 contained prey items. Decapods (crustaceans) were identified in 41% of stomachs and made up similar percentages within each Furbearer Management Unit (FMU). The order Perciformes composed the majority (62%) of fish prey across all stomach samples. Coastal Plain river otters primarily consumed crustaceans (50%) and fish (40%). Piedmont and Mountain river otters consumed fish (32% and 42%, respectively) most often followed by crustaceans (62% and 50%, respectively). Prey selection was similar between the sexes. Out of 368 samples, metabarcoding DNA examination was able to reliably match 164 prey items to species, 5 classes, 18 orders, 25 families, and 42 genera. Fishes made up 33% of the identifications, particularly Perciformes (13%), Cypriniformes (7%), and Siluriformes (5%). Twelve percent of identifications was made up by Amphibia, split evenly by Anura and Urodela. No birds or reptiles were detected in the Mountain or Piedmont FMUs, and no mammals were detected in the Coastal Plain or Mountain FMU. Overall, river otters in North Carolina consume a large variety of prey that varied regionally. The manual examination provided identifications that were not provided by the DNA examination (i.e., crayfish, brown snakes), while the DNA examination provided a more accurate identification of the broad array of prey items. To understand the composition of annual river otter diets we encourage managers to expand research to evaluate river otter diets year‐round and incorporate additional noninvasive methods (e.g., scat surveys) throughout the year.

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

AbstractDiet of the otter inhabiting a forest stream in SW Poland was studied in order to show the impact of the otter on fish 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 fish component of the diet (together 70.1%). The most numerous fish species consumed were Perca fluviatilis (24.1%) and Rutilus rutilus (12.1%). The otter diet was most diversified 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 proportions of Gobio gobio and Esox lucius were statistically different in those two years compared. This study confirms previous findings that the otter is a generalist piscivorous predator hunting opportunistically on locally and seasonally most common fish species, mainly of low economic importance.

Eurasian otters prefer to prey on religious released non-native fish on the Qinghai-Tibetan Plateau.

Religious wildlife release is prevalent worldwide, especially in Asia countries. It is one of the anthropogenic pathways to cause biological invasions. Religious fish release is common on the Qinghai-Tibetan Plateau, yet few studies have assessed the influences of religious fish release on local species. In Yushu, a city on the Qinghai-Tibetan Plateau, we interviewed local people, conducted fish trap surveys in local rivers, and examined the diet of Eurasian otters Lutra lutra using a fecal DNA metabarcoding approach. We found that fish release started at least in 1980-1990s in Yushu. Tibetan residents released fish in large amounts and released fish were usually exotic commercial fish purchased from market. Despite such long-term and intensive fish release activities, released fish were few in local rivers. On the other hand, Eurasian otters mainly prey on fish and released fish accounted for ~20% of relative read abundance of prey DNA in otters' diet, indicating their high preference on released fish. Our study suggested that religious fish release may provide additional food resources for otters, whereas otters, as a top predator in local rivers, may deplete non-native fish once they were released and, therefore, reduce the probability of colonization of released fish, although further studies are required to assess otters' impact. Our study revealed otters' diet in Yushu, providing basic information for local otter management and conservation. Furthermore, it represents a case showing that native predators prey on religious released animals, implying a probable direction for controlling invasive species through native predator conservation.

Macronutrient composition of sea otter diet with respect to recolonization, life history, and season in southern Southeast Alaska.

The sea otter (Enhydra lutris) population of Southeast Alaska has been growing at a higher rate than other regions along the Pacific coast. While good for the recovery of this endangered species, rapid population growth of this apex predator can create a human-wildlife conflict, negatively impacting commercial and subsistence fishing. Previous foraging studies throughout the sea otter range have shown they will reduce invertebrate prey biomass when recolonizing an area. The goal of this study was to examine and quantify the energy content of sea otter diets through direct foraging observations and prey collection. Our study area, Prince of Wales Island in southern Southeast Alaska, exhibits a gradient of sea otter recolonization, thus providing a natural experiment to test diet change in regions with different recolonization histories. Sea otter prey items were collected in three seasons (spring, summer, and winter) to measure caloric value and lipid and protein content. We observed 3523 sea otter dives during the spring and summer. A majority of the sea otter diet consisted of clams. Sea otters in newly recolonized areas had lower diet diversity, higher energetic intake rates (EIR, kcal/min), and prey had higher energy content (kcal/g). Females with pups had the highest diet diversity and the lowest EIR. Sea otter EIR were higher in the fall and winter vs. spring and summer. Sea cucumber energy and lipid content appeared to correspond with times when sea otters consumed the highest proportion of sea cucumbers. These caloric variations are an important component of understanding ecosystem-level effects sea otters have in the nearshore environment.

Can the Eurasian otter (Lutra lutra) be used as an effective sampler of fish diversity? Using molecular assessment of otter diet to survey fish communities

The Eurasian otter Lutra lutra is a generalist carnivore that is widely distributed in many aquatic ecosystems. Based on its inherent attributes of opportunistic foraging behaviour and broad dietary range, it is naturally considered a potential sampler of the diversity of aquatic vertebrates. To test the ability and efficiency of otters as a diversity sampler, we used DNA metabarcoding to investigate the composition in vertebrates of the diet of otters that inhabit a forest stream area in northeast China. Twenty vertebrate prey taxa were detected in 98 otter spraints. Otter diet mainly comprised aquatic fishes (59.4%) and amphibians (39.0%). We also used traditional approaches to investigate fish communities at 60 sampling sites in the same area to determine the relationship between fish population composition in the environment and otter diet. The comparison revealed that 28 species of fish were distributed in this area, of which five are simultaneously detected in otter spraints. This indicates that molecular analysis of the diet of otters is not an ideal approach for investigating fish diversity, at least when using the 12SV5 primer pair. Based on a review of the available molecular research on otter diet, we conclude that the low species resolution may be due to the presence of many closely-related prey species in native habitats and lack of suitable barcodes. Considering the remarkable power of diet metabarcoding analysis in capturing elusive and rare species, it represents an approach that can compensate for the defects associated with fishing methods and we suggest that it can be used as an auxiliary means of measuring traditional fish diversity.

Exploring the biotic resistance of the invasive green crab (Carcinus maenas) by examining the diet of North American river otters (Lontra canadensis)

Exploring the biotic resistance of the invasive green crab (Carcinus maenas) by examining the diet of North American river otters (Lontra canadensis)

Exploring the biotic resistance of the invasive green crab (Carcinus maenas) by examining the diet of North American river otters (Lontra canadensis): suppl. figure

Exploring the biotic resistance of the invasive green crab (Carcinus maenas) by examining the diet of North American river otters (Lontra canadensis): suppl. figure

Habitat selection and diet of the Asian small-clawed otter in Karlapat Wildlife Sanctuary, Odisha, India

ABSTRACT Conservation of habitats and flagship species, such as the Asian small-clawed otter, is one of the most effective ways to conserve aquatic biodiversity. The study was conducted at Karlapat wildlife sanctuary, Odisha, India, and aims to determine the habitat and diet preferences of Asian small-clawed otters. Presence-absence of Asian small-clawed otters and associated habitat variables were recorded along 1-km sections of streams and rivers. We used principal component analysis (PCA) and logistic regression to examine habitat variables influencing otter presence along streams and rivers. Diet analysis of Asian small-clawed otters was conducted to calculate the percent frequency of occurrence and score-bulk estimate of each food item in the study area. We found that the presence of Asian small-clawed otters was related to denser canopy, higher shrub cover, and rocky stretches. A logistic regression model showed that Asian small-clawed otters significantly selected for higher canopy cover. Crabs were found to be the most preferred food item (>80%) in the diet of Asian small-clawed otters. These findings shed light on the regional-scale habitat selection and diet of Asian small-clawed otters and indicate important species-habitat relationships, thus providing valuable information for conservation management and land-use planning.

Alien prey in the diet of the indigenous Eurasian otter in Vistula River, Poland

Alien prey in the diet of the indigenous Eurasian otter in Vistula River, Poland

Spraints demonstrate small population size and reliance on fishponds for Eurasian otter (Lutra lutra) in Hong Kong

AbstractLack of data on population sizes and resource requirements are major impediments to the effective conservation of rare species globally. The conservation of the Eurasian otter (Lutra lutra) in Hong Kong reflects many of these key challenges for elusive and difficult‐to‐study mammals. It is a rare carnivore that has narrowly escaped extirpation, now surviving within a human‐dominated environment. Using sign surveys and spraint analysis, we recorded only 40 fresh spraints from 246 otter signs locations, over 4 months of intensive sampling across 2 years. Records were restricted to the Mai Po wetlands, confirming this as the core area for Hong Kong's otter population. Molecular analysis and microsatellite genotyping identified a minimum of seven individuals, two pairs of which were likely related. The genetic and sign data together strongly indicate a small population. Fish dominated the otter diet, highlighting the importance of fishpond habitats as a premium foraging resource. Given the rapid changes surrounding the Mai Po area (especially the new Northern Metropolis Development Strategy), maintaining quality and connected habitats, in addition to sustaining commercial fishponds will be key to otter recovery and long‐term population viability in Hong Kong.

Better sturdy or slender? Eurasian otter skull plasticity in response to feeding ecology.

Otters are semi-aquatic mammals specialized in feeding on aquatic prey. The Eurasian otter Lutra lutra is the most widely distributed otter species. Despite a low degree of genetic variation across its European range, the population from Great Britain exhibits distinct genetic structuring. We examined 43 skulls of adult Eurasian otters belonging to 18 sampling localities and three genetic clusters (Shetlands, Wales and Scotland). For each sample location, information regarding climate was described using bioclimatic variables from WorldClim, and information on otter diet was extracted from the literature. By using photogrammetry, 3D models were obtained for each skull. To explore any evidence of adaptive divergence within these areas we used a three dimensional geometric morphometric approach to test differences in skull size and shape between areas with genetically distinct populations, as well as the influence of diet, isolation by distance and climate. Males were significantly larger in skull size than females across all the three genetic clusters. Skull shape, but not size, appeared to differ significantly among genetic clusters, with otters from Shetland exhibiting wider zygomatic arches and longer snouts compared to otters from Wales, whereas otters from Scotland displayed intermediate traits. A significant relationship could also be found between skull shape variation, diet as well as climate. Specifically, otters feeding on freshwater fish had more slender and short-snouted skulls compared to otters feeding mostly on marine fish. Individuals living along the coast are characterised by a mixed feeding regime based on marine fish and crustaceans and their skull showed an intermediate shape. Coastal and island otters also had larger orbits and eyes more oriented toward the ground, a larger nasal cavity, and a larger distance between postorbital processes and zygomatic arch. These functional traits could also represent an adaptation to favour the duration and depth of diving, while the slender skull of freshwater feeding otters could improve the hydrodynamics.

Deciphering Eurasian otter (Lutra lutra L.) and seal (Phoca vitulina L.; Halichoerus grypus F.) diet: Metabarcoding tailored for fresh and saltwater fish species.

Long-lived top predators shape biodiversity structure in their ecosystems and predator-prey interactions are critical in decoding how communities function. Studies on the foraging ecology of seals and Eurasian otters in Western Europe are outdated and most studies solely performed traditional hard part analysis. Molecular metabarcoding can be used as an innovative noninvasive diet analysis tool, which has proven efficient and complementary to hard part analysis, however, lacking application in the wider North Sea area. In this study, DNA from digesta, collected between 2014-2020, were used to identify fish species in the diet of 47 Eurasian otters, 54 harbour seals and 21 grey seals by applying a next-generation metabarcoding approach. A newly designed 16S rRNA primer, providing the best coverage of >130 local marine and freshwater fish species, was used to amplify prey DNA from seal scats and otter gut content sampled from the North Sea and regional freshwater bodies. Frequent fish species included tench, ninespine stickleback and white bream in otters; hooknose and common roach in grey seals and Pleuronectidae and sand gobies in harbour seals. Bipartite network analysis showed a strong overlap of harbour and grey seal diets. Otter diet intersected with both seal species in terms of freshwater species. This study provides new knowledge about dietary composition and community assemblage of fish prey in otters and seals in the North Sea and regional freshwaters, and a new molecular tool to elucidate predator-prey interactions and interspecies competition in complex and changing ecosystems under pressure from anthropogenic activities.

Assessing the Diet of a Predator Using a DNA Metabarcoding Approach

The diet of top predators is vital information needed to determine their ecological function and for their conservation management. However, the elusive habit and low population density of many predators constrains determination of their diets. While the morphological identification of scat contents is the traditional method, DNA metabarcoding has lately proven a more efficient and accurate method of identifying prey taxa. We applied DNA metabarcoding to analyzing the diet of the Eurasian otter (Lutra lutra), a top predator in freshwater ecosystems, using 12S and 16S rRNA mitochondrial primers target vertebrate prey. Diet did not vary among different data removal thresholds of 0.1, 1, 3, and 5%, comprising fishes (>90%), amphibians and birds (>2%), and occasionally mammals (<2%). Both 12S and 16S primers revealed similar otter diets, indicating that a single set of primers with a higher threshold is cost-effective for detecting the main prey taxa. Using 12S primers and a 5% threshold, we found no seasonal variation of otter diet in the Tangjiahe National Nature Reserve. A different prey community was found outside the reserve, which resulted in different prey composition for otters. However, prey taxon richness was not different between otters in- and outside the reserve. Otters preferred Schizothorax spp., the largest-sized fish species in the reserve, whereas they mainly preyed on Triplophysa bleekeri, a small-sized fish species, outside the reserve. Otters’ flexible feeding strategy reflect their high adaptability. However, greater human disturbance outside the reserve may present significant challenges to otters by altering prey communities and reducing prey profitability. Combining fecal DNA metabarcoding and local fish survey will provide opportunities for more detailed studies on the impact of different levels of human disturbances on prey communities and otters.

Dryland river restoration via carnivore reintroduction: Nonnative fauna dominate diets of river otters reintroduced to the Upper Rio Grande

Dryland rivers often harbor abundant populations of nonnative species that threaten native fauna and negatively alter ecosystem structure and function. Native semiaquatic carnivores, such as river otters (Lontra canadensis), that could predate nonnative fauna have been extirpated from many dryland rivers. During 2008–2010, river otters were reintroduced to the Upper Rio Grande (URG), a principal dryland river in the arid southwestern USA that has been colonized by dozens of nonnative fish and invertebrates. We conducted a scat-based food habits study and investigated whether otter reintroduction could be useful for managing nonnative aquatic fauna via predation. Across seasons, nonnative crayfish and fish were the dominant prey items in otter scats; crayfish frequency of occurrence was highest (61–95%), followed by nonnative suckers (7–26%) and trout (3–28%). After accounting for heterogeneous sampling effort between seasons, multi-level models estimated 0.77–0.93 probabilities that scats contained crayfish, compared to 0.05–0.21 and 0.03–0.14 probabilities for suckers and trout, respectively. Reintroducing river otters, and possibly other semiaquatic carnivores, to degraded dryland rivers might be an effective conservation action for controlling nonnative faunal populations within the biomic approach to river restoration, which could reestablish trophic cascades that improve ecosystem structure and function.