Patterns of mortality in southern sea otters (Enhydra lutris nereis) from 1998-2001

Detailed postmortem examination of southern sea otters (Enhydra lutris nereis) found along the California (USA) coast has provided an exceptional opportunity to understand factors influencing survival in this threatened marine mammal species. In order to evaluate recent trends in causes of mortality, the demographic and geographic distribution of causes of death in freshly deceased beachcast sea otters necropsied from 1998-2001 were evaluated. Protozoal encephalitis, acanthocephalan-related disease, shark attack, and cardiac disease were identified as common causes of death in sea otters examined. While infection with acanthocephalan parasites was more likely to cause death in juvenile otters, Toxoplasma gondii encephalitis, shark attack, and cardiac disease were more common in prime-aged adult otters. Cardiac disease is a newly recognized cause of mortality in sea otters and T. gondii encephalitis was significantly associated with this condition. Otters with fatal shark bites were over three times more likely to have pre-existing T. gondii encephalitis suggesting that shark attack, which is a long-recognized source of mortality in otters, may be coupled with a recently recognized disease in otters. Spatial clusters of cause-specific mortality were detected for T. gondii encephalitis (in Estero Bay), acanthocephalan peritonitis (in southern Monterey Bay), and shark attack (from Santa Cruz to Point A?o Nuevo). Diseases caused by parasites, bacteria, or fungi and diseases without a specified etiology were the primary cause of death in 63.8% of otters examined. Parasitic disease alone caused death in 38.1% of otters examined. This pattern of mortality, observed predominantly in juvenile and prime-aged adult southern sea otters, has negative implications for the overall health and recovery of this population.     

The distribution of nuclear genetic variation and historical demography of sea otters

The amount and distribution of population genetic variation is crucial information for the design of effective conservation strategies for endangered species and can also be used to provide inference about demographic processes and patterns of migration. Here, we describe variation at a large number of nuclear genes in sea otters Enhydra lutris ssp. We surveyed 14 variable microsatellite loci and two genes of the major histocompatibility complex (MHC) in up to 350 California sea otters Enhydra lutris nereis , which represents ∼10% of the subspecies' population, and 46 otters from two Alaskan sites. We utilized methods for detecting past reductions in effective population size to examine the effects of near extinction from the fur trade. Summary statistic tests largely failed to find a signal of a recent population size reduction (within the past 200 years), but a Bayesian method found a signal of a strong reduction over a longer time scale (up to 500 years ago). These results indicate that the reduction in size began long enough ago that much genetic variation was lost before the 19th century fur trade. A comparison of geographic distance and pairwise relatedness for individual otters found no evidence of kin-based spatial clustering for either gender. This indicates that there is no population structure, due to extended family groups, within the California population. A survey of population genetic variation found that two of the MHC genes, DQB and DRB, had two alleles present and one of the genes, DRA, was monomorphic in otters. This contrasts with other mammals, where they are often the most variable coding genes known. Genetic variation in the sea otter is among the lowest observed for a mammal and raises concerns about the long-term viability of the species, particularly in the face of future environmental changes.     

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.     

Understanding the inter-specific dynamics of two co-existing predators in the Tierra del Fuego Archipelago: the native southern river otter and the exotic American mink

Knowledge about interactions between endangered native southern river otters (Lontra provocax) and introduced American mink (Neovison vison) is essential for effective management of both species. We evaluated competition for spatial and trophic niches between otter and mink in overlapping and non-overlapping areas, comparing distribution, habitat preference, diet and mink marking behavior. We surveyed otter and mink signs along 250 km of Beagle Channel coastline. Habitat suitability models were constructed based on species presence/absence and habitat characteristics, using generalized linear models. Feces were collected for diet analyses. Otters used forested coasts with 12°–32° shoreline slope and without human influence, and our evidence suggests they were not affected by mink presence. Mink preferred forested and shrubland coasts with 10°–28° shoreline slope. Neither human influence nor otter presence affected mink habitat occupation, but in the presence of otters, mink left fewer signs. Otters consumed more aquatic prey than mink, and mink modified their diet in the presence of otters, consuming more exotic small terrestrial mammals and less fish as well as shifting to smaller and shallower fish species that are less consumed by otters. Mink showed more plastic, generalist behavior than otters, being more tolerant of human presence, using more habitat types and having greater diet breadth. At the same time, otters apparently affect mink adversely and could help limit their invasion in sympatric areas. Conservation and recovery of otters, therefore, may produce a secondary benefit of simultaneously reducing the effect of mink, thereby providing an additional way to control this exotic predator’s population.     

Habitat selection by the Cape clawless otter (Aonyx capensis) in rivers in the Western Cape Province, South Africa

We radio‐tracked seven Cape clawless otters (Aonyx capensis) (Schinz, 1821) in two rivers in the Western Cape Province, South Africa, providing data on their habitat selection. Habitat type was investigated at a scale that enabled us to separate the effects of types of riparian vegetation, geomorphology and anthropogenic influences. Otters selected areas with boulders and/or reed beds, which provided high crab density and shelter. Direct observations showed that they used two foraging modes depending on the habitat selected. Otters could select open water within c. 8 m of the shore, dive and surface with or without prey. Otherwise hunting involved them moving into shallow water (c. 0.2 m deep), and walking along the substrate feeling for prey with their forefeet. Disturbed possible prey items were then caught with the forefeet.     

Use of salmonid carcasses by vertebrate scavengers

The use of salmon Salmo salar carrion by otters Lutra lutra and other scavengers along the River Dee in north-east Scotland was studied by radio-tagging and individual marking of fish carcasses. More carcasses were available on the Dee than on tributary streams used for spawning, indicating that salmon returned to the river after spawning and died there. The amount of salmon carrion available to terrestrial and avian scavengers along the Dee varied from 6.7 kg. km^-1 on an upstream study area to 36 kg. km^-1 downstream. Fish carcasses in the Dee were moved by spates up to 20 km but in streams used for spawning less than 1 km. Of 86 carcasses examined in 1990/91, 64 were available to terrestrial and avian scavengers on the bank or awash and of these 45 had been fed upon by otters and 16 by birds. In 1991/92, 23 of 30 carcasses were available to terrestrial and avian scavengers. All had been fed upon, 19 by otters, four by birds. Other carcasses, in shallow water, were not available to terrestrial and avian scavengers. Subsequent scavenging was mainly by otters and continued for up to three weeks after the carcasses were found. Heron Ardea cinerea , great black-backed gull Larus marinus and crow Corvus corone also scavenged salmon carcasses along the Dee. Great black-backed gulls were the most frequent scavengers, but heron (dominant to black-backed gull) was a major scavenger in 1990/91. Crows, subordinate to other scavengers, waited, often in pairs, upon dominant scavengers. There were more scavenging birds downstream and numbers did not change between years. Of 20 salmon carcasses placed in spawning areas eight were probably, two possibly, removed by otters. Otters continued to scavenge carcasses for up to a month. Scavenging by foxes Vulpes vulpes and birds followed the removal of fish carcasses from the water by otters. Radio transmitters were removed by otters and left lying alongside carcasses.     

Do alien North American mink compete for resources with native South American river otter in Argentinean Patagonia?

American mink Mustela vison , originally bred in fur farms, have become established in areas occupied by native endangered Southern river otter Lontra provocax , in Patagonia. In accordance with European experience, this biological invasion in South America raises questions about the interaction between invasive mink and native otter, from the viewpoints of both community assembly and conservation. We set out (1) to find which aspects of habitat structure were related to the distribution of signs of both this invasive species and Southern river otter Lontra provocax , in Argentinean Patagonia and their most common prey and (2) to test general predictions of niche partitioning between these two species. Based on surveys of 447 of 600 m transects for otter and mink scats/footprints along the waterside of lakes and rivers in the Andean Patagonian region, we compared diet composition (from scat analysis) and micro-habitat preferences (from field signs) of the two species. Otters were more specialist than mink in habitat use and diet. Mink used different habitats in other river basins where otters were absent. Where they occurred together in the basin of the Limay River, the distributions of their signs were similar, and mink diet was more similar to that of otters. There was no detectable difference in otter diet before and after mink arrival in the Limay basin. Contrary to the prediction of niche partitioning, and to the findings of European studies, resource use by mink was more similar to that of otters where the species occurred sympatrically than where they were allopatric.     

Diet diversity and breeding of top predators are determined by habitat stability and structure: a case study with the Eurasian otter (<Emphasis Type="Italic">Lutra lutra</Emphasis> L.)

A study on Eurasian otter was conducted in order to establish if feeding ecology and breeding of this European freshwater top predator were affected by the habitat complexity or stability. The work was based on the comparison of contrasting environmental settings. Significant gradients were found for otter diet parameters and breeding, both also changing according to habitat gradient patterns (water capacity and permanence during droughts, habitat stability, and habitat complexity). The otter diet was less diverse in the most stable (and complex) habitats, eating more fish. Otters also breed more regularly in such more stable courses, with more suitable fish availability. The step toward lower habitat stability can put otters in a less advantageous position in front of generalist predators, foraging more frequently outside or in the edge of aquatic ecosystems. Implications for otters and other similar top predator’s conservation are discussed.     

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.     

Predation on adult Atlantic salmon, Salmo salar L., by otters, Lutra lutra (L.), within the River Dee system, Aberdeenshire, Scotland

Predation on adult salmon, Salmo salur L., by otters, Lurra lutra (L.), varied seasonally on the R. Dee, Aberdeenshire, Scotland, being highest during the spawning season in winter. Predation is described for some tributaries of the river. Male fish were caught by otters more often than females, and it is suggested that they were most vulnerable during their extensive excursions up and down stream, particularly as they negotiated shallow riffles. Otters appeared to prey upon healthy fish rather than those infested with Saprolegnia sp. but there was no difference in the size, freshwater- or sea-age offish killed by otterscompared with 'kelts' which had died non-violently. At least some of the otters obtained most of their daily food requirements by catching a single salmon per night.
Considerable numbers of adult salmon may be killed by otters during the spawning period but it is suggested that, because most are males, this is unlikely to affect the breeding success of the salmon population. Most predation occurs outside the fishing season and so is unlikely to reduce the numbers of salmon caught by anglers.     

DIET AND FORAGING BEHAVIOR OF SEA OTTERS IN SOUTHEAST ALASKA

Abstract: Direct observations of feeding sea otters ( Enhydra lutris ) at 11 sites in southeast Alaska showed infaunal clams to be the primary prey utilized by otters throughout the region. Foraging dive times associated with clam and sea urchin prey were significantly longer than those for more easily captured prey (crabs and mussels). Dive times and surface intervals were also generally correlated with water depth or apparent difficulty in obtaining buried prey. Male otters, which fed more extensively on clams than females, made significantly longer foraging dives than females. Foraging success remained high, even at sites where prey numbers were found to be very low during a related study. The very deeply burrowing geoduck clam ( Panope abrupta ), while common at several otter feeding sites, was rarely captured by otters. These results, combined with those of a companion study on prey numbers, indicate that butter clams ( Saxidomus giganteus ) account for the majority of the sea otter diet in southeast Alaska, and that sea urchins may represent relatively short-term prey in comparison to infaunal bivalves in regions where both prey types co-exist. Furthermore, the importance of butter clams in the sea otter diet and the tendency for this bivalve to retain chronically high levels of paralytic shellfish poisoning toxins in southeast Alaska increases the probability that toxic phytoplankton blooms influence sea otter distribution in this region.     

Translocations maintain genetic diversity and increase connectivity in sea otters,Enhydra lutris

Sea otters, Enhydra lutris, were once abundant along the nearshore areas of the North Pacific. The international maritime fur trade that ended in 1911 left 13 small remnant populations with low genetic diversity. Subsequent translocations into previously occupied habitat resulted in several reintroduced populations along the coast of North America. We sampled sea otters between 2008 and 2011 throughout much of their current range and used 19 nuclear microsatellite markers to evaluate genetic diversity, population structure, and connectivity between remnant and reintroduced populations. Average genetic diversity within populations was similar: observed heterozygosity 0.55 and 0.53, expected heterozygosity 0.56 and 0.52, unbiased expected heterozygosity 0.57 and 0.52, for reintroduced and remnant populations, respectively. Sea otter population structure was greatest between the Northern and Southern sea otters with further structuring in Northern sea otters into Western, Central, and Southeast populations (including the reintroduced populations). Migrant analyses suggest the successful reintroductions and growth of remnant groups have enhanced connectivity and gene flow between populations throughout many of the sampled Northern populations. We recommend that future management actions for the Southern sea otter focus on future reintroductions to fill the gap between the California and Washington populations ultimately restoring gene flow to the isolated California population.     

Why do river otters scent-mark? An experimental test of several hypotheses

We tested several alternative hypotheses about the function of scent marking by the North American river otter, Lontra canadensis. Otters may mark at latrine sites with spraints (faeces) to (1) signal species identity, (2) advertise their reproductive status, (3) establish and maintain territories, and (4) communicate social status and identity to group members. Olfactory preference tests were conducted at the Alaska Sealife Center in Seward, Alaska, on a group of 15 wild-caught male otters in February 1999. We found that male otters investigated otter scent more than sealion faeces. The male otters also showed a preference for male scent over the scent of anoestrous females. No preference for the scent of unfamiliar males, compared with the scent of familiar males, was observed, and no preference for the scent of close kin was detected. However, an investigation of dominant relationships of the captive otters showed that dominant males spent more time investigating male scent than did subordinate males. Thus, spraints deposited at latrine sites may function to communicate social status of males.     

KILLER WHALES AND MARINE MAMMAL TRENDS IN THE NORTH PACIFIC—A RE-EXAMINATION OF EVIDENCE FOR SEQUENTIAL MEGAFAUNA COLLAPSE AND THE PREY-SWITCHING HYPOTHESIS

Springer et al . (2003) contend that sequential declines occurred in North Pacific populations of harbor and fur seals, Steller sea lions, and sea otters. They hypothesize that these were due to increased predation by killer whales, when industrial whaling's removal of large whales as a supposed primary food source precipitated a prey switch. Using a regional approach, we reexamined whale catch data, killer whale predation observations, and the current biomass and trends of potential prey, and found little support for the prey-switching hypothesis. Large whale biomass in the Bering Sea did not decline as much as suggested by Springer et al ., and much of the reduction occurred 50–100 yr ago, well before the declines of pinnipeds and sea otters began; thus, the need to switch prey starting in the 1970s is doubtful. With the sole exception that the sea otter decline followed the decline of pinnipeds, the reported declines were not in fact sequential. Given this, it is unlikely that a sequential megafaunal collapse from whales to sea otters occurred. The spatial and temporal patterns of pinniped and sea otter population trends are more complex than Springer et al . suggest, and are often inconsistent with their hypothesis. Populations remained stable or increased in many areas, despite extensive historical whaling and high killer whale abundance. Furthermore, observed killer whale predation has largely involved pinnipeds and small cetaceans; there is little evidence that large whales were ever a major prey item in high latitudes. Small cetaceans (ignored by Springer et al .) were likely abundant throughout the period. Overall, we suggest that the Springer et al . hypothesis represents a misleading and simplistic view of events and trophic relationships within this complex marine ecosystem.     

Morphological and thermal properties of mammalian insulation: the evolutionary transition to blubber in pinnipeds

Carnivora includes three independent evolutionary transitions to the marine environment: pinnipeds (seals, sea lions, and walruses), sea otters, and polar bears. Among these, only the pinnipeds have retained two forms of insulation, an external fur layer and an internal blubber layer for keeping warm in water. In this study we investigated key factors associated with the transition to the use of blubber, by comparing blubber characteristics among the pinnipeds. Characteristics included gross morphology (blubber thickness), fat composition (fatty acid profiles, percentage lipid, and water), and thermal conductivity. Sea lions, phocids, and walrus, which have lower fur densities than fur seals, have thicker blubber layers than fur seals (P < 0.001). Comparisons of lipid content, water content, and fatty acid composition indicated significant differences in the composition of the inner and outer regions of the blubber between groups (P < 0.001), consistent with the hypothesis that phocids and sea lions utilize the outer layer of their blubber primarily for thermal insulation, and the inner layer for energy storage. Fur seals, by contrast, rely more on their fur for thermal insulation, and utilize their blubber layer primarily for energy storage. Comparing across carnivore species, differences in total insulation (fur and/or blubber) are influenced substantially by body size and habitat, and to a lesser extent by latitudinal climate. Overall, these results indicate consistent evolutionary trends in the transition to blubber and evidence for convergent evolution of thermal traits across lineages. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, ??, ??–??.     

The pathology and seawater performance of farmed Atlantic salmon infected with glochidia of Margaritifera margaritifera

The pathology of glochidial infection of the freshwater mussel Margaritifera margaritifera was examined in farmed Atlantic salmon Salmo salar in fresh water and for 150 days after transfer of salmon to sea water. Prevalence of infection in fresh water was 95%, mean abundance 134 per fish and mean infection intensity 140. Prevalence in sea water was 80–94% in the first 7 weeks after transfer but glochidia were absent, apart from remains, after 50 days in sea water. Glochidia on salmon in fresh water were associated with localized hyperplasia and fusion of secondary gill lamellae. Focally extensive epithelial hyperplasia and fusion of secondary lamellae were present 4–10 days after transfer to sea water. Twenty-three days after transfer, small nodules with a more discrete appearance were present suggesting partial resolution of tissue response; hyperplastic responses associated with glochidia were much reduced after 50 days. Plasma chloride in infected fish 10 days after transfer was 153 mmol. 1⁻¹, significantly higher than fish without infection, suggesting poorer adaptation to sea water. No mortalities due to glochidial infection in sea water were recorded and there was no significant difference in mean weight between infected and control fish.     

Scent marking by otters (Lutra lutra): signaling the use of resources

Otters (Lutra lutra) deposit feces as scent marks ("spraints")throughout their range, and observations on this behavior inShetland were used to test the hypothesis that carnivores ingroup ranges use scent marking to signal priority of use ofresources to other group members. Sprainting was seasonal (highrates coinciding with low prey availability), and there wasno significant, overall difference in sprainting rates betweenotters of different sex or status. There were no concentrationsof spraints near group territorial boundaries. Sprainting wasassociated with the beginning of feeding bouts, as well as withthe utilization of other resources, such as fresh water anddens ("holts"). More than 30% of spraints were deposited inplaces that flooded within hours, and the spraints were functionalonly for a short time. It is argued that the temporal patternof use and subsequent replenishment of resources makes it advantageousfor otters to signal to other group members when they are exploitinga "patch" and for other members to avoid resources already partlydepleted by a prior arrival. With such a signaling system thereis no need for actual aggressive encounters to reinforce themessage of scent marking. Sprainting could be the mechanismfor the observed spaced-out use of resources among the inhabitantsof a group territory.     

Causes and consequences of marine mammal population declines in southwest Alaska: a food-web perspective

Populations of sea otters, seals and sea lions have collapsed across much of southwest Alaska over the past several decades. The sea otter decline set off a trophic cascade in which the coastal marine ecosystem underwent a phase shift from kelp forests to deforested sea urchin barrens. This interaction in turn affected the distribution, abundance and productivity of numerous other species. Ecological consequences of the pinniped declines are largely unknown. Increased predation by transient (marine mammal-eating) killer whales probably caused the sea otter declines and may have caused the pinniped declines as well. Springer et al. proposed that killer whales, which purportedly fed extensively on great whales, expanded their diets to include a higher percentage of sea otters and pinnipeds following a sharp reduction in great whale numbers from post World War II industrial whaling. Critics of this hypothesis claim that great whales are not now and probably never were an important nutritional resource for killer whales. We used demographic/energetic analyses to evaluate whether or not a predator–prey system involving killer whales and the smaller marine mammals would be sustainable without some nutritional contribution from the great whales. Our results indicate that while such a system is possible, it could only exist under a narrow range of extreme conditions and is therefore highly unlikely.     

The otter (Lutra lutra L.) in western France

In a survey of western France during October-November 1980, signs of otters were found at 46 (15%) of the 315 sites investigated. Otters were largely restricted to four areas, three of these being in the uplands (of Brittany, the Massif Central and the Pyrenees) and one in the coastal lowlands (of the Landes-Gironde). The average number of signs found per successful site was 1 6, significantly lower than for Scodand and Ireland but similar to the figure for England. The percentage of positive sites and the scattered nature of the signs in western France was also broadly comparable witJi the English situation. This is surprising since most of the factors which adversely affect otters appear to be less oppressive there dian in England. Only water pollution is noticeably more extreme and this seems to offer the best, overall explanation for the nature of the decline in the area studied.     

Evidence for food competition between mink (Mustela vison) and otter (Lutra lutra) on Scottish islands

Food competition between American mink and otters was measured by comparing the diets of sympatric mink and otter populations with those of allopatric populations. Niche breadth was narrower for otters than mink. Niche breadth was wider for both mink and otters on islands where they co-existed in comparison to that of the allopatric populations. Niche overlap was lower in sympatric populations on islands with mammalian prey, however, niche overlap was not reduced on small islands without mammalian prey. Obtained data suggest that mink and otter compete for food resources and, when alternative prey sources are available, mink become more generalist predators to avoid competition with otters. However, when alternative prey sources are not available, both species become more generalist.