Fluctuating asymmetry and genetic diversity in sea otters (Enhydra lutris)
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Report↗
Geology topics
James L. Bodkin
Publications and source records attributed to James L. Bodkin.
An aerial survey method to estimate sea otter abundance
Sea otters ( Enhydra lutris ) occur in shallow coastal habitats and can be highly visible on the sea surface. They generally rest in groups and their detection depends on factors that include sea conditions, viewing platform, observer technique and skill, distance, habitat and group size. While visible on the surface, they are difficult to see while diving and may dive in response to an approaching survey platform. We developed and tested an aerial survey method that uses intensive searches within portions of strip transects to adjust for availability and sightability biases. Correction factors are estimated independently for each survey and observer. In tests of our method using shore-based observers, we estimated detection probabilities of 0.52-0.72 in standard strip-transects and 0.96 in intensive searches. We used the survey method in Prince William Sound, Alaska to estimate a sea otter population size of 9,092 (SE = 1422). The new method represents an improvement over various aspects of previous methods, but additional development and testing will be required prior to its broad application.
Conference Paper↗
Estimating age of sea otters with cementum layers in the first premolar
We assessed sources of variation in the use of tooth cementum layers to determine age by comparing counts in premolar tooth sections to known ages of 20 sea otters ( Enhydra lutris ). Three readers examined each sample 3 times, and the 3 readings of each sample were averaged by reader to provide the mean estimated age. The mean (SE) of known age sample was 5.2 years (1.0) and the 3 mean estimated ages were 7.0 (1.0), 5.9 (1.1) and, 4.4 (0.8). The proportion of estimates accurate to within ± 1 year were 0.25, 0.55, and 0.65 and to within ± 2 years 0.65, 0.80, and 0.70, by reader. The proportions of samples estimated with >3 years error were 0.20, 0.10, and 0.05. Errors as large as 7, 6, and 5 years were made among readers. In few instances did all readers uniformly provide either accurate (error <1 yr) or inaccurate (error >1 yr) counts. In most cases (0.85), 1 or 2 of the readers provided accurate counts. Coefficients of determination (R2) between known ages and mean estimated ages were 0.81, 0.87, and 0.87, by reader. The results of this study suggest that cementum layers within sea otter premolar teeth likely are deposited annually and can be used for age estimation. However, criteria used in interpreting layers apparently varied by reader, occasionally resulting in large errors, which were not consistent among readers. While large errors were evident for some individual otters, there were no differences between the known and estimated age-class distribution generated by each reader. Until accuracy can be improved, application of this ageing technique should be limited to sample sizes of at least 6-7 individuals within age classes of ≥ 1 year.
Alaska, California↗
Sea Otter Enhydra lutris
The sea otter, Enhydra lutris , is the largest member of the Mustelidae family and is the only one which lives entirely in marine waters. Sea otters are unique among marine mammals because, unlike whales, dolphins and seals, they do not have a layer of fat or blubber to keep them warm in the cool oceans of the North Pacific. Instead, sea otters depend on dense fur that traps tiny air bubbles to insulate them from the cold water. To stay warm, they also must maintain a very high metabolic rate, requiring the sea otter to eat about 25% of its body weight per day. Sea otters eat mostly invertebrates - clams, crabs, urchins, and mussels - found in shallow coastal waters.
Alaska↗
Population genetic studies of the sea otter ( Enhydra lutris ): A review and interpretation of available data
C urrent information about the utility of genetic markers for estimating population structuring in sea otters ( Enhydra lutris ) is reviewed. Analyses of spatial population structuring with biochemical and molecular genetic markers are discussed in the context of the species' ecology and history of exploitation. Studies that have used a diversity of genetic markers including allozymes, mitochondrial DNA (mtDNA), and multilocus minisatellites revealed that geographically spearated populations of sea otters are highly differentiated, though little evidence for phylogeographic structuring was suggested. Analyses of population relationships based on mtDNA haplotype frequency distribution suggested that populations can be separated into four major groups: (1) California; (2) Prince William Sound, Alaska; (3) Kodiak Island, Alaska, and islands of the Aleutian archipelago, including the Commander Islands; and (4) the Kuril Islands. Populations from locales separated by large geographic distances often shared haplotypes, suggesting recent common ancestry and some degree of historical gene flow. THe large differences among populations in nuclear and mtDNA gene frequency suggested strong constrains on contemporary gene flow and/or considerable drift in gene frequencies due to population bottlenecks. No evidence for microgeographic structuring was noted. Levels of genetic diversity within populations varied greatly across the species range but were not related to contemporary estimates of population size.
Conference Paper↗
Monitoring the status of wild sea otter populations: field studies and techniques
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Endangered Species UPDATE↗
Comparative demography of sea otter populations
Population trends are poorly documented and demographic information is typically lacking for many carnivorous mammals. The sea otter ( Enhydru lutris ) has a well known history of decline and recovery, and while many other species have declined as precipitously, few have recovered so spectacularly. Generally speaking, northern populations (remnants within the range of E. l. lutris and E. l. kenyoni ) have recovered at high rates while recovery of the southern sea olter ( E. I. nereis ), which is listed as legally threatened under the Endangered Species Act (ESA), has progressed more slowly. Our purpose is to contrast trends in abundance and demographic patterns between Southern California and northern (Washington, British Columbia, Alaska, and Asia) sea otter populations. Specifically, we provide (1) a brief review of the main findings to date: (2) a summary of ongoing and planned studies; and (3) recommendations for future research. A more detailed account of these and other issues concerning the conservation and management of sea otters is provided in the U.S. Fish and Wildlife Service's Recovery Plan for the Califomia Sea Otter (U.S. Fish and Wildlife Service 1996).
Endangered Species UPDATE↗
Sea otter predation on Dungeness crabs in Glacier Bay, Alaska
Sea otter ( Enhydra lutris ) populations are increasing and expanding into areas of Southeast Alaska where Dungeness crabs ( Cancer magister ) are commercially harvested. Although sea otters are known to feed on Dungeness crabs, their effects upon crab demography have not been quantified. We censused Dungeness crabs in adjacent areas within Glacier Bay National Park and Preserve with and without sea otters in April 1993, 1994, and 1995 to investigate the effects of sea otter predation. Two different crab survey techniques were used. Commercial crab pots with sealed escape rings and baited similarly were soaked for 24 hours to measure CPUE in 0-25 m depths. A minimum of 50 pots was used for each site; 25 additional pots were used in 1994 and 1995 to sample crabs to 95 m depths. Divers censused crabs along belt transects (2 m x 100 m) laid perpendicular to shore from 0 m depth to 18 m; 15 transects were surveyed at each site on each date. Sea otter numbers were estimated from aerial surveys and boat observers. Prey types and feeding rates of sea otters were made from telescopic observations. In Outer Dundas Bay, where 46, 39, and 40 sea otters were present in 1993, 1994, and 1995 respectively, CPUE of Dungeness crabs in pots was 0.04, 0.04, and 0.2 crabs per day, respectively, at depths to 25 m, but was higher (4.0 crabs per day) at deeper depths (95 m). Only two Dungeness crabs were observed in 45 diver transects surveyed during three years in Outer Dundas Bay, however Dungeness crabs were a common prey item (15%) of sea otters. Approximately 16 km N in Inner Dundas Bay, where no sea otters were observed any year, CPUE of Dungeness in pots was 2.7, 1.8, and 2.5 crabs per day for 1993, 1994, and 1995, respectively. Adult crab density estimated from diver transects was 236, 78, and 50 crabs/ha in 1993, 1994, and 1995, respectively. Significantly lower crab densities and CPUE occurred in the site occupied by sea otters. Our data imply that sea otter predation decreases Dungeness crab abundance and may induce movement of crabs to greater depths. A depth refuge for crabs from sea otter predation may vary with local bathymetry.
Alaska↗
Sea otters in the northern Pacific Ocean
About 250 years ago sea otters ( Enhydra lutris ) were distributed continuously from central Baja California, north and west along the Pacific Rim to Machatka Peninsula in Russia, and south along the Kuril Island to northern Japan (Kenyon 1969; Fig. 1a). Several hundred thousand sea otters may have occurred in the north Pacific region when commercial hunting began in the 18th century (Riedman and Estes 1990). At least two attributes of the sea otter have influenced humans, likely for as long as they have resided together along the coast of the north Pacific Ocean. First, sea otters rely on a dense fur, among the finest in the world, for insulation in the cold waters of the Pacific Ocean. The demand for sea otter fur led to their near extinction in the 19th century. The fur harvest, begun about 1740 and halted by international treaty in 1911, left surviving colonies, each likely numbering less than a few hundred animals, in California, south-central Alaska, and the Aleutian, Medney, and Kuril Islands (Fig. 1a). These individuals provided the nucleus for the recovery of the species. Today more than 100,000 sea otters occur throughout about 75% of their original range (fig. 1b). Immigration has resulted in near-complete occupation of the Aleutian and Kuril archipelagos and the Alaska peninsula. Successful translocations have resulted in viable populations in southeast Alaska, Washington, and British Columbia. Large amounts of unoccupied habitat remain along the coasts of Russia, Canada, the United States, and Mexico. The second potential source of conflict between sea otters and humans is that sea otters prey on and often limit some benthic invertebrate populations. Because some of these invertebrates are aso used by humans (Estes and VanBlaricom 1985), human perceptions about the effects of sea otter foraging on invertebrates sometimes differ. By limiting populations of herbivorous invertebrates ( e.g. , sea urchins [Echinoidea]) otters help maintain the integrity of kelp forest communities. At the same time, sea otter predation on other marine invertebrates can lead to direct competition with humans for resources. These interactions add complex dimensions to the conservation and management of sea otters, in large part because of wide-ranging social, ecological, and economic consequences of sea otter foraging. Long-term data on abundance and distribution are available for relatively few sea otter populations. Here we summarize such data from three populations: Being Island, Russia; Prince William Sound, Alaska; and Olympic Peninsula, Washington. The Bering Island population resulted from natural emigration and represents complete recovery. Prince William Sounds represents near recovery of a remnant population, whereas the Washington population was established via translocations from Alaska and is just beginning to recover. We will compare growth rates and current status among these populations. Because of its unique status and growth characteristics, the California sea otter is not treated in this article.
Book chapter↗
An overview of sea otter studies
The Exxron Valdez oil spill (EVOS) on 24 March 1989 threatened extensive areas of prime sea otter ( Enhydra lutris ) habitat along the coasts of south-central Alaska. The spill occurred in northeastern Prince William Sound (PWS), and oil moved rapidly south and west through PWS into the Gulf of Alaska. Much of the coastline of western PWS was heavily oiled, and the slick eventually spread as far southwest as Kodiak Island and the Alaska Peninsula (Galt and Payton 1990; Morris and Loughlin, Chapter 1). All coastal waters affected by the spill were inhabited by sea otters. Concern for the survival of sea otters following the oil spill was immediate and well founded. Sea otters are particularly vulnerable to oil contamination because they rely on pelage rather than blubber for insulation, and oiling drastically reduces the insulative value of the fur (Costa and Kooyman 1982; Siniff et al. 1982; Geraci and Williams 1990). Within days of the spill, recovery of oiled live otters and carcasses began. During the several months following the spill, sea otters became symbolic of the mortality associated with the spilled oil, and of the hope for rescue and recovery of injured wildlife (Batten 1990). An extensive sea otter rescue and rehabilitation effort was mounted in the weeks and months following the spill. Handling and treatment of the captive sea otters posed an enormous and difficult challenge, given the large number of otters held at the facilities and minimal prior experience in caring for oiled sea otters. Rehabilitation of sea otters was a separate effort from the postspill studies designed to evaluate injury to the otter populations and is not addressed in this chapter only as it relates to evaluation of damage assessment studies. Detailed information on the rehabilitation effort is presented in Bayha and Kormendy (1990) and Williams and Davis (1990). Sea otters retained a high profile in the Natural Resource Damage Assessment (NRDA) studies largely because the initial injury to the sea otter population was readily demonstrable, but also because of concerns about long-term damages. The scope of the postspill studies to assess oil-related damages to sea otters was extensive: From 1989 through 1993, more than $3,000,000 was spent, and more than 20 scientists were involved in a comprehensive research program. The studies were predominantly directed at sea otter populations in PWS. Damages to sea otters generally can be classified as either acute, defined as spill-related deaths occurring during the spill, or chronic, defined as longer term lethal or sublethal oil-related injuries. Studies of acute damages focused on estimating the total initial loss of sea otters. Characterization of the pathologies associated with exposure to oil was a secondary goal of studies of acute effects. Chronic or longer term damages may have resulted from sublethal initial exposure or continued exposure to hydrocarbons persisting in the environment. Studies of chronic effects included evaluating abundance and distribution, survival and reproduction rates, foraging behavior, and pathological, physiological, and toxicological changes in the years following the spill. The objective of this chapter is to review the studies conducted on sea otters in response to the EVOS and to synthesize the major findings of those studies relative to injury to the sea otter population associated with exposure to oil. We also provide recommendations for research to improve our understanding of the effects of future oil spills on Sea otter populations.
Alaska↗
An intersection model for estimating sea otter mortality along the Kenai Peninsula
We developed an intersection model to integrate parameters estimated from three distinct data sets that resulted from the Exxon Valdez oil spill: (1) the distribution, amount, and movements of spilled oil; (2) the distribution and abundance of sea otters along the Kenai Peninsula; and (3) the estimates of site-specific sea otter mortality relative to oil exposure from otters captured for rehabilitation and from collected carcasses. In this chapter, we describe the data sets and provide examples of how they can be used in the model to generate acute loss estimates. We also examine the assumptions required for the model and provide suggestions for improving and applying the model.
Alaska↗
Age-specific reproduction in female sea otters (Enhydra lutris) from south-central Alaska: Analysis of reproductive tracts
We estimated age at sexual maturity and age-specific reproductive rates by examining carcasses and reproductive tracts from 177 female sea otters (Enhydra lutris ). Carcasses were recovered from south-central Alaska, Primarily from western Prince William Sound, as a result of the T/V Exxon Valdez oil spill in 1989. We found 65% of our sample to be sexually mature. Sexual maturity was first attained at age 2. The proportion of sexually mature animals increased from 30% at age 2 to 100% at age 5. Annual reproductive rates increased from 22% at age 2 to 78% at age 5 and remained relatively stable (75-88%) through to age 15. the sex ratio (female:male) of 49 fetal sea otters was 18:37 and differed significantly from parity. Females younger than 8 tended to produce more female fetuses, while older mothers did not. Our estimates of the reproductive characteristics of female sea otters obtained by examination of reproductive tracts were similiar to those reported in the literature based on in situ observations of marked individuals.
Alaska↗
Annual and Spatial Variation of the Kelp Forest Fish Assemblage at San Nicolas Island, California
The kelp forest fishes of San Nicolas Island, California were studied from 1981-1986 to examine the causes of among-site and among-year variation in the fish assemblages. Fish counts and seven physical and biological variables were recorded at six sites around the island every spring and fall. Over the study period, a total of 45 fish species from 18 families were recorded, though members of nive families dominated at all sites. Among-site variation was considereable with two sites on the south side of the island having two to four times as many non-schooling fishes as the other four sites. Three variables, based on stepwise multiple regression techniques, were important predictors of site-specific fish abundance: 1) vertical relief; 2) sand cover and 3) understory algal cover. The total number of fishes varied interannually by a factor of three. Due to recruitment occuring each spring, there was a strong seasonal component to the variation in fish abundance. The extent of seasonal and interannual variaton of fish abundance is an indication of the variable nature of recruitment to this area. Over the 6 yr period, there were three distinct groupings of fish assemblages correspondong to pre- (Fall 1981 - Fall 1982), during spring (Spring 1983 - Spring 1984) and post El Nino (Fall 1984 - Fall 1986) sampling dates. During El Nino sampling period, there was considerable recruitment of southern affinity fish species, increasing both the abundance and diversity of the fish assemblages. Large-scale oceanographic processes, coupled with site-specific features of the reef habitat, produce a moderately diverse, though relatively abundant fish fauna at San Nicolas Island.
California↗
Molt frequency and size class distribution in the California spiny lobster (Panulirus interruptus), at San Nicolas Island, California
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California Fish and Game↗
Patterns of seabird and marine mammal carcass deposition along the central California coast, 1980-1986
At monthly intervals from February 1980 through December 1986, a 14.5-km section of central California coastline was systematically surveyed for beach-cast carcasses of marine birds and mammals. Five hundred and fifty-four bird carcasses and 194 marine mammal carcasses were found. Common murres, western grebes, and Brandt's cormorants composed 45% of the bird total. California sea lions, sea otters, and harbor seals composed 90% of the mammal total. Several factors appeared to affect patterns of carcass deposition. The El Niño – Southern Oscillation (ENSO) of 1982–1983 was the dominant influence in terms of interannual variation in carcass deposition. During this ENSO, 56% of the seabirds and 48% of the marine mammals washed ashore. Patterns of intra-annual variation were species specific and were related to animal migration patterns, reproduction, and seasonal changes in weather. Nearshore currents and winds influenced the general area of carcass deposition, while beach substrate type and local patterns of sand deposition influenced the location of carcass deposition on a smaller spatial scale. Weekly surveys along a 1.1-km section of coastline indicated that 62% of bird carcasses and 41% of mammal carcasses remained on the beach less than 9 days. Cause of death was determined for only 8% of the carcasses. Oiling was the most common indication of cause of death in birds (6%). Neonates composed 8% of all mammal carcasses.
Canadian Journal of Zoology↗
Extent, causes and timing of moose calves mortality in western interior Alaska
We examined the causes and timing of moose ( Alces alces ) calf mortality during 1988 90 in 2 locations in western interior Alaska. Annual survival rates of all calves in 1988 (0.34, n = 42) and 1989 (0.29, n=47) on the Nowitna National Wildlife Refuge (NWR) and in 1990 (0.25, n = 62) on the Koyukuk NWR were not significantly different. Survival rates of male and female calves were not significantly different during any year. Annual survival of single calves was significantly higher than that for twins in 1989 (0.56 vs. 0.20) and 1990 (0.37 vs. 0.21), but not in 1988. Black bears ( Ursus americanus ) killed 40% of all calves. Wolves ( Canis lupus ) killed 9% of the calves, unknown predators killed 8%, grizzly bears ( Ursus arctos ) killed 3%, and 5% died from other causes.
Alaska↗
Procedures and rationale for marking sea otters captured and treated during the T/V Exxon Valdez oil spill
Four methods were used for marking sea otters ( Enhydra lutris ) captured and treated during the response to the T/V Exxon Valdez oil spill. Colored and numbered flipper tags were placed on each sea otter that was brought to the otter treatment centers. These tags allowed individual recognition and permitted the tracking of individuals through the treatment and holding processes. Recovery of tagged carcasses may provide a crude measure of the fates of rehabilitated otters. Seven sea otters were instrumented with radio transmitters attached to flipper tags as part of a pilot release program. The results of that study were inconclusive. Forty-five sea otters were implanted with radio transmitters as part of a study to assess the fate of rehabilitated sea otters. Specific objectives of the study include estimating survived rates and monitoring the reproductive success of the sample of rehabilitated sea otters and comparing the results with similar variables in a control population. Transponder chips were injected in the perianal region of all but seven of the instrumented sea otters, and all were tagged with red flipper tags. The advantages and disadvantages of each type of tag are discussed.
Alaska↗
Effects of kelp forest removal on associated fish assemblages in central California
Visual surveys along subtidal belt transects were used to compare fish assemblages on an experimental and a control site before and after the removal of a canopy-forming kelp forest. The giant kelp Macrocystis pyrifera (L.) C.A. Agardh was removed at the holdfast from ≈ 1 ha of high relief structurally complex rock substratum. The abundance of seven species of fish, of which five were considered midwater species, significantly declined after the kelp was removed. No increases in abundance were noted within the experimental site. On the control site, two declines in abundance were observed, while two species increased in abundance. These results indicate that the presence of a giant kelp forest may increase the abundance and species diversity of the fish assemblages over a high relief rocky reef in central California, U.S.A.
Journal of Experimental Marine Biology and Ecology↗