Foraging strategies and prey switching in the California sea otter

Keystone interdependence: Sea otter responses to a prey surplus following the collapse of a rocky intertidal predator

The sea star Pisaster ochraceus and sea otters (Enhydra lutris) are two predators capable of shaping rocky intertidal and kelp forest community structure and functioning. In 2013, a sea star wasting event decimated populations of Pisaster along the west coast of North America. The collapse of this species in the rocky intertidal revealed an unexpected relationship between two keystone predators. In this study, we show how the loss of Pisaster along the Monterey Peninsula, CA, USA led to an increase in mussel (Mytilus californianus) size and expansion into lower tidal zones. Before the sea star wasting event, the local sea otter population fluctuated around a near equilibrium. However, in the absence of Pisaster, sea otters increased their dietary intake on mussels, which contributed in part to a local population-level rise. These results demonstrate how the loss of a keystone predator in one ecosystem may impart population-level changes to another.

The effect of a harmful algal bloom (Karenia selliformis) on the benthic invertebrate community and the sea otter (Enhydra lutris) diet in eastern Hokkaido

In recent decades, the locally extinct sea otter (Enhydra lutris lutris) has been recolonizing the coast of eastern Hokkaido. Their diet includes benthic invertebrates such as bivalves, sea urchins, snails, and chitons. In the fall of 2021, a harmful algal bloom (HAB) of Karenia selliformis occurred across Hokkaido's northern and eastern coasts, leading to a massive mortality of sea urchins. This dinoflagellate produces a neurotoxin (gymnodimine) implicated in shellfish poisoning. To determine the effect of the HAB on the marine community, we conducted benthic surveys using SCUBA and visually monitored the prey items of the sea otters in the affected area from 2020 to 2023. Following the HAB, we observed an 82% decrease in benthic sea urchin density (number m2), leading to their complete absence from the diet of sea otters. Conversely, bivalve density increased six-fold, accompanied by a nearly two-fold rise in their percentage in the sea otters' diet. Minimal changes were observed in the density of chitons and snails, with no significant alteration in the sea otters' diet. Despite these changes, the impact of the HAB on otters' dietary preferences was temporary, as the percentage of dietary sea urchins began recovering one year later. Sea otters augmented their diet with bivalves to compensate for the reduced availability of sea urchins during the HAB with no apparent effects on the number of sea otters or their health. Our results highlight the adaptability of sea otters to adjust their diet according to prey availability.

Data needs for sea otter bioenergetics modeling

Sea otters are keystone predators whose recovery and expansion from historical exploitation throughout their range can serve to enhance local biodiversity, promote community stability, and buffer against habitat loss in nearshore marine systems. Bioenergetics models have become a useful tool in conservation and management efforts of marine mammals generally, yet no bioenergetics model exists for sea otters. Previous research provides abundant data that can be used to develop bioenergetics models for this species, yet important data gaps remain. Here we review the available data that could inform a bioenergetics model, and point to specific open questions that could be answered to more fully inform such an effort. These data gaps include quantifying energy intake through foraging by females with different aged pups in different quality habitats, the influence of body size on energy intake through foraging, and determining the level of fat storage that is possible in sea otters of different body sizes. The more completely we fill these data gaps, the more confidence we can have in the results and predictions produced by future bioenergetics modeling efforts for this species.

Tool use increases mechanical foraging success and tooth health in southern sea otters (Enhydra lutris nereis)

Although tool use may enhance resource utilization, its fitness benefits are difficult to measure. By examining longitudinal data from 196 radio-tagged southern sea otters (Enhydra lutris nereis), we found that tool-using individuals, particularly females, gained access to larger and/or harder-shelled prey. These mechanical advantages translated to reduced tooth damage during food processing. We also found that tool use diminishes trade-offs between access to different prey, tooth condition, and energy intake, all of which are dependent on the relative prey availability in the environment. Tool use allowed individuals to maintain energetic requirements through the processing of alternative prey that are typically inaccessible with biting alone, suggesting that this behavior is a necessity for the survival of some otters in environments where preferred prey are depleted.

Revealing the extent of sea otter impacts on bivalve prey through multi-trophic monitoring and mechanistic models

Sea otters are apex predators that can exert considerable influence over the nearshore communities they occupy. Since facing near extinction in the early 1900s, sea otters are making a remarkable recovery in Southeast Alaska, particularly in Glacier Bay, the largest protected tidewater glacier fjord in the world. The expansion of sea otters across Glacier Bay offers both a challenge to monitoring and stewardship and an unprecedented opportunity to study the top-down effect of a novel apex predator across a diverse and productive ecosystem. Our goal was to integrate monitoring data across trophic levels, space, and time to quantify and map the predator-prey interaction between sea otters and butter clams Saxidomus gigantea, one of the dominant large bivalves in Glacier Bay and a favoured prey of sea otters. We developed a spatially-referenced mechanistic differential equation model of butter clam dynamics that combined both environmental drivers of local population growth and estimates of otter abundance from aerial survey data. We embedded this model in a Bayesian statistical framework and fit it to clam survey data from 43 intertidal and subtidal sites across Glacier Bay. Prior to substantial sea otter expansion, we found that butter clam density was structured by an environmental gradient driven by distance from glacier (represented by latitude) and a quadratic effect of current speed. Estimates of sea otter attack rate revealed spatial heterogeneity in sea otter impacts and a negative relationship with local shoreline complexity. Sea otter exploitation of productive butter clam habitat substantially reduced the abundance and altered the distribution of butter clams across Glacier Bay, with potential cascading consequences for nearshore community structure and function. Spatial variation in estimated sea otter predation processes further suggests that community context and local environmental conditions mediate the top-down influence of sea otters on a given prey. Overall, our framework provides high-resolution insights about the interaction among components of this food web and could be applied to a variety of other systems involving invasive species, epidemiology or migration.

Archaeological and Contemporary Evidence Indicates Low Sea Otter Prevalence on the Pacific Northwest Coast During the Late Holocene

The historic extirpation and subsequent recovery of sea otters (Enhydra lutris) have profoundly changed coastal social-ecological systems across the northeastern Pacific. Today, the conservation status of sea otters is informed by estimates of population carrying capacity or growth rates independent of human impacts. However, archaeological and ethnographic evidence suggests that for millennia, complex hunting and management protocols by Indigenous communities limited sea otter abundance near human settlements to reduce the negative impacts of this keystone predator on shared shellfish prey. To assess relative sea otter prevalence in the Holocene, we compared the size structure of ancient California mussels (Mytilus californianus) from six archaeological sites in two regions on the Pacific Northwest Coast, to modern California mussels at locations with and without sea otters. We also quantified modern mussel size distributions from eight locations on the Central Coast of British Columbia, Canada, varying in sea otter occupation time. Comparisons of mussel size spectra revealed that ancient mussel size distributions are consistently more similar to modern size distributions at locations with a prolonged absence of sea otters. This indicates that late Holocene sea otters were maintained well below carrying capacity near human settlements as a result of human intervention. These findings illuminate the conditions under which sea otters and humans persisted over millennia prior to the Pacific maritime fur trade and raise important questions about contemporary conservation objectives for an iconic marine mammal and the social-ecological system in which it is embedded.

Social organization of otters in relation to their ecology

Otter species are known to fluctuate intraspecifically from a solitary lifestyle to group-living arrangements. By examining what is known about habitat use and foraging style in otters of 13 different species, based on 93 studied sites, we assessed (1) the relationship between social habits and preferred habitats, (2) the relationship between species and prey preferences, and (3) the effect of predator avoidance on their social organization in order to assess the socio-ecological factors influencing otters. Females remain the core of their social stability. We show the major influence of habitats and feeding strategies (i.e. socio-ecology) of otters. The different species of solitary otters most often inhabit linear environments, such as freshwater ecosystems or wave-exposed marine coasts, and their habitat is often subject to disturbances that fragment their functional continuity. Social otters are more often found in extensive habitats with high plant cover, regular food resources and in areas with large predators compared to solitary species. The maintenance of regular resources and the fact that the main trophic resources are replenished rapidly might be determining factors driving sociality. Group-living and bachelor congregations among otters can also respond to pressure from large predators. This suggests that foraging, habitat use and the presence of large predators may be the drivers of sociality in otters. We conclude that most otters have a greater social potential than previously assumed, which is confirmed by their various vocalizations recently described.

Informing sea otter reintroduction through habitat and human interaction assessment

Sea otters Enhydra lutris have been absent from Oregon, USA, following their extirpation over a century ago. Stakeholder groups and native tribes are advocating for reintroduction to restore historic populations. We investigated the potential for successful reintroduction by: (1) estimating expected equilibrium sea otter densities as a function of habitat variables to assess sea otter habitat in Oregon; and (2) spatially relating areas of high expected densities to human activities (e.g. fisheries, recreation, vessel activity, protected areas) to anticipate potential disturbance or fishery resource competition. We estimated that 4538 (1742-8976; 95% CI) sea otters could exist in Oregon, with higher expected abundance (N = 1551) and densities (x̄ = 2.45 km-2) within the southern region. Most core habitat areas (97%), representing clusters of high expected densities, overlapped with some form of human activity. While commercial shipping and tow lanes overlapped little (1%) with core habitat areas, recreational activities (58%) and fisheries (76%) had a higher degree of overlap, posing higher disturbance risk. We anticipate higher resource competition potential with the commercial red sea urchin fishery (67% of harvest areas) than the commercial Dungeness crab fishery (9% of high-catch crabbing grounds). Our study presents the first published carrying capacity estimate for sea otters in Oregon and can provide population recovery targets, focus attention on ecological and socioeconomic considerations, and help to inform a recovery plan for a resident sea otter population. Our findings suggest current available habitat may be sufficient to support a sea otter population, but resource managers may need to further investigate and consider whether current human activities might conflict with reestablishment in Oregon, if plans for a reintroduction continue.

Sex and occupation time influence niche space of a recovering keystone predator

Predators exert strong effects on ecological communities, particularly when they re-occupy areas after decades of extirpation. Within species, such effects can vary over time and by sex and cascade across trophic levels. We used a space-for-time substitution to make foraging observations of sea otters (Enhydra lutris) across a gradient of reoccupation time (1-30 years), and nonmetric multidimensional scaling (nMDS) analysis to ask whether (a) sea otter niche space varies as a function of occupation time and (b) whether niche space varies by sex. We found that niche space varied among areas of different occupation times. Dietary niches at short occupation times were dominated by urchins (Mesocentrotus and Strongylocentrotus spp; >60% of diets) in open habitats at 10-40 m depths. At longer occupation times, niches were dominated by small clams (Veneroida; >30% diet), mussels (Mytilus spp; >20% diet), and crab (Decapoda; >10% diet) in shallow (<10 m) kelp habitats. Diet diversity was lowest (H' = 1.46) but energy rich (~37 kcal/min) at the earliest occupied area and highest, but energy poor (H' = 2.63, ~9 kcal/min) at the longest occupied area. A similar transition occurred through time at a recently occupied area. We found that niche space also differed between sexes, with bachelor males consuming large clams (>60%), and urchins (~25%) from deep waters (>40 m), and females and territorial males consuming smaller, varied prey from shallow waters (<10 m). Bachelor male diets were less diverse (H' = 2.21) but more energy rich (~27 kcal/min) than territorial males (H' = 2.54, ~13 kcal/min) and females (H' = 2.74, ~11 kcal/min). Given recovering predators require adequate food and space, and the ecological interactions they elicit, we emphasize the importance of investigating niche space over the duration of recovery and considering sex-based differences in these interactions.

Active touch in sea otters: in-air and underwater texture discrimination thresholds and behavioral strategies for paws and vibrissae

Sea otters (Enhydra lutris) are marine predators that forage on a wide array of cryptic, benthic invertebrates. Observational studies and anatomical investigations of the sea otter somatosensory cortex suggest that touch is an important sense for detecting and capturing prey. Sea otters have two well-developed tactile structures: front paws and facial vibrissae. In this study, we use a two-alternative forced choice paradigm to investigate tactile sensitivity of a sea otter subject's paws and vibrissae, both in air and under water. We corroborate these measurements by testing human subjects with the same experimental paradigm. The sea otter showed good sensitivity with both tactile structures, but better paw sensitivity (Weber fraction, c=0.14) than vibrissal sensitivity (c=0.24). The sea otter's sensitivity was similar in air and under water for paw (cair=0.12, cwater=0.15) and for vibrissae (cair=0.24, cwater=0.25). Relative to the human subjects we tested, the sea otter achieved similar sensitivity when using her paw and responded approximately 30-fold faster regardless of difficulty level. Relative to non-human mammalian tactile specialists, the sea otter achieved similar or better sensitivity when using either her paw or vibrissae and responded 1.5- to 15-fold faster near threshold. Our findings suggest that sea otters have sensitive, rapid tactile processing capabilities. This functional test of anatomy-based hypotheses provides a mechanistic framework to interpret adaptations and behavioral strategies used by predators to detect and capture cryptic prey in aquatic habitats.

Sudden collapse of a mesopredator reveals its complementary role in mediating rocky reef regime shifts

While changes in the abundance of keystone predators can have cascading effects resulting in regime shifts, the role of mesopredators in these processes remains underexplored. We conducted annual surveys of rocky reef communities that varied in the recovery of a keystone predator (sea otter, Enhydra lutris) and the mass mortality of a mesopredator (sunflower sea star, Pycnopodia helianthoides) due to an infectious wasting disease. By fitting a population model to empirical data, we show that sea otters had the greatest impact on the mortality of large sea urchins, but that Pycnopodia decline corresponded to a 311% increase in medium urchins and a 30% decline in kelp densities. Our results reveal that predator complementarity in size-selective prey consumption strengthens top-down control on urchins, affecting the resilience of alternative reef states by reinforcing the resilience of kelp forests and eroding the resilience of urchin barrens. We reveal previously underappreciated species interactions within a 'classic' trophic cascade and regime shift, highlighting the critical role of middle-level predators in mediating rocky reef state transitions.

Increased olfactory search costs change foraging behaviour in an alien mustelid: a precursor to prey switching?

If generalist predators are to hunt efficiently, they must track the changing costs and benefits of multiple prey types. Decisions to switch from hunting preferred prey to alternate prey have been assumed to be driven by decreasing availability of preferred prey, with less regard for accessibility of alternate prey. Olfactory cues from prey provide information about prey availability and its location, and are exploited by many predators to reduce search costs. We show that stoats Mustela erminea, an alien olfactory predator in New Zealand, are sensitive to the search costs of hunting both their preferred rodent prey (mice) and a less desirable alternate prey (locust). We manipulated search costs for stoats using a novel form of olfactory camouflage of both prey, and found that stoats altered their foraging strategy depending on whether mice were camouflaged or conspicuous, but only when locusts were also camouflaged. Stoats gave up foraging four times more often when both prey were camouflaged, compared to when mice were conspicuous and locusts camouflaged. There were no differences in the foraging strategies used to hunt camouflaged or conspicuous mice when locusts were easy to find. Consequently, camouflaged mice survived longer than conspicuous mice when locusts were hard to find, but not when locusts were easy to find. Our results demonstrate that predators can integrate search costs from multiple prey types when making foraging decisions. Manipulating olfactory search costs to alter foraging strategies offers new methods for understanding the factors that foreshadow prey switching.

Evaluating potential conservation conflicts between two listed species: sea otters and black abalone

Population consequences of endangered species interacting as predators and prey have been considered theoretically and legally, but rarely investigated in the field. We examined relationships between spatially variable populations of a predator, the California sea otter, Enhydra lutris nereis, and a prey species, the black abalone, Haliotis cracherodii. Both species are federally listed under the Endangered Species Act and co-occur along the coast of California. We compared the local abundance and habitat distribution of black abalone at 12 sites with varying densities of sea otters. All of the populations of abalone we examined were in the geographic area currently unaffected by withering disease, which has decimated populations south of the study area. Surprisingly, our findings indicate that sea otter density is positively associated with increased black abalone density. The presence of sea otters also correlated with a shift in black abalone to habitat conferring greater refuge, which could decrease illegal human harvest. These results highlight the need for a multi-species approach to conservation management of the two species, and demonstrate the importance of using field-collected data rather than simple trophic assumptions to understand relationships between jointly vulnerable predator and prey populations.

Divergent Skull Morphology Supports Two Trophic Specializations in Otters (Lutrinae)

Variation in terrestrial mammalian skull morphology is known to constrain feeding performance, which in turn influences dietary habits and ultimately fitness. Among mustelids, otters have evolved two feeding specializations: underwater raptorial capture of prey (mouth-oriented) and capture of prey by hand (hand-oriented), both of which have likely associations with morphology and bite performance. However, feeding biomechanics and performance data for otters are sparse. The first goal of this study was to investigate the relationships between feeding morphology and bite performance among two mouth-oriented piscivores (Pteronura brasiliensis and Lontra canadensis) and two hand-oriented invertebrate specialists (Enhydra lutris and Aonyx cinerea). Since other vertebrate taxa that are mouth-oriented piscivores tend to possess longer skulls and mandibles, with jaws designed for increased velocity at the expense of biting capability, we hypothesized that mouth-oriented otters would also possess long, narrow skulls indicative of high velocity jaws. Conversely, hand-oriented otters were expected to possess short, blunt skulls with adaptations to increase bite force and crushing capability. Concomitant with these skull shapes we hypothesized that sea otters would possess a greater mandibular bluntness index, providing for a greater mechanical advantage compared to other otter species investigated. A second goal was to examine morphological variation at a finer scale by assessing variation in cranial morphology among three sea otter subspecies. Since diet varies among these subspecies, and their populations are isolated, we hypothesized that the magnitude of mandibular bluntness and concomitant mechanical advantage, as well as occlusal surface area would also vary within species according to their primary food source (fish versus hard invertebrates). Functional expectations were met for comparisons among and within species. Among species the phylogeny suggests a deeply rooted transition to alternative foraging types. Yet within foraging types alternative species were also strongly variable, suggesting either selective differences in the extent or nature of realized foraging mode, or an accumulation of non-adaptive changes during the long independent evolutionary history. At the finest scale, variation among subspecies indicates that trophic adaptation occurred rapidly, making it interesting that we happened to find both deeply and shallowly-rooted transformations associated with diet type in otter species and subspecies.

The interaction of intraspecific competition and habitat on individual diet specialization: a near range-wide examination of sea otters

The quantification of individuality is a common research theme in the fields of population, community, and evolutionary ecology. The potential for individuality to arise is likely context-dependent, and the influence of habitat characteristics on its prevalence has received less attention than intraspecific competition. We examined individual diet specialization in 16 sea otter (Enhydra lutris) populations from southern California to the Aleutian Islands in Alaska. Because population histories, relative densities, and habitat characteristics vary widely among sites, we could examine the effects of intraspecific competition and habitat on the prevalence of individual diet specialization. Using observed diet data, we classified half of our sites as rocky substrate habitats and the other half containing a mixture of rocky and unconsolidated (soft) sediment substrates. We used stable isotope data to quantify population- and individual-level diet variation. Among rocky substrate sites, the slope [±standard error (SE)] of the positive significant relationship between the within-individual component (WIC) and total isotopic niche width (TINW) was shallow (0.23 ± 0.07) and negatively correlated with sea otter density. In contrast, the slope of the positive WIC/TINW relationship for populations inhabiting mixed substrate habitats was much higher (0.53 ± 0.14), suggesting a low degree of individuality, irrespective of intraspecific competition. Our results show that the potential for individuality to occur as a result of increasing intraspecific competition is context-dependent and that habitat characteristics, which ultimately influence prey diversity, relative abundance, and the range of skillsets required for efficient prey procurement, are important in determining when and where individual diet specialization occurs in nature.

Testing the nutritional-limitation, predator-avoidance, and storm-avoidance hypotheses for restricted sea otter habitat use in the Aleutian Islands, Alaska

Sea otters (Enhydra lutris) inhabiting the Aleutian Islands have stabilized at low abundance levels following a decline and currently exhibit restricted habitat-utilization patterns. Possible explanations for restricted habitat use by sea otters can be classified into two fundamentally different processes, bottom-up and top-down forcing. Bottom-up hypotheses argue that changes in the availability or nutritional quality of prey resources have led to the selective use of habitats that support the highest quality prey. In contrast, top-down hypotheses argue that increases in predation pressure from killer whales have led to the selective use of habitats that provide the most effective refuge from killer whale predation. A third hypothesis suggests that current restricted habitat use is based on a need for protection from storms. We tested all three hypotheses for restricted habitat use by comparing currently used and historically used sea otter foraging locations for: (1) prey availability and quality, (2) structural habitat complexity, and (3) exposure to prevailing storms. Our findings suggest that current use is based on physical habitat complexity and not on prey availability, prey quality, or protection from storms, providing further evidence for killer whale predation as a cause for restricted sea otter habitat use in the Aleutian Islands.

The effects of sublethal levels of 2,4-dichlorophenoxyacetic acid herbicide (2,4-D) on feeding behaviors of the crayfish O. rusticus

The widespread use of herbicides across the globe has increased the probability of synthetic chemicals entering freshwater habitats. On entering aquatic habitats, these chemicals target and disrupt both physiological and behavioral functioning in various aquatic organisms. Herbicides, such as 2,4-dichlorophenoxyacetic acid (2,4-D), can have negative impacts on chemoreception because these receptor cells are in direct contact with water-soluble chemicals in the environment. Studies focusing on lethal concentration (LC50) levels may understate the impact of herbicides within aquatic habitats because damage to the chemoreceptors can result in modified behaviors or lack of appropriate responses to environmental or social cues. The purpose of this experiment was to determine whether exposure to sublethal levels of 2,4-D alters the foraging behaviors of crayfish Orconectes rusticus. We hypothesized that crayfish exposed to greater concentrations of 2,4-D would be less successful in locating food or on locating food would consume smaller amounts possibly due to an inability to recognize the food odors in the contaminated waters. Crayfish were exposed to three sublethal levels of 2,4-D for 96 h and placed into a Y-maze system with a fish gelatin food source placed randomly in the right or left arm. Average walking speed, average time spent in the correct arm, and percent consumption were analyzed. Our data show that crayfish were impaired in their ability to forage effectively. These inabilities to locate and consume adequate amounts of food could result in lower body weights and decreased fitness in populations of crayfish exposed to 2,4-D in natural habitats.

Sea otters homogenize mussel beds and reduce habitat provisioning in a rocky intertidal ecosystem

Sea otters (Enhydra lutris) are keystone predators that consume a variety of benthic invertebrates, including the intertidal mussel, Mytilus californianus. By virtue of their competitive dominance, large size, and longevity, M. californianus are ecosystem engineers that form structurally complex beds that provide habitat for diverse invertebrate communities. We investigated whether otters affect mussel bed characteristics (i.e. mussel length distributions, mussel bed depth, and biomass) and associated community structure (i.e. biomass, alpha and beta diversity) by comparing four regions that varied in their histories of sea otter occupancy on the west coast of British Columbia and northern Washington. Mussel bed depth and average mussel lengths were 1.5 times lower in regions occupied by otters for >20 years than those occupied for <5 yrs. Diversity of mussel bed associated communities did not differ between regions; however, the total biomass of species associated with mussel beds was more than three-times higher where sea otters were absent. We examined alternative explanations for differences in mussel bed community structure, including among-region variation in oceanographic conditions and abundance of the predatory sea star Pisaster ochraceus. We cannot discount multiple drivers shaping mussel beds, but our findings indicate the sea otters are an important one. We conclude that, similar to their effects on subtidal benthic invertebrates, sea otters reduce the size distributions of intertidal mussels and, thereby, habitat available to support associated communities. Our study indicates that by reducing populations of habitat-providing intertidal mussels, sea otters may have substantial indirect effects on associated communities.

Structure and mechanism of diet specialisation: testing models of individual variation in resource use with sea otters

Studies of consumer-resource interactions suggest that individual diet specialisation is empirically widespread and theoretically important to the organisation and dynamics of populations and communities. We used weighted networks to analyze the resource use by sea otters, testing three alternative models for how individual diet specialisation may arise. As expected, individual specialisation was absent when otter density was low, but increased at high-otter density. A high-density emergence of nested resource-use networks was consistent with the model assuming individuals share preference ranks. However, a density-dependent emergence of a non-nested modular network for 'core' resources was more consistent with the 'competitive refuge' model. Individuals from different diet modules showed predictable variation in rank-order prey preferences and handling times of core resources, further supporting the competitive refuge model. Our findings support a hierarchical organisation of diet specialisation and suggest individual use of core and marginal resources may be driven by different selective pressures.

Using stable isotopes to investigate individual diet specialization in California sea otters (Enhydra lutris nereis)

Differences in diet composition among conspecifics (dietary specialization) have been documented across a broad range of taxonomic groups and habitats, and such variation at the individual level is increasingly recognized as an important component of diversity in trophic interactions. Accurate identification of individual dietary specialization, however, requires longitudinal dietary records that are labor-intensive and cost-prohibitive to obtain for many species. Here we explore the use of stable isotopes (delta13C and delta15N) as a promising technique for detecting and quantifying patterns of individual dietary specialization. Southern sea otters (Enhydra lutris nereis) offer a unique opportunity for testing this approach because (1) they consume a wide variety of prey that span multiple trophic levels, habitats, and ecologically defined functional groups; and (2) individual diet specialization can be validated with existing observational data. We analyzed the isotopic composition of sea otter vibrissae (n = 31) in order to characterize inter- and intra-individual variation in sea otter diets at Monterey Bay, California, USA. At the population level, sea otters showed substantial variation in both delta13C and delta15N values, occupying nearly all of the "isotopic space" created by the diversity of isotopic signatures of potential prey taxa. Most of the variation in sea otter vibrissae was accounted for by differences between individuals, with much less contributed by within-individual variation. A majority of sea otters (approximately 80%) showed relatively little temporal variability in isotopic composition, suggesting that the proportional composition of most individuals' diets is relatively constant over time; a few individuals (approximately 20%) exhibited a high degree of intra-vibrissa isotopic variability, suggesting seasonal shifts in diet composition. These results and our interpretation of them were supported by long-term observational data on the diets of radio-tagged sea otters from the same population (n = 23). Our results demonstrate that stable isotopes can provide an efficient tool for measuring individual- and population-level dietary breadth and may be useful for studying populations where longitudinal data on individuals would otherwise be impossible to acquire. This will be critical for examining the causes and consequences of dietary variation within and among consumer populations, thereby improving our understanding of these important ecological and evolutionary processes at the community level.

Prey choice and habitat use drive sea otter pathogen exposure in a resource-limited coastal system

The processes promoting disease in wild animal populations are highly complex, yet identifying these processes is critically important for conservation when disease is limiting a population. By combining field studies with epidemiologic tools, we evaluated the relationship between key factors impeding southern sea otter (Enhydra lutris nereis) population growth: disease and resource limitation. This threatened population has struggled to recover despite protection, so we followed radio-tagged sea otters and evaluated infection with 2 disease-causing protozoal pathogens, Toxoplasma gondii and Sarcocystis neurona, to reveal risks that increased the likelihood of pathogen exposure. We identified patterns of pathogen infection that are linked to individual animal behavior, prey choice, and habitat use. We detected a high-risk spatial cluster of S. neurona infections in otters with home ranges in southern Monterey Bay and a coastal segment near San Simeon and Cambria where otters had high levels of infection with T. gondii. We found that otters feeding on abalone, which is the preferred prey in a resource-abundant marine ecosystem, had a very low risk of infection with either pathogen, whereas otters consuming small marine snails were more likely to be infected with T. gondii. Individual dietary specialization in sea otters is an adaptive mechanism for coping with limited food resources along central coastal California. High levels of infection with protozoal pathogens may be an adverse consequence of dietary specialization in this threatened species, with both depleted resources and disease working synergistically to limit recovery.

Food limitation leads to behavioral diversification and dietary specialization in sea otters

Dietary diversity often varies inversely with prey resource abundance. This pattern, although typically measured at the population level, is usually assumed to also characterize the behavior of individual animals within the population. However, the pattern might also be produced by changes in the degree of variation among individuals. Here we report on dietary and associated behavioral changes that occurred with the experimental translocation of sea otters from a food-poor to a food-rich environment. Although the diets of all individuals were broadly similar in the food-rich environment, a behaviorally based dietary polymorphism existed in the food-poor environment. Higher dietary diversity under low resource abundance was largely driven by greater variation among individuals. We further show that the dietary polymorphism in the food-poor environment included a broad suite of correlated behavioral variables and that the individuals that comprised specific behavioral clusters benefited from improved foraging efficiency on their individually preferred prey. Our findings add to the growing list of examples of extreme individuality in behavior and prey choice within populations and suggest that this phenomenon can emerge as a behavioral manifestation of increased population density. Individuality in foraging behavior adds complexity to both the fitness consequences of prey selection and food web dynamics, and it may figure prominently as a diversifying process over evolutionary timescales.

The effect of foraging parameters on the probability that a dive is successful

In this paper, we investigate the foraging decisions of an animal that dives to obtain its food. It might seem reasonable to use the probability that the diver is successful in any dive as an indicator of habitat quality. We use a dynamic model of optimal prey choice to show that this interpretation of diving success is not generally valid. In particular, we show that diving success is not directly proportional to the overall rate of gain that can be achieved in an environment. Furthermore, some environmental factors can have a non-monotonic effect on the probability of success. For example, as the travel time to the foraging area increases, the probability of success may first increase and then decrease. We point out that the same conclusions are likely to apply in the context of mate choice, i.e. the probability of getting a mate may not be an indicator of the quality of the environment in terms of reproductive success.

Prey selection by harbour seals, Phoca vitulina, in relation to variations in prey abundance

This study compared the composition of the diet of harbour seals (Phoca vitulina) feeding in the Moray Firth, Scotland, with the abundance of their fish prey estimated from dedicated fishery surveys in January 1992 and 1994 and June 1992. Prey-size selection was also examined in these three time periods and in January 1991. In each period, the most abundant fish species contributed most to the diet. However, the relative abundances of the remaining species in the sea showed little similarity to their contribution to the seals' diet. Diet composition was almost totally dominated by either pelagic species or species dwelling on or strongly associated with the seabed, depending upon the relative abundance of pelagic schooling prey. Most fish consumed were 10–16 cm in length, although larger cod and herring were taken. With the exception of cod, the extent of size selection was dependent upon the use of correction factors that accounted for otolith erosion due to digestion.

Seasonal and between-year variations in the diet of harbour seals in the Moray Firth, Scotland

Predictions concerning the effect of seals upon prey stocks require an understanding of temporal variations in diet composition. This study examined the extent of between-year and seasonal variations in the diet of harbour seals (Phoca vitulina) from the Moray Firth, Scotland, between 1989 and 1992. Analyses of fish otoliths and cephalopod beaks collected from 1129 faecal samples were used to derive estimates of the contribution made by each of 35 prey species, based on the number and mass consumed. The key prey, by mass, were sand eels (Ammodytidae) (47%), lesser octopus (Eledone cirrhosa) (27%), whiting (Merlangius merlangus) (6%), flounder (Platichthys flesus) (5%), and cod (Gadus morhua) (4%). Between-year and seasonal fluctuations in the contributions of these species were observed. Sand eels contributed 86–20% in summer and 91–49% in winter. Lesser octopus contributed 0–62% in summer and < 5% in winter, whilst whiting and cod contributed 2–34% in winter and 1–4% in summer. In contrast to 1988, clupeids were unimportant in winter. Differences in diet composition appeared to reflect local changes in the availability of food, especially overwintering clupeids. Results indicate that dietary information obtained from short-term studies can be a poor indicator of subsequent diet composition and should be treated with caution when used to predict the effect of seals on prey populations.

Foraging patterns of California sea otters as indicated by telemetry

Foraging behavior was studied in 38 sea otters (Enhydra lutris) implanted with radio transmitters. The observed foraging behavior of instrumented individuals was similar to that of uninstrumented otters observed in previous studies: dive duration varied with prey type but not with prey size, dive success was highest for small prey, and the length of surface intervals increased with prey size. However, telemetry revealed that some otters foraged farther offshore and made longer dives than was indicated by visual observations. Individuals within age–sex classes varied in several aspects of foraging behavior, including the duration of dives and length of surface intervals. There were no overall differences between the dive durations or surface intervals during the day and during the night, though some individuals had longer dives or surface intervals during either the day or the night. There were differences in the foraging behavior of the various age–sex classes, the most striking being those between juvenile males and females. Juvenile males foraged much farther offshore [Formula: see text] in deeper water [Formula: see text] than other otters and made long dives during both the day [Formula: see text] and the night [Formula: see text]. Juvenile females fed for longer periods than other otters.