Prey consumption of Steller sea lions (Eumetopias jubatus) off Alaska: How much prey do they require?

The effects of seasonal and regional differences in diet composition on the food requirements of Steller sea lions (Eumetopias jubatus) were estimated by using a bioenergetic model. The model considered differences in the energy density of the prey, and differences in digestive efficiency and the heat increment of feeding of different diets. The model predicted that Steller sea lions in southeast Alaska required 45–60% more food per day in early spring (March) than after the breeding season in late summer (August) because of seasonal changes in the energy density of the diets (along with seasonal changes in energy requirements). The southeast Alaska population, at 23,000 (±1660 SD) animals (all ages), consumed an estimated 140,000 (±27,800) t of prey in 1998. In contrast, we estimated that the 51,000 (±3680) animals making up the western Alaska population in the Gulf of Alaska and Aleutian Islands consumed just over twice this amount (303,000 [±57,500] t). In terms of biomass removed in 1998 from Alaskan waters, we estimated that Steller sea lions accounted for about 5% of the natural mortality of gadids (pollock and cod) and up to 75% of the natural mortality of hexagrammids (adult Atka mackerel). These two groups of species were consumed in higher amounts than any other. The predicted average daily food requirement per individual ranged from 16 (±2.8) to 20 (±3.6) kg (all ages combined). Per capita food requirements differed by as much as 24% between regions of Alaska depending on the relative amounts of low–energy-density prey (e.g. gadids) versus high–energy-density prey (e.g. forage fish and salmon) consumed. Estimated requirements were highest in regions where Steller sea lions consumed higher proportions of low–energy-density prey and experienced the highest rates of population declin

Diets of Steller sea lions (Eumetopias jubatus) in Southeast Alaska, 1993−1999

The diet of Steller sea lions (Eumetopias jubatus) was determined from 1494 scats (feces) collected at breeding (rookeries) and nonbreeding (haulout) sites in Southeast Alaska from 1993 to 1999. The most common prey of 61 species identified were walleye pollock (Theragra chalcogramma), Pacific herring (Clupea pallasii), Pacific sand lance (Ammodytes hexapterus), Pacific salmon (Salmonidae), arrowtooth flounder (Atheresthes stomias), rockfish (Sebastes spp.), skates (Rajidae), and cephalopods (squid and octopus). Steller sea lion diets at the three Southeast Alaska rookeries differed significantly from one another. The sea lions consumed the most diverse range of prey categories during summer, and the least diverse during fall. Diet was more diverse in Southeast Alaska during the 1990s than in any other region of Alaska (Gulf of Alaska and Aleutian Islands). Dietary differences between increasing and declining populations of Steller sea lions in Alaska correlate with rates of population change, and add credence to the view that diet may have played a role in the decline of sea lions in the Gulf of Alaska and Aleutian Islands

Evaluating strategies for managing anthropogenic mortality on marine mammals : an R implementation with the package RLA

Funding: ADERA provided support for salaries (MA).Bycatch, the undesirable and non-intentional catch of non-target species in marine fisheries, is one of the main causes of mortality of marine mammals worldwide. When quantitative conservation objectives and management goals are clearly defined, computer-based procedures can be used to explore likely population dynamics under different management scenarios and estimate the levels of anthropogenic removals, including bycatch, that marine mammal populations may withstand. Two control rules for setting removal limits are the Potential Biological Removal (PBR) established under the US Marine Mammal Protection Act and the Removals Limit Algorithm (RLA) inspired from the Catch Limit Algorithm (CLA) developed under the Revised Management Procedure of the International Whaling Commission. The PBR and RLA control rules were tested in a Management Strategy Evaluation (MSE) framework. A key feature of PBR and RLA is to ensure conservation objectives are met in the face of the multiple uncertainties or biases that plague real-world data on marine mammals. We built a package named RLA in the R software to carry out MSE of control rules to set removal limits in marine mammal conservation. The package functionalities are illustrated by two case studies carried out under the auspices of the Oslo and Paris convention (OSPAR) (the Convention for the Protection of the Marine Environment of the North-East Atlantic) Marine Mammal Expert Group (OMMEG) in the context of the EU Marine Strategy Framework Directive. The first case study sought to tune the PBR control rule to the conservation objective of restoring, with a probability of 0.8, a cetacean population to 80% of carrying capacity after 100 years. The second case study sought to further develop a RLA to set removals limit on harbor porpoises in the North Sea with the same conservation objective as in the first case study. Estimation of the removals limit under the RLA control rule was carried out within the Bayesian paradigm. Outputs from the functions implemented in the package RLA allows the assessment of user-defined performance metrics, such as time to reach a given fraction of carrying capacity under a given level of removals compared to the time needed given no removals.Publisher PDFPeer reviewe

The political biogeography of migratory marine predators

During their migrations, marine predators experience varying levels of protection and face many threats as they travel through multiple countries' jurisdictions and across ocean basins. Some populations are declining rapidly. Contributing to such declines is a failure of some international agreements to ensure effective cooperation by the stakeholders responsible for managing species throughout their ranges, including in the high seas, a global commons. Here we use biologging data from marine predators to provide quantitative measures with great potential to inform local, national and international management efforts in the Pacific Ocean. We synthesized a large tracking data set to show how the movements and migratory phenology of 1,648 individuals representing 14 species-from leatherback turtles to white sharks-relate to the geopolitical boundaries of the Pacific Ocean throughout species' annual cycles. Cumulatively, these species visited 86% of Pacific Ocean countries and some spent three-quarters of their annual cycles in the high seas. With our results, we offer answers to questions posed when designing international strategies for managing migratory species

Growth and bioenergetic models for stellar sea lions (eumetopias jubatus) in Alaska

The primary goal of my study was to develop a bioenergetic model to predict the food requirements of Steller sea lions (Eumetopias jubatus). An important component of the bioenergetic model was a physical growth model. Growth models were constructed using morphometric measurements of males (> 1 year old), females (> 1 year old), and pregnant females with a foetus that had been shot on rookeries, haulouts, and in the coastal waters of southeastern Alaska, the Gulf of Alaska and along the Bering Sea ice edge between 1976 and 1989. A Richards model best described age related growth in body length and mass. Males grew (in length) over a longer period than females and exhibited a growth spurt in mass which coincided with sexual maturity. Sexual dimorphism in both body length and mass was significant by 3 years of age. The average predicted standard lengths of males and females older than 12 years were 3.04 m and 2.32 m respectively, while the average predicted weights were 681 kg and 273 kg respectively. Residuals of the size at age models indicated seasonal changes in growth rates. Young animals (<6 years old) and adult males grew little during the breeding season (May - July), and adult males did not resume growth until sometime after November. The bioenergetic model was used to estimate the food requirements of the Alaskan Steller sea lion population in the 1990's and to examine how these food requirements varied seasonally and spatially. Input included age/sex-specific energy requirements, population size/composition, and diet composition/energy content by date and region of Alaska. Error in model predictions was calculated using uncertainty in parameter values and Monte Carlo simulation methods. Food requirements were generally lowest in the summer and highest in the winter and spring mainly due to changes in activity budgets and the energy content of the diet. The mean daily food requirement of pregnant females was only marginally greater than the mean daily food requirement of non-pregnant females of the same age, but the mean daily food requirement of females nursing pups was about 70% greater than females of the same age without pups. Per capita population food requirements differed by up to 12% among regions of Alaska due to differences in the energy content of the diet. Steller sea lion predation was small relative to total walleye pollock natural mortality, but accounted for a large part of total Atka mackerel natural mortality. Of the bioenergetic, population, and diet parameters, uncertainty in bioenergetic parameters resulted in the largest error in model predictions. The model provided both a quantitative estimate of the Alaskan Steller sea lion population's food requirements and direction for future research.Science, Faculty ofZoology, Department ofGraduat

Food consumption by sea lions: Existing data and techniques

Knowing the quantity of prey that sea lions consume is a prerequisite for assessing the role of sea lions in aquatic ecosystems and the potential for competition to occur with fisheries. We reviewed the different approaches that have been used to estimate the food requirements for the six species of sea lions. We reviewed data on the quantity of food consumed by sea lions in captivity, and examined how consumption varied by species, body size, and season. We also reviewed and quantified available information on the energetics of sea lions and assessed the potential application of these data to parameterize an existing bioenergetic model that was developed to estimate the food requirements of Steller sea lions. Our study provided ranges of estimates of food consumption for sea lions that can be used in various modeling strategies to assess the impact of sea lions on prey populations, including commercially exploited fish species. The approaches reviewed in our study shared common difficulties arising from the quantity and quality of data, and the integration of data across scales and species. Our modeling exercise, in particular, identified the major uncertainties involved in estimating the food requirements of each sea lion species using an energetics approach. Our results provide direction for future research aimed at improving the accuracy and comparability of estimates of food consumption for sea lions.</p

Food consumption by sea lions: Existing data and techniques

Knowing the quantity of prey that sea lions consume is a prerequisite for assessing the role of sea lions in aquatic ecosystems and the potential for competition to occur with fisheries. We reviewed the different approaches that have been used to estimate the food requirements for the six species of sea lions. We reviewed data on the quantity of food consumed by sea lions in captivity, and examined how consumption varied by species, body size, and season. We also reviewed and quantified available information on the energetics of sea lions and assessed the potential application of these data to parameterize an existing bioenergetic model that was developed to estimate the food requirements of Steller sea lions. Our study provided ranges of estimates of food consumption for sea lions that can be used in various modeling strategies to assess the impact of sea lions on prey populations, including commercially exploited fish species. The approaches reviewed in our study shared common difficulties arising from the quantity and quality of data, and the integration of data across scales and species. Our modeling exercise, in particular, identified the major uncertainties involved in estimating the food requirements of each sea lion species using an energetics approach. Our results provide direction for future research aimed at improving the accuracy and comparability of estimates of food consumption for sea lions.</p

Quantifying errors associated with using prey skeletal structures from fecal samples to determine the diet of Steller's sea lion (<em>Eumetopias jubatus</em>)

We examined the digestion and passage times of bones and other hard parts from pollock, herring, salmon, and sandlance recovered from two juvenile captive Steller's sea lions (Eumetopias jubatus) subjected to varying activity levels. Key bones that could be identified to species were distributed over an average of 3.2 scats (range 1-6) following a single meal, with pollock remains Occurring in significantly more scats than other species. Relying on otoliths alone to determine the presence of prey resulted in significantly fewer prey being identified than if other structures were also used (such as vertebrae, jaw bones, and teeth), particularly for salmon. Using either technique, there were significant differences in the likelihood that bones would be recovered from the series of scats produced following a meal, with pollock recovery exceeding herring (by three-fold) and sandlance (by eight-fold). Differences between species were reduced when recovery was calculated on a per scat basis rather than over multiple scats. Active animals passed greater numbers of bones, but the overall effect on prey recovery estimates was not significant. Defecation times of prey structures from a meal were variable and ranged from an initial 2-56 h to a final 28-148 h. The time interval to pass 95% of recovered structures varied by a factor of two among prey species, and was highest for pollock due to retention beyond 65 h.</p

Quantifying errors associated with using prey skeletal structures from fecal samples to determine the diet of Steller's sea lion (<em>Eumetopias jubatus</em>)

We examined the digestion and passage times of bones and other hard parts from pollock, herring, salmon, and sandlance recovered from two juvenile captive Steller's sea lions (Eumetopias jubatus) subjected to varying activity levels. Key bones that could be identified to species were distributed over an average of 3.2 scats (range 1-6) following a single meal, with pollock remains Occurring in significantly more scats than other species. Relying on otoliths alone to determine the presence of prey resulted in significantly fewer prey being identified than if other structures were also used (such as vertebrae, jaw bones, and teeth), particularly for salmon. Using either technique, there were significant differences in the likelihood that bones would be recovered from the series of scats produced following a meal, with pollock recovery exceeding herring (by three-fold) and sandlance (by eight-fold). Differences between species were reduced when recovery was calculated on a per scat basis rather than over multiple scats. Active animals passed greater numbers of bones, but the overall effect on prey recovery estimates was not significant. Defecation times of prey structures from a meal were variable and ranged from an initial 2-56 h to a final 28-148 h. The time interval to pass 95% of recovered structures varied by a factor of two among prey species, and was highest for pollock due to retention beyond 65 h.</p