The development of diving capacity and behavior in Steller sea lions in Alaska: exploring how behavior and physiology interact in an altricial otariid

A Collaboration with Dr. Lorrie Rea, Alaska Department of Fish and Game

Graduate Students Involved: Julie Richmond, Michael Rehberg

Project #1: High-resolution foraging behavior and movement patterns of Steller sea lion juveniles in regions of population stability and decline

Funded by National Marine Fisheries Service

Project Summary: The ongoing decline in the western population of Steller sea lions (Eumetopias jubatus) has placed marine mammal management in direct conflict with several important commercial fisheries, and has focused attention on identifying habitat and prey resources critical for population survival. Research into the causes of the decline has focused largely on two factors: reduced prey availability (due either to fisheries competition or environmental change) and/or declines in the survivorship of juveniles - the age class most susceptible to changes in the prey landscape due to their lack of foraging experience, smaller size, and reduced dive capacity. Determining how juvenile sea lions select foraging locations and make foraging decisions in light of biological and environmental conditions is critical if we are to understand how changes in fish abundance will impact condition and survival. It is also critical if we are to assess when in the year juveniles are most sensitive to prey availability, where (relative to their haul out sites, rookeries, and designated critical areas) the bulk of juvenile foraging takes place, and whether differences in population trajectories are correlated with variation in individual foraging effort (as would be expected if prey availability differed).  While several management actions have been taken based on assumptions about how sea lions utilize their habitat, to date the data collected has been unsuitable for use in the fine-scale analysis techniques that are most appropriate for addressing the above issues. However, recent advances in tag technology now make it possible to head-mount small, high-resolution satellite-relay data loggers (SRDLs) on juvenile sea lion. These tags (manufactured by Sea Mammal Research Unit) will be custom designed for this project so that they collect and transmit the highly detailed dive data necessary to determine how individual animals manage their time and energy resources when foraging (analyses which require information about the depth, duration, sequence, and shape of individual dives).  In addition, because their smaller size allows them to be head-mounted, we should receive significantly more and higher quality locations from tagged animals, which in turn will allow us to construct habitat use maps with much greater spatial and temporal resolution. The need for more detailed information on how Steller sea lions select and utilize foraging grounds, and the absence of the high-resolution dive data necessary to do this has been highlighted by the recognition that the Western population decline is not uniform, and that counts around in the Dutch Harbor area are increasing.  

Therefore this work will utilize high-resolution SRDLs deployed on juvenile in area of decline and increase in order to collect the data necessary to address the following hypotheses: 1) that Steller sea lion juveniles in both regions select foraging areas with particular environmental features likely to aggregate prey resources; 2) that juvenile sea lions diving in regions with recently declining and increasing counts demonstrate different foraging behaviors and effort; and 3) that differences in foraging behavior and habitat use of Steller sea lion juveniles are correlated with age, nutritional and physiological status. In addressing these hypotheses, this research will enable us to link biological (prey composition, distribution, and abundance) and physical characteristics of the foraging environment (temperature, bathymetry, location) with the foraging behavior and strategies of a free-ranging marine vertebrate predator. These links can be built at a much finer scale than previously due to the use new technology, and so can provide detailed 3-dimensional maps of sea lion habitat use patterns in regions where populations are increasing and decreasing. In particular, we hope to determine whether juveniles in the declining population must work harder (as judged by dive duration, frequency, or pattern) in order to find prey resources. This data would be of the appropriate type and resolution of data with which to test the hypothesis that shifts in predator-prey relationships are contributing to the ongoing decline. Finally, because this research will be carried out in close collaboration with Alaska Department of Fish and Game and the National Marine Fisheries Service, there will be immediate links between data collected and sea lion and fisheries management strategies.

Project #2: The role of physiological constraint in the acquisition of foraging ability: development of diving capacity in juvenile Steller Sea Lions

Funded by: Cooperative Institute for Arctic Research

Project Summary: Knowledge of the mechanisms by which juvenile mammals acquire the tools necessary to become competent predators is crucial to our understanding of how physiological processes influence behavioral strategies. As one possible cause of the decline in the western Alaska population of Steller sea lions (Eumetopias jubatus) is a reduction in juvenile survival, it seems particularly important to understand how physiological status influences juvenile foraging patterns for this species. While previous work in other pinnipeds has clearly demonstrated that the smaller body oxygen stores and higher mass specific metabolic rates of juveniles limits their behavioral repertoire, and under certain conditions can reduce foraging efficiency and restrict prey availability, no similar work has been undertaken on Steller sea lions. This project will specifically examine the diving capacity of juvenile Steller sea lions in order to determine how the maturation of blood and muscle oxygen stores might influence foraging capacity and diving performance. To do so, this work will center around three main hypotheses: first, that while juvenile sea lions are dependent on maternal nutritional support they have lower mass specific oxygen stores than do adults; second, that oxygen stores are of similar size to those of adults (on a mass specific basis) by the time that independent foraging has begun; and third, that reduced diving capacity has the potential to limit prey selection. To address these questions, we will measure the blood and muscle oxygen stores of juveniles, and then work closely with ADFG and NMFS biologists to correlate physiological status with body condition, nutritional status, and recorded diving patterns. By comparing data from animals in the Western stock with those in the more stable Southeastern stock, we hope to determine whether juveniles in the declining population must operate closer to their physiological limit in order to obtain the necessary prey resources, which in turn would provide support for the hypothesis that shifts in predator-prey relationships are contributing to the ongoing decline.


Research Products to date

  1. Rehberg, M.J. and J.M. Burns. (2008). Differences in diving and swimming behavior in pup and juvenile Steller sea lions (Eumetopias jubatus) in Alaska. Canadian Journal of Zoology 86:539-553. PDF

  2. Richmond, J.P, J.M. Burns, L.D. Rea (2006) Ontogeny of total body oxygen stores and aerobic dive potential in the Steller sea lion (Eumetopias jubatus). Journal of Comparative Physiology B. 176: 535-545. PDF

  3. Richmond, J.P., J.M. Burns, L.D. Rea, K. Mashburn. (2005). Postnatal ontogeny of erythropoietin and hematology in free-ranging Steller sea lions (Eumetopias jubatus). General and Comparative Endocrinology. 141:240-247. PDF