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

• Main Author: Winship, Arliss J.
• Format: Thesis
• Language: English
• Published: 2000
• Subjects: Atka; Bering Sea; Gulf of Alaska; Sea ice; Alaska
• Online Access: http://hdl.handle.net/2429/10566

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 of Zoology, Department of Graduate