Sources of variation in the foraging behavior and demography of the sea otter, Enhydra lutris

I investigated demography and foraging behavior of the southern sea otter (Enyhdra lutris nereis), in order to examine the individual and population-level consequences of alternative foraging strategies, and to evaluate the importance of food-limitation as a factor limiting population growth. I developed a maximum likelihood-based analytical method to estimate historical age/sex-specific vital rates, and spatial/temporal variation in vital rates, using carcass age-structure and population census databases. To estimate current demographic trends I conducted a mark-recapture study, measuring survival and reproduction of 115 radio-tagged study animals between 2001 and 2004. Together, these analyses indicate that survival has decreased substantially between the early 1990s and the present, and is lowest in the north-central portion of the sea otter's range. The greatest decrease was for adult (>4 years of age) females: variation in survival of this age/sex class is primarily responsible for regulating population growth and driving population trends.; I also collected foraging data from 60 marked sea otters, and found pronounced individual dietary specialization (greater than that reported previously). Cluster analysis indicated that individual diets could be grouped into three general "diet types"; differences in foraging behavior between these diet types suggest that they represent distinct foraging strategies. The rate of energy gain while foraging was low for the population as a whole, but showed a high degree of variation. Foraging strategies differed with respect to mean energy gain; however, because the mean and within-animal variation in rate of energy gain were positively correlated, all three strategies result in similar probabilities of exceeding a critical rate of energy gain on any given day, and there is likely a trade-off between the mean and variance in the rate of energy gain. Correlational selection is also important in maintaining multiple foraging strategies within the population: a multivariate fitness surface fit to foraging behavior indicates multiple fitness peaks corresponding to alternative foraging strategies. I suggest that food-limitation is likely an important ultimate factor restricting population growth in the center of the sea otters' range in California, but that the existence of alternative foraging strategies may obscure expected patterns of density dependence.