Sea otters (Enhydra lutris) and Steller sea lions (Eumetopias jubatus) in the North Pacific: Evaluating mortality patterns and assessing population status at multiple time scales.

摘要

Population status relative to food resources is a basic yet elusive metric to determine for any natural population. Age-specific mortality patterns are one indicator because of the characteristic changes that occur as a population becomes food limited. In general, density dependent (i.e. bottom-up) forces increase mortality rates, especially in the juvenile and old age classes of long-lived, K-selected species. In contrast, top-down forces can result in age-indiscriminant increases in mortality rates. Other perturbations (e.g. exposure to contaminants) can cause more complex changes to age-specific mortality rates that do not affect a population uniformly, as only individuals that utilize contaminated habitats are deleteriously affected. I developed source-sink models to test for lingering survival effects from the 1989 Exxon Valdez oil spill on the sea otter population in western Prince William Sound, Alaska. I used maximum likelihood methods to find the most likely ways survival rates have changed since the spill using age-structure and population census data bases. My results indicate that 35-40% of the western Prince William Sound sea otter population was still suffering negative effects 16 years after the Exxon Valdez oil spill. Young animals were most affected in the first few years after the spill, but by 1993 their survival rates had returned to normal. After a lag of 5-6 years, survival began to drop indicating reduced life-expectancy for the portion of the population utilizing oil contaminated habitats.;Body size and body condition are an alternative indicator of population status because they are influenced by the relative abundance and accessibility of resources in the environment via "phenotypic plasticity." Intuitively, as resources become limited, body condition declines resulting in reduced growth rates and smaller body size at maturity while the opposite occurs as the productivity of the environment increases or non-density dependent population declines reduce competition among survivors. I examined sea otter and Steller sea lion morphometric data sets for evidence of both immediate (time of sampling) and historic resource availability. To accomplish this, I fit a standard growth function to cross-sectional size data, but modified the growth function to allow detection of trends in average size at age over time. This approach demonstrated that (1) structural size trends do not track population size trends, but the structural size record can inform mechanisms behind population change (i.e. bottom-up vs. top-down), and (2) when building growth curves from cross-sectional data, decreasing trends in structural size typical of populations reaching K could easily be misinterpreted as "indeterminate growth." For sea otters, I also found a body condition index that was sensitive to environmental conditions and patterns of age- and sex-specific body condition that reflect population status at the time of sampling.;Growth curves and body condition indices gave a more complete picture of population status than either of these metrics alone, and they proved useful in illuminating the mechanism behind the Gulf of Alaska Steller sea lion population decline. Specifically, the decline was likely initiated by top-down mechanisms sometime during or before the 1950s resulting in an increasing trend in structural size during the 1960s and early 1970s. However, beginning in 1978 and into the early 1980s reduced juvenile growth rates were evidence of resource limitation affecting sea lions, likely due to regime shift or El Nino events. However, this signature appeared short lived and was evident in both the eastern Gulf of Alaska and SE Alaska (i.e. increasing population), but not in populations further west (i.e. declining population) suggesting bottom-up forces alone could not explain the population decline. I concluded that top-down forcing was likely the major contributor to the Steller sea lion population declines. However, I could not rule out bottom-up and top-down forces working synergistically through increased predation risk resulting from resource limitation and/or trait-mediated effects (i.e. predator avoidance strategies) that decreased sea lion habitat use or feeding efficiency.