GROWTH AND EQUILIBRIUM IN SEA OTTER POPULATIONS

Counts through time were compiled for 5 Enhydra lutris populations in the NE Pacific Ocean that were below equilibrium density: Attu Island, SE Alaska, British Columbia, Washington State, and C California. Similar data were obtained from the equilibrium density population at Amchitka Island in 1971 and 1986. Rate of increase for the 4 northern populations was 17-20% year-1. Density- or size-dependent changes in rate of increase could not be demonstrated for any of these populations. The California population has undergone 3 apparent growth phases: the early 1900s to the mid-1970s when it increased about 5% yr-1; mid-1970s to mid-1980s when it declined c5% yr-1; and the mid-1980s to 1988 when it increased c7% yr-1. An exponential growth model accounted for 92-98% of the variation in counts through time in all cases. Population increase at Attu Island was achieved largely by range expansion as opposed to increased density. Range expansion in lineal and areal habitat occurred at 11% and 13% yr-1, respectively. Despite similarities in island size and physical environment, the most conservative estimates of population density at Amchitka Island were >3 × greater than maximum density estimates for Attu Island. Surveys of Amchitka Island from the mid-1930s through the mid-1980s indicate that the population increased to a peak in the 1940s; declined abruptly thereafter; and subsequently increased to a new and higher equilibrium in the 1960s, where it has since remained. Population data, together with information on sea otter foraging and benthic community structure at Attu and Amchitka, suggest that multiple population equilibria exist in this system, emanating from complex trophic interactions low in the food web. The lower population equilibrium may be achieved largely or exclusively on an invertebrate diet consisting principally of herbivorous sea urchins. When unregulated by sea otter predation, the rocky benthos is deforested by sea urchin grazing. As growing otter populations compete increasingly for food, grazing intensity declines and the system shifts to one dominated by kelp beds, in turn leading to increased production, a shift in habitat structure, and population increases of kelp bed fishes. Apparently this new food resource elevates the sea otter population to a higher and more stable equilibrium. -from Author