Identifying factors that influence sea otter (Enhydra lutris) population
density can provide insight into why it varies spatially and temporally
and when a recovering population has reached an equilibrium density
because of food resources (i.e., carrying capacity K). Although food
availability is widely recognized as an important extrinsic factor
affecting sea otter density, how do we determine when a population has
reached K? The goal of this study was to estimate K for Simpson Bay,
Alaska by measuring the abundance of edible bivalves, the primary prey
for sea otters for over 40 years. We then compared prey abundance and
estimated replacement rate (i.e., the mean age of bivalves predated by
sea otters) to estimated annual prey consumption based on the mean
population density for the past 18 years. On average, 110 adult sea
otters (5.2 km−2) have occupied Simpson Bay annually since 2001
consuming an estimated 176,660 kg of bivalves. The total mass (standing
stock) of the major bivalves (predominately butter clams and stained
macomas) was 785,730 kg, so adult sea otters consumed about 22%
annually. Based on these observations and calculations, the estimated
annual number of sea otters occupying Simpson Bay appears to be at or
near K based on the replacement rate of food resources. However, other
intrinsic (e.g., male territoriality and emigration) and extrinsic
(e.g., predation, disease, human-related mortality) factors may
influence equilibrium density, which varies spatially and temporally,
resulting in a mosaic of subpopulations with different densities, rates
of growth and discontinuous distributions. Understanding the balance
among these factors may be one of the most challenging ecological
questions for sea otter conservation and management as populations
recover from their range-wide decimation during the Maritime Fur Trade
in the late 18th and 19th centuries
Trends and Carrying Capacity of Sea Otters in Southeast Alaska