Indices Used to Assess Status of Sea Otter Populations: A Comment

Garshelis, David; Ames, Jack; Hardy, Robert; Wendell, Frederick

Estes et al. (1986) reported that sea otters (Enhydra lutris) in California fed less than otters in a food-limited population at Amchitka Island, Alaska, and California otters spent similar amounts of time foraging in various parts of their range. They thereby concluded that otters in California were below carrying capacity throughout their present range. We show that these data are a weak basis for this conclusion. Time spent foraging may have been consistent among the study sites in California because none of these areas was recently occupied by otters. Time budget comparisons between California and Alaska are unjustified because the data are derived from visual scan sampling, which is constrained to daylight hours and is variably biased (depending on location) against foraging activity. Comparisons across distant geographic areas are further hampered because time budgets are affected not only by food abundance, but also by weather, body size, and available prey. Other purported indicators of population status also are equivocal: (1) it is common for otters in the food-limited Amchitka population to eat fish, whereas piscivory is rare in Aleutian populations with more abundant invertebrate prey, but it does not necessarily follow that because California otters do not eat fish they must have ample food; (2) although many California otters drown in fishing nets, net mortality cannot explain the lack of population growth near the center of the range; and (3) the claim that range expansion in California has ceased is not only incorrect, but geographic expansion is probably unrelated to resource availability in the center of the range. We contend that the indices used by Estes et al. (1986) were inadequate to assess population status and would be of dubious value in guiding management strategies.

Indices Used to Assess Status of Sea Otter Populations: A Comment

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