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

Abstract

The California sea otter (Enhydra lutris) population, after increasing for more than half a century, stabilized and probably declined from the mid-1970's to the mid-1980's. Estes et al. (1986) suggested that the stabilization and decline were not due to food limitation. Garshelis et al. (1990) challenged this suggestion, although in doing so they misrepresented arguments made by Estes et al. (1986), provided no evidence for alternative hypotheses, and offered no constructive recommendations for a better means of population assessment. While acknowledging some of the points made by Garshelis et al. (1990), I believe the collective evidence presented by Estes et al. (1986) provided a reasonable basis for rejecting the food-limitation hypothesis, and point out that recent increases in the California sea otter population following a legislated reduction in net entanglement mortality is strong evidence against the food-limitation hypothesis. J. WILDL. MANAGE. 54(2):270-272 I will preface my response to Garshelis et al. (1990) with a brief history of the sea otter population in California and a summary of the arguments my colleagues and I made concerning the status of that population in the mid-1980's. The California sea otter population was hunted to the brink of extinction and may have contained as few as 50 individuals at the beginning of this century (Ralls et al. 1983). Protection was provided in 1911, and the population subsequently increased. Although the growth characteristics of the California population are poorly known through the 1940's, there were an estimated 500-600 individuals in 1950. Thereafter, the population increased steadily at about 5% per year and in 1976 contained an estimated 1,789 otters (U.S. Fish Wildl. Serv. 1982). Although further increases were expected, the population then ceased growing and probably declined somewhat (Estes 1990). However, the extent to which this lack of growth was due to limiting resources versus disturbances was uncertain. My colleagues and I concluded that further population growth probably was not food limited (Estes et al. 1986) because (1) diurnal activity budgets of the California population were similar to those of other sea otter populations known to be below equilibrium density; (2) estimated entanglement mortality from fishing gear was roughly equivalent to an expected growth increment, given the estimated population size at that time and the earlier observed rate of increase of about 5% per year; (3) the diet of the population was similar, in that piscivory was lacking, to other populations known to be below carrying capacity; and (4) suitable habitat for increased expansion existed at both ends of the population's range. This last point is especially problematic to the resource-limitation hypothesis because several other sea otter populations in the North Pacific Ocean have grown to sizes far exceeding that of the California population, with no measurable decline in growth rate (Estes 1990). Although no single line of reasoning was very strong, we thought the collective evidence was compelling. Garshelis et al. (1990) have disagreed. However, despite their censure and our reply, the argument now is immaterial. In 1985 the State of California limited the set-net fishery in central California, thus reducing the number of sea otters lost to entanglement. The sea otter population has since increased at a rate of about 7% per year, a trend that has persisted through 1989 (R. J. Jameson and J. A. Estes, unpubl. data). I consider this to be nearly definitive evidence that the observed lack of increase in the California sea otter population from the mid1970's to the mid-1980's was not food limited, although conceivably my colleagues and I drew the right conclusion for the wrong reasons. Garshelis et al. (1990) were highly critical of our data and our reasoning, challenging each of our major arguments and even points we did not make. I agree with many of their technical comments, some of which were acknowledged in our original paper (Estes et al. 1986). I even agree with them that our evidence was inconclusive, a point also acknowledged in our original paper. However, I differ with them on 2 main issues. First, although the evidence we presented admittedly was insufficient to permit