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NOAA Technical Memorandum NMFS-AFSC-192

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Harvesting young-of-the-year from large mammal populations: An application of systemic management


We evaluate the current commercial harvest of harp seal (Pagophilus groenlandicus) and proposed subsistence harvests of northern fur seal (Callorhinus ursinus) pups based on intra-specific comparisons. These comparisons utilize a pattern derived from 167 cases of estimated consumption rates by large mammals. In all cases, the predation rates involve large mammal prey less than 1 year of age. Recent harvests of harp seal pups are exceeded by 20 (about 12%) of the estimated consumption rates among the nonhuman species. Although this is not statistically significant, further analysis may find this harvest to be unsustainable when we account for other factors such as the number of other predators, the number of prey each predator consumes, trophic level, the biodiversity of the system, and differences involving terrestrial predators preying on marine prey.

Initially, there is no scientific basis for rejecting a proposed subsistence harvest of 150 northern fur seal pups on St. George Island, Alaska, or a comparable harvest of 1,125 pups on St. Paul Island. The proposed harvests of fur seal pups are exceeded by 162 (97%) of the 167 cases of predation among nonhuman predatory species. In both cases, therefore, the harvests represent consumption rates in the lower extremes of predation rates observed for nonhuman species. Further explicit consideration of relevant factors could lead to a slight reduction in the assurance that the harvest of 150 pups is sustainable, but there is a low likelihood that such a harvest would prove to be unsustainable. This determination accounts for the complexity of the ecosystems involved owing to the integrative nature of the patterns used in the evaluation. The northern fur seal population is declining, raising concerns about the addition of a subsistence harvest to the mortality this species experiences. In management, a declining population reflects all of the contributing factors to include all human influence. Management is necessary to relieve systems of the abnormal effects of our activities whenever human influence is found to be unusual, abnormal, or pathological. Such influence involves the full spectrum of human activities to include the effects of abnormal fisheries harvests, pollution, and CO2 production. As long as harvests of the declining species are not abnormally large, other abnormal human influences are the preferred focus of management.

The comparisons used in our study make the conclusions regarding northern fur seals seem conservative, largely because most examples of predation among nonhuman predators exceed the harvest rates proposed for this species. Overtly accounting for other factors could lead to the conclusion that proposed harvests are less conservative than initially indicated. We show, for example, that directly accounting for the number of competing predators is likely to show the proposed harvest to be exceeded by a smaller portion of the sample among nonhuman predators than when this factor is not explicitly treated. Nevertheless, the mortality rates of the proposed harvest are small enough that it is likely that they fall in the lower end of the spectrum of most subsets of data for consumption rates among other large mammals – subsets that would emerge through explicit consideration of more specific management questions.

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