U.S. Department of Commerce
Date of this Version
1-2009
Abstract
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 species (Fowler and Hobbs 2002, Belgrano and Fowler 2008). This involves implementation of the principles of management (embodied in systemic management, Fowler 2003).
Here we evaluate three harvests: 1) the harvest of harp seal pups in the northwest Atlantic, 2) the proposed harvest of northern fur seal pups on St. George Island, Alaska, and 3) a hypothetical harvest of northern fur seal pups on St. Paul Island comparable to that proposed for St. George (the latter two islands are the two largest in the Pribilof Archipelago in the Bering Sea where the largest portion of the global population of northern fur seals breed). These evaluations are based on comparisons with observed consumption rates by other large mammal predators in their take of juveniles (less than 1 year of age) from large mammal populations. The choice of these sets of comparisons overtly accounts for the a priori knowledge that the predator and prey (humans and fur/harp seals) are both large mammals and that the prey are young-of-the-year. Our analyses of these data exemplify part of the decision-making process in systemic management: the process of directly accounting for factors such as number of predatory species, body size, taxonomic status, and trophic level. More specific comparisons will have to await the results of research to provide information that is missing in current data: other knowledge of these or similar systems. The need for research is defined by the management question being addressed as explained in the discussion section below, with examples. Refining such questions (asking more specific management questions) leads to defining further research.
Comments
Published by U.S. DEPARTMENT OF COMMERCE.