Response of Pinniped Populations to Directed Harvest, Climate Variability, and Commercial Fishery Activity: A Comparative Analysis

Abstract

Hypotheses concerning the decline of Steller sea lions in the Gulf of Alaska (GOA) were evaluated based on comparison of data to three other ecosystems that had similar environmental and commercial fishery characteristics. We focus on examining the effects of commercial pinniped harvest, commercial fisheries, and environmental changes. Of the four pinniped species included in this study, only the Steller sea lion population has exhibited a sharp decline in population number. Comparative analysis indicated that the Gulf of Alaska pinniped population has not experienced any unique large-scale perturbations compared to the other ecosystems. Commercial pinniped harvest occurred in all four ecosystems. The history of harvest was shortest for GOA Steller sea lions and the numbers taken were lower than in the Barents Sea and Benguela Current ecosystems. Exploitation rates, though only calculated for years when both harvest numbers and total population size were available, also indicated that the Steller sea lions have experienced very little commercial harvest pressure compared to the other pinniped populations in the three other ecosystems. The age group of animals killed was comparable throughout all ecosystems. Large-scale variability in water temperature was common to all four ecosystems. Although the periodicity of the changes varied among ecosystems, they all appeared to be driven by low pressure systems. The variability of the oscillations in water temperature was lowest for the eastern Pacific ocean (i.e., the GOA and California Current ecosystems) and highest in the Benguela Current. Commercial fisheries played a major part in all four ecosystems. The main species in pinniped diets were often the target of commercial fishing activity leading to potential conflicts between the 2 types of predators (i.e., pinnipeds and commercial fisheries). Exploitation rates in the GOA were comparable to or less than rates in the other ecosystems while the rates were highest in the Barents Sea. Statistical analysis showed that GOA pollock exploitation rates were significantly different from the rates of most other species. Healthy pinniped populations were present in all the ecosystems in this study except for the GOA despite the presence of much commercial fishing activity. This suggests the need for more detailed analysis of the possible role of commercial fisheries in the GOA ecosystem and the management actions taken to alleviate its effects.