Walleye Pollock (Theragra Chalcogramma) Distribution In The Eastern Bering Sea Related To Fishery And Environmental Factors

Abstract

Walleye pollock (Theragra chalcogramma) in the eastern Bering Sea (EBS) support the largest single-species fishery in the world. Pollock also play an important role in the EBS ecosystem as an important prey species. The decline of the western population of Steller sea lions during the 1980s and 1990s raised concerns about the potential competition between the pollock fishery and the sea lion population. My research focused on pollock distribution related to the fishery and physical environment at different temporal and spatial scales using fisheries acoustic data and observer data in the winter fishing season during 2002-2006. Temperature and wind played important roles in determining the pollock distribution in winter, especially from late February to March. The changes in spatial structure during the fishing season suggested that the fishery probably influenced pollock distribution by removing some portion of the local population and perhaps even smoothing out the aggregated distribution of pollock. At a small scale, pollock schools became smaller and denser. At the meso-scale, the distances between schools increased. At a larger scale, range estimates from variography increased which indicated that the spatial correlation among pollock extended to greater distances after fishing. Fishing behavior was also studied using Levy flight theory and its relation to pollock distribution in the EBS. Fishing behavior was significantly correlated to the fractal dimension of fish which measures the degree of pollock clustering, rather than to pollock spatial concentration or density in the EBS. The observer data were also included to analyze the effect of fish distribution on fishing behavior at the school scale. The results indicated that school density rather than the school size played an important role in fishing behavior. Finally, catch depletion analysis was used to examine the potential local depletion. While frequentist and Bayesian methods confirmed that the fishery caused slight local depletion in some areas in the EBS, the magnitude was less than that before sea lion protection measures were put into place in 1999 to spread out the fishery in space and time.