Localized depletion of three Alaska rockfish species

Localized depletion of three Alaska rockfish species

D. Hanselman, P. Spencer, K. Shotwell, and R. Reuter

Localized depletion of three Alaska rockfish speciesThis is part of Biology, Assessment, and Management of North Pacific Rockfishes
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The distributions of some rockfish species in Alaska are clustered. Their distribution and relatively sedentary movement patterns could make localized depletion of rockfish an ecological or conservation concern. Alaska rockfish have varying and little-known genetic stock structures. Rockfish fishing seasons are short and intense and usually confined to small areas. If allowable catches are set for large management areas, the genetic, age, and size structures of the population could change if the majority of catch is harvested from small concentrated areas. In this study, we analyzed data collected by the North Pacific Observer Program from 1991 to 2004 to assess localized depletion of Pacific ocean perch (Sebastes alutus), northern rockfish (S. polyspinis), and dusky rockfish (S. variabilis). The data were divided into blocks with areas of approximately 10,000 km2 and 5,000 km2 of consistent, intense fishing. We used two different block sizes to consider the size for which localized depletion could be detected. For each year, the Leslie depletion estimator was used to determine whether catch-per-unit-effort (CPUE) values in each block declined as a function of cumulative catch. We examined trends in CPUE over time using NMFS survey data for depleted areas. The results showed significant depletions among all species in several areas and years. Pacific ocean perch exhibited localized depletion most often, but not continuing into the next year, possibly indicating more migration than expected. Northern and dusky rockfish showed depletions less often, and occasional significant increases also occurred. Northern rockfish showed significant longer-term fishery depletion in one area. Results could be affected by hyperstability, migration, or target switching. The importance of the results depends on genetic stock structure, and importance of an area for reproductive success.

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