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Groundfish Assessment Program

Tracking Red King Crab Seasonal Movements in the Eastern Bering Sea

red king crab
Figure 1.  A red king crab with an attached archival tag that will collect depth and temperature data.  Photo by Jan Haaga.

In December 2009, a total of 135 adult male red king crab (Paralithodes camtschaticus) were tagged with electronic archival tags and released in the eastern Bering Sea for the purpose of tracking their seasonal migrations. The tagging was conducted off the fishing vessel Arctic Fury which was chartered for the 1-week period immediately following the 2009 commercial red king crab fishery.

The archival tags were programmed to collect measurements of depth at 1-minute intervals and temperature at 30-minute intervals. Archival tags were attached to traditional spaghetti tags which were looped though the isthmus muscle of the crab (Fig. 1).

The isthmus tagging procedure on red king crab has been utilized by the Alaska Department of Fish and Game (ADF&G) for more than 30 years to allow for tag retention through the molting stage. Tag returns are expected when the commercial crab fishery begins again in fall/winter of 2010.

This cooperative project is designed to resolve an important question relevant to the management of red king crab. Male red king crab perform seasonal migrations in Bristol Bay that extend from offshore feeding areas occupied in summer and fall to relatively shallow, inshore mating areas occupied in spring. It is currently unknown, however, whether all mature males participate in this migration or just males that will not molt ("skip-molt males").

This distinction is important because a primary strategy for managing the commercial fishery is to adjust the harvest rate to preserve a sufficient number of males to mate all mature females, based on the assumption that all mature males participate in mating each year. We intend to test this assumption by determining the annual migratory trajectories of both individuals that are likely to molt (old shell) and those that are unlikely to molt (new shell).

Migration trajectories will be constructed from daily position estimates which, in turn, will be obtained from depth data collected by the archival tags. Since red king crab always stay in contact with the bottom, the depth data include a record of tidal height fluctuations which spatially vary in amplitude and timing throughout Bristol Bay.

Daily positions will be determined by comparing the records of tag depth to tide heights predicted by a tidal simulation model for elements of a spatial grid in a process known as tidal pattern matching. Migration trajectories will then be constructed from the daily positions using cubic-spline smoothing. The comparison of migratory tracks between old- and new-shell males should indicate whether or not all males participate in the breeding migration.

By Dan Nichol and Dave Somerton

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