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Groundfish Assessment Program

Male Snow Crab Migration in the Eastern Bering Sea

figure 1, click to enlarge
Figure 1.  Click image to enlarge

During March 2011, RACE Division scientist Dan Nichol completed another round of snow crab (Chionoecetes opilio) tagging in an ongoing study to track their seasonal migrations in the eastern Bering Sea. The research, a cooperative project involving the RACE Division, the Alaska Department of Fish and Game (ADF&G), the Bering Sea Fisheries Research Foundation (BSFRF), and the Marine Conservation Alliance Foundation, is designed to address the question of whether or not adult males migrate from offshore wintering areas northwest of the Pribilof Islands (where the fishery occurs) to more inshore areas where mature females reside.

Figure 2.  A snow crab tagged with an electronic archival tag, just prior to release.  Photo by Dan Nichol.

A seasonal offshore migration of mature males, which is inferred from the change in their apparent spatial distribution between the NMFS summer bottom trawl survey and the winter commercial fishery, has been corroborated by a mark-recovery tagging program conducted by the ADF&G. Although this movement of mature males into deeper water during winter is firmly established, the timing and other particulars of a return inshore migration of these males have not been demonstrated. Since mature females remain in the shallower areas throughout the year, the specifics of this return migration are important because they are critical to understanding whether males continue to participate in breeding throughout their lives.

The archival tags, which record depth at 1-minute intervals and temperature at 30-minute intervals, are attached to crabs that are then released. When the tags are subsequently recovered, migration pathways of the individual crabs can be reconstructed from the stored depth data. Because the depth sensors are sensitive enough to record the change in tide heights (pressure differences produced from the changing water elevations above the crab), the timing and shape of the tidal signature from a tag can be compared to a tidal simulation model using a technique called "tidal pattern matching" to then locate where an individual crab is on any particular day after being released.

figure 3, click to enlarge
Figure 3.  Click image to enlarge

In April 2010, a total of 120 male snow crabs were fitted with electronic archival tags aboard the fishing vessel Kiska Sea and released. An additional 157 were tagged aboard the fishing vessel Pacific Sun and released in March 2011 (Fig. 1). The archival tags are attached to spaghetti tags which are wrapped around the carapace of the crabs between the first and second walking legs (Fig. 2). Because male snow crab do not molt after they reach maturity, there is no concern with molting and the attachment of tags.

To date, 23 tagged snow crabs have been recaptured by the commercial fishery and returned, more than 9 months after their release. These initial tag depth records show that some but not all the crab made an inshore migration and returned offshore (Fig. 3), which could be interpreted as some but not all adult males return for mating every year. We expect to have a better picture of the overall migration patterns of the snow crabs once we get additional returns from the 2012 fishery and once we’ve applied the tidal pattern matching analysis to examine the specific routes undertaken.

By Dan Nichol and Dave Somerton

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