AFSC/RACE/GAP/Nichol: Archival tag depth and temperature data from snow crab

Seasonal migration of commercial-size (=102 mm carapace width [CW]), morphometrically mature (MM) snow crabs (Chionoecetes opilio) from the eastern Bering Sea was examined in relation to the summer distribution of mature females to identify spatiotemporal overlap of males and females and determine the likelihood of mating associations for specific reproductive stages. Depth variation associated with this migration was examined to determine whether seasonal migrations contribute to previously recognized spatial differences in distributions of commercial-size males caught in the winter fishery and in the National Marine Fisheries Service summer bottom trawl survey. Depth data from 33 data storage tags attached to commercial-size MM males during 2010 and 2011 indicated that most males moved inshore during spring—a movement that would allow them to mate with multiparous females but not with pubescent-primiparous females. Smaller tagged males (100–102 mm CW) underwent more extensive inshore migrations, and several of them traveled more than 100 km in one direction. Both tagging and distribution data indicated that most commercial-size MM males remained predominantly on the outer shelf throughout the year (despite some inshore movements during spring) and, therefore, these males did not contribute greatly to the spatial differences observed between winter and summer.

To examine the migration of commercial-size (=102 mm carapace width [CW]), morphometrically mature (MM) snow crabs (Chionoecetes opilio) from the eastern Bering Sea.

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Nichol, D.G. and Somerton, D.A. 2015. Seasonal migrations of morphometrically mature male snow crab (Chionoecetes opilio) in the eastern Bering Sea in relation to mating dynamics. Fishery Bulletin 113: 313-326.

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Methodology:

A total of 277 morphometrically mature male snow crabs (96–134 mm CW) were tagged and released with pressure-and-temperature-recording DSTs on 18–22 April 2010 (n=120) and 7–8 March 2011 (n=157), near the end of each fishing season to ensure that tagged crabs were not recaptured until the following year. Tagging operations occurred on the winter snow crab fishing grounds northwest of the Pribilof Islands at approximately 57°35' N in 2010 and about 100 km farther north at 58°30' N in 2011 (Fig. 1). DSTs were attached to spaghetti tags that were wrapped around the carapace of the crabs between the first and second walking legs. Because male snow crabs do not molt after they reach maturity, the effect of tagging on their behavior and mortality was assumed to be negligible. In April 2010, an additional 221 snow crabs were tagged and released with numbered spaghetti tags without a DST attached as a control to test whether the additional DST attachment affected capture rate, as well as to help examine site fidelity.

Crabs were captured and tagged aboard commercial pot fishing vessels Kiska Sea (in 2010) and Pacific Sun (in 2011) during normal commercial fishing operations, and they were released within 10 min of capture at the same location. All tagged male crabs were large-clawed with new to slightly worn hard shells, conditions that predominated in the catches during tagging. Only crabs that possessed all their limbs were selected for tagging. Tagged crabs were recaptured by commercial crab pot vessels during the following winter and spring snow crab fisheries in 2011 and 2012, and a tag reward program was implemented to provide an incentive to return tags. Tag recapture locations were provided by the fishermen who returned tags.

The DSTs, Cefas G5 Long Life tags with 2 MB of memory (Cefas Technology Limited, Lowestoft, UK), measured depth (pressure) at 1-min intervals, with an accuracy of ±2 m and precision of <0.08 m, and temperature at 30-min intervals, with an accuracy of ±0.1°C and precision of 0.03°C. DSTs were bullet-shaped with dimensions of 8 × 31 mm and weighed 1g in water.