Size grading studies help refine hatchery grow-out strategies

As hatchery crabs grow, their molting becomes increasingly asynchronous, resulting in a range of sizes.

A large-scale nursery grow-out experiment was carried out at the Alutiiq Pride Shellfish Hatchery in Seward, as a part of Ben Daly’s Ph.D. research at the University of Alaska Fairbanks. Juvenile red king crabs are highly cannibalistic, making long-term holding in the hatchery difficult. Crabs were sorted by size to reduce cannibalism and allow crabs to reach later stages with higher survival rates. Small, large, and a mixture of crab sizes were reared at low, moderate, and high stocking densities. The small crabs achieved high survival at all stocking densities, while the large and mixed crabs had high survival at low densities but low survival at the moderate and high densities. After crabs of a range of sizes were reared together at high densities, all remaining crabs were large suggesting larger individuals prey on smaller individuals. Smaller crabs that are reared without larger individuals avoid the negative effects of dominance. These data suggest that sorting crabs by size increases overall hatchery productivity. Small crabs can be held at high densities; however, densities must be significantly lower for larger crabs to achieve similar survival rates. Future hatchery grow-out strategies to maximize production will incorporate sorting crabs by size and varying stocking densities depending on size class.

News Flash is edited by Ben Daly. AKCRRAB is a research and rehabilitation project sponsored by the Alaska Sea Grant College Program, UAF School of Fisheries and Ocean Sciences, NOAA Fisheries, the Alutiiq Pride Shellfish Hatchery, community groups, and industry members. For more information go to http://seagrant.uaf.edu/research/projects/initiatives/king_crab/general.