Increasing hatchery production of juvenile red king crabs (Paralithodes camtschaticus) through size grading

Abstract

Cannibalism is problematic in hatchery production of many crustaceans and can be exacerbated by differential growth, size variability, and asynchronous molting. We conducted two hatchery experiments in Seward, Alaska, USA to investigate effects of size grading on survival and growth of juvenile red king crabs (Paralithodes camtschaticus). We reared larvae and subsequent juveniles until juveniles were eight weeks post-settlement. For each experiment, these eight-week old juvenile crabs (approximately 2.0 to 4.5mm carapace width) were sorted using a 3.3mm mesh screen into: “small,” “large,” and “ungraded” size classes. In the diet experiment, the three size classes were stocked at a density of 600crabsm−2 and reared either on a control diet of commercial mariculture feeds or the control diet supplemented with astaxanthin and calcium for 53days. In the density experiment, the three size classes were stocked at densities of 400, 900, and 1400crabsm−2 and fed the control diet plus astaxanthin and calcium for 31days. Survival in both experiments was strongly influenced by size grading. Generally, small crabs had higher survival than large and ungraded crabs. Diet was not a significant factor in weight or survival. Small crabs had relatively high survival at all stocking densities, but all size classes had decreased survival with increasing density, likely from cannibalism. Size graded crabs reared at elevated densities yielded improved biomass per rearing area (gm−2) compared to ungraded populations, suggesting lower survival rates may achieve the goal of optimizing hatchery production. Coupled with appropriate stocking densities, size grading could be used in laboratory and hatchery rearing protocols for red king crab and other likely cannibalistic crustaceans to maximize survival, improve hatchery efficiency, and increase the financial viability of large-scale stock enhancement or aquaculture programs.