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Abstract: Stock enhancement through the release of cultured juveniles has been suggested as a possible recovery tool for depleted red (Paralithodes camtschaticus) and blue (Paralithodes platypus) king crab populations in Alaska, USA. Considerable progress has been made in the past decade in red king crab culture technology, but similar technologies are less developed for blue king crabs. As part of a stock enhancement feasibility study, hatchery-raised blue king crab juveniles were cultured from larvae of ovigerous females collected off Saint Matthew Island, Alaska. Juvenile instars were then cultured in indoor nursery containers for 42days at stocking densities of 500, 1000, 2000, 4000, and 6000crabsm−2 starting at metamorphosis to the first instar stage (C1). At day 42, survival was above 90% at all the densities, but had a slightly decreasing trend that ranged from 96% at 500crabsm−2 to 90% at 6000crabsm−2. The numbers of crabs cannibalized increased with density, but the overall increase in crab biomass offset the negative effects of cannibalism. Survival rates and biomass production for the first 42days post-settlement is much higher for blue king crabs compared to red king crabs reared under identical conditions. The low rate of cannibalism implies that blue king crabs are well suited for large-scale hatchery culture. Our results suggest that these two closely related crab species can be reared at different stocking densities immediately flowing settlement and that high densities will maximize hatchery efficiency for blue king crabs. Future studies should investigate longer term nursery culture, beyond 42days, to refine protocols for extended blue king crab rearing. Improving nursery techniques will boost the productivity and financial viability of a large-scale stock enhancement program. [Copyright &y& Elsevier]

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. [Copyright &y& Elsevier]