| Summary: | Cannibalism is problematic in hatchery production of many crustaceans and can be exacerbated by differential growth, size variability, and asynchronous molting. We conducted two large-scale experiments in Seward, Alaska, USA to investigate effects of size grading on hatchery production (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 were sorted into: “small” (<3.3 mm carapace width (CW)), “large” (>3.3 mm CW), and “ungraded” (approximately 2.0 to 4.5 mm CW) size classes. In the diet experiment, the three size classes were stocked at a density of 600 crabs m-2 and reared either on a control diet of commercial mariculture feeds or a control diet supplemented with astaxanthin and calcium for 53 days. In the density experiment, the three size classes were stocked at densities of 400, 900, and 1400 crabs m-2 and fed the control diet plus astaxanthin and calcium for 31 days. 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 growth or survival of crabs in the first experiment. In the second experiment, large and ungraded crabs had higher growth and decreased survival with increasing density, likely from cannibalism. Small crabs had high survival and low growth at all stocking densities. These results suggest that larger juveniles must be held at reduced densities to maximize survival, while smaller individuals can be held at higher densities. Coupled with appropriate stocking densities, size grading should 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.
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