Search

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: We tested the effects of two rearing temperatures (8 and 11°C) on survival, larval period, and health of red king crab (Paralithodes camtschaticus) larvae in eight hatchery-scale 1200L tanks as part of a stock enhancement feasibility study. Larvae were stocked at a density of 50larvaeL−1. Survival did not vary from the newly hatched first stage zoea to the glaucothoe and first juvenile crab stage (C1) between the two temperatures. Survival averaged 50% from stocking to the glaucothoe stage and 20% from stocking to C1. Overall, 480,000 Z1s were stocked and 96,200 C1s were produced. Larval periods were significantly shorter at the higher temperature, averaging 21days from stocking to glaucothoe and 35days from stocking to C1 at 11°C compared to 30days to glaucothoe and 50days to C1 at 8°C. Specific attributes of larval health measured in this study included the size and relative number of lipid droplets in the anterior region of the larval gut as a measure of stored energy reserves and percentage of the surface of the larval exoskeleton fouled with filamentous bacteria. Quantitative assessment of accumulated lipids as measured by maximum lipid droplet diameters and relative numbers of lipid droplets in the anterior region of the gut was not significantly different between larvae reared at the two temperatures, suggesting that larvae were able to accumulate similar quantities of lipid energy reserves at both temperatures. Filamentous bacterial fouling was greater during the late Z4 stage in the larvae reared at 8°C compared to those reared at 11°C, which was likely due to the longer intermolt duration at the lower temperature allowing more time for bacterial accumulation on the exoskeleton. We found that red king crab larvae can be cultured in a substantially shorter time period without compromising survival or health by increasing rearing temperature from 8 to 11°C. [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]

Abstract: Juvenile red king crab (Paralithodes camtschaticus) mass rearing was conducted in Seward, Alaska, USA in a king crab stock enhancement feasibility study. Hatchery-raised juveniles were cultured from larvae of 12 ovigerous females collected from Bristol Bay, Alaska, USA. Juvenile instars were cultured in nursery grow-out containers in two phases: (1) C1–C3 juveniles and (2) C3–C6 juveniles. Experiments lasted for 42 and 44 days, respectively, and tested the suitability of various diets, stocking densities and substrates in terms of survival rate and growth. The first experiment (C1–C3) compared fully-factorial treatments of three diets (Cyclop-eeze®, enriched Artemia nauplii, or Zeigler™ shrimp feed), three stocking densities (500 m−2, 1000 m−2, or 2000 m−2), and two substrates (none or a combination of artificial seaweed, gillnet, and mechanical biofilter medium). The second experiment (C3–C6) used a mixed diet and compared fully-factorial treatments of two stocking densities of 800 m−2 and 1600 m−2 and two substrates (as above). No one food produced the highest survival and growth to C3. Cyclop-eeze® yielded highest survival (62.7%) with low growth (wet weight 8.63 mg and CW 2.04 mm). Crabs on shrimp nursery feed had the highest wet weight (10.0 mg) and CW (2.14 mm) but with low survival (44.5%). The highest stocking density resulted in a decrease in survival in both experiments. Lower stocking densities of 500 m−2 and 800 m−2 had relatively high survival of 58.7% and 48.7%, respectively, while the 1600 m−2 and 2000 m−2 densities had survival of 30.5% and 44.7%, respectively. Growth appeared to be inhibited at the highest density (2000 m−2) in the C1 to C3 stages, as crabs at this density were smaller and weighed less than at the 500 m−2 or 1000 m−2 density. Complex artificial substrate increased survival and reduced growth in both experiments, likely due to reduced cannibalism and increased time spent foraging. Among substrate treatments, highest abundances of crabs were observed on artificial seaweed, which may more closely resemble preferred natural substrate. These results suggest that culturing red king crab juveniles at low to intermediate stocking densities with a mixed diet in the presence of artificial substrate provides good survival. Intermediate densities may provide for greater production without great loss in survival. [Copyright &y& Elsevier]