Size grading juvenile RKC (size-diet)

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.

在诸多甲壳类动物的孵化场养殖生产中，同类相食是一类棘手问题，而生长差异、个体大小不均以及蜕皮不同步等因素会进一步加剧该现象。我们于美国阿拉斯加州苏厄德开展了两项大型试验，旨在探究规格分级对幼体红王蟹（Paralithodes camtschaticus）孵化场养殖生产（存活率与生长性能）的影响。我们将幼虫及后续幼体持续饲养至附着后8周的幼蟹阶段。每项试验中，我们将该阶段的8周龄幼蟹划分为三个规格组别：“小规格组”（头胸甲宽<3.3 mm，carapace width, CW）、“大规格组”（头胸甲宽>3.3 mm，carapace width, CW）以及“未分级组”（头胸甲宽约2.0至4.5 mm）。饲料试验中，三个规格组均以600只·m⁻²的密度放养，分别饲喂商用海水养殖饲料对照日粮，以及添加了虾青素与钙的对照日粮，饲养周期为53天。密度试验中，三个规格组分别以400、900及1400只·m⁻²的密度放养，饲喂添加虾青素与钙的对照日粮，饲养周期为31天。两项试验结果均显示，规格分级对幼蟹存活率存在显著影响。总体而言，小规格幼蟹的存活率显著高于大规格组与未分级组。第一项试验中，饲料类型对幼蟹的生长性能与存活率未产生显著影响。第二项试验中，随着放养密度升高，大规格组与未分级组的生长性能提升，但存活率出现下降，该现象大概率由同类相食行为导致。而小规格组在所有受试放养密度下均保持了较高的存活率与较低的生长速率。上述研究结果表明，若要最大化幼蟹存活率，需降低大规格幼蟹的放养密度；而小规格个体则可采用更高的放养密度。结合适宜的放养密度，在红王蟹及其他易发生同类相食的甲壳类动物的实验室与孵化场饲养规程中，应引入规格分级操作，以实现存活率最大化、提升孵化场养殖效率，并增强规模化种群增殖或水产养殖项目的财务可行性。