Program for feed calculations.

Fish that are produced in hatcheries often produce fewer surviving adult offspring than do wild fish when both spawn in the wild. This difference in fitness is likely due to inadvertent selection for adaptation to the hatchery environment. Size at release is positively correlated with survival at sea. Therefore, selection should favor traits that promote fast growth in the hatchery even if those traits are maladaptive in the wild. In that case, changing hatchery conditions to reduce the variance in size at release would reduce the opportunity for domestication selection. Here we test whether grading by size and raising each size group separately can substantially reduce the variance in size at release. We graded a mix of 15 full-sibling families of winter run steelhead (Oncorhynchus mykiss) juveniles (initial n = 375 fish/tank) into small, medium, and large body size groups. We then promoted growth in the initially-small fish with excess food and low densities, and slowed growth in the initially-large fish with restricted feedings and high densities. We successfully held back the growth of the initially-large fish, but the initially-small fish never caught up with the controls, despite being raised under ideal conditions. This result suggests that inherent physiological or behavioral factors, rather than interactions with larger fish, restrict the growth of small fish. Both the total variance among individual fish and the variance among family means was reduced by the grading treatment, although only the reduction in total variance among individuals was statistically significant when comparing intraclass correlation values (ICC). The reduced variance due to slowing the growth of the initially-large fish indicates that grading could possibly be a tool used by hatcheries to lessen selection on salmonids. However, it would first be necessary to better understand the factors limiting growth in the smaller fish.当孵化场繁育的鱼类与野生鱼类均在野外产卵时，前者产出的存活成年后代数量往往少于后者。这种适合度（fitness）差异可能源于针对孵化场环境适应的无意选择。放流时的体型（size at release）与海洋存活率呈正相关。因此，即便某些性状在野生环境中存在适应不良的问题，自然选择仍会青睐那些能促进孵化场内快速生长的性状。在此背景下，通过调整孵化场饲养条件以降低放流体型的方差，可减少驯化选择（domestication selection）发生的概率。本研究旨在验证：按体型分级并单独饲养各规格组，是否可显著降低放流体型的方差。我们将15个全同胞家系（full-sibling families）的冬季洄游钢头鳟（Oncorhynchus mykiss）幼鱼（初始放养密度为每缸375尾）按体型大小划分为小、中、大三个组别。随后，我们通过过量投喂与低密度饲养，促进初始小型组幼鱼的生长；同时通过限饲与高密度饲养，抑制初始大型组幼鱼的生长。实验结果显示，我们成功抑制了初始大型组幼鱼的生长，但初始小型组幼鱼即便在理想饲养条件下，也始终未能追上对照组的生长水平。该结果表明，限制小型鱼生长的因素是其内在的生理或行为特性，而非与体型更大个体的互动。分级处理不仅降低了个体间的总变异，也降低了家系均值间的变异；但通过组内相关值（intraclass correlation values, ICC）的比较分析可见，仅个体间总变异的降低具有统计学显著性。通过抑制初始大型组幼鱼生长所带来的变异降低，表明体型分级处理或许可作为孵化场减轻鲑科鱼类（salmonids）驯化选择的一种可行手段。不过，在推广该方法前，仍需进一步明确限制小型鱼生长的具体机制。