Evaluation of potential overwinter mortality of age-0 walleye and appropriate age-1 sampling gear

Potential recruitment of age-0 Walleye Sander vitreus to adults is often indexed by the relative abundance of age-0 individuals during their first summer or fall. However, relationships between age-0 and adult Walleye abundance are often weak or nonsignificant in many waters. Overwinter mortality during the first year of life has been hypothesized as an important limitation to Walleye recruitment in lakes, but limited evidence of such mortality exists, likely due to difficulties in sampling age-1 Walleye during spring. The objectives of this study were to: 1) compare results from nighttime electrofishing to index relative abundance of age-1 Walleyes with relative abundance indices of minifyke nets in four eastern South Dakota lakes; 2) determine whether size-selective mortality was occurring in those four lakes; and 3) if size-selective mortality was occurring in these lakes, determine whether that mortality was attributed to body condition. We sampled four natural lakes in eastern South Dakota 2 wk after ice-off in 2013 and 2014. Precision of nighttime electrofishing (coefficient of variation = 216.6) was greater than that estimated for minifyke nets (coefficient of variation = 338.5) across both years. We detected no differences in length-frequency distributions of collected spring age-1 Walleye between the two gears. Age-0 fall relative abundance indices from electrofishing were significantly greater (P < 0.01) than spring age-1 nighttime electrofishing indices of relative abundance at three of the four study lakes, indicating that overwinter mortality may occur at a substantial rate during the first year of life for Walleye in these systems. Quantile–quantile regression plots showed evidence of size-selective mortality in three of four lakes sampled. However, body condition of age-0 Walleye appeared to have little to no influence on overwinter mortality. Instead, we suggest that smaller-sized walleye may be more vulnerable to overwinter predation. Collectively, these results provide evidence of previously hypothesized overwinter mortality within the first year for Walleye and indicate possibilities for indexing potential adult recruitment of Walleye just after this critical period.

0龄白斑狗鱼（Walleye，Sander vitreus）向成体的种群补充潜力，通常可通过其首个夏季或秋季的0龄个体相对丰度进行评估。然而，在多数水域中，0龄与成体白斑狗鱼的丰度关联往往较弱，甚至无统计学意义。有假说指出，生命首年的越冬死亡率是湖泊中白斑狗鱼种群补充的重要限制因素，但相关实证较为匮乏，这大概率源于春季采样1龄白斑狗鱼存在较大困难。
本研究的目标如下：1）在南达科他州东部的4个湖泊中，对比夜间电渔法（nighttime electrofishing）与小型刺网（minifyke nets）的相对丰度指数，以评估1龄白斑狗鱼的相对丰度；2）明确上述4个湖泊中是否存在体型选择性死亡现象；3）若存在体型选择性死亡，探究其是否与个体身体状况相关。
本研究于2013年和2014年的冰融后2周，对南达科他州东部的4个天然湖泊开展了采样。两年间，夜间电渔法的抽样精度（变异系数=216.6）高于小型刺网（变异系数=338.5）。两种采样工具采集的春季1龄白斑狗鱼的体长频率分布无显著差异。
在4个研究湖泊中的3个里，夜间电渔法得到的0龄秋季相对丰度指数显著高于春季1龄夜间电渔法相对丰度指数（P < 0.01），这表明该研究系统中白斑狗鱼生命首年的越冬死亡率可能处于较高水平。分位数-分位数（quantile–quantile）回归图显示，4个采样湖泊中的3个存在体型选择性死亡的证据。但0龄白斑狗鱼的身体状况似乎对越冬死亡率几乎无影响。据此，我们推测体型更小的白斑狗鱼可能更易在越冬期间遭受捕食。
综上，本研究结果为此前提出的白斑狗鱼生命首年越冬死亡假说提供了实证支持，并表明可在这一关键时期后，通过采样评估白斑狗鱼潜在的成体种群补充潜力。