Department of Fisheries and Oceans, Charlottetown, PE, C1A 7M8, Canada.
J Fish Biol. 2009 Jun;74(9):2094-114. doi: 10.1111/j.1095-8649.2009.02290.x.
Temperate-zone anguillid eels use both saline (marine or brackish) and fresh waters during their continental phase, but use of fresh waters is paradoxical because on average these fishes grow more rapidly in saline than in fresh waters. Based on data from anguillid eels whose habitat-residency histories had been determined by Sr:Ca otolithometry, superiority of growth rates in saline water is much greater in American eels Anguilla rostrata in north-eastern North America (mean saline:fresh growth rate ratio 2.07) than in European Anguilla anguilla, Japanese Anguilla japonica and shortfinned Anguilla australis eels (range of mean ratios 1.12-1.14). Data from A. rostrata in the Hudson Estuary, U.S.A., and Prince Edward Island, Canada, were used to test adaptive explanations of catadromous migrations. The hypothesis that lower mortality in fresh water offsets faster growth in saline water was not supported because loss (mortality + emigration ) rates did not vary between saline and fresh zones of the Hudson Estuary. Hypotheses that anguillid eels move to fresh water to escape from larger anguillid eels in saline water or to evaluate habitat quality were not supported by size and age distributions. Catadromy in temperate-zone anguillid eels increases the diversity of occupied habitats and therefore lowers fitness variance caused by environmental fluctuations. Catadromy in temperate-zone anguillid eels could be due to natural selection for maximum geometric mean fitness which is sensitive to fitness variance. Temperate-zone catadromy might also be maladaptive, at least in local areas, due to shifts over time in selective pressures or to inability of panmictic genetic systems to adapt to local conditions.
温带鳗鲡在大陆阶段既利用盐水（海洋或半咸水）又利用淡水，但利用淡水是自相矛盾的，因为这些鱼类在盐水中的生长速度平均比在淡水中快。根据通过锶钙耳石测年法确定了栖息地居留史的鳗鲡的数据，在北美的东北美洲美洲鳗 Anguilla rostrata（盐水：淡水生长速度比 2.07）比欧洲鳗 Anguilla anguilla、日本鳗 Anguilla japonica 和短鳍鳗 Anguilla australis（平均比值范围 1.12-1.14）中，盐水生长速度的优越性要大得多。来自美国哈德逊河口和加拿大爱德华王子岛的 A. rostrata 的数据用于测试洄游的适应性解释。低死亡率在淡水中抵消了在盐水中更快的生长速度的假设不成立，因为哈德逊河口盐水区和淡水区的死亡率（死亡+外迁）率没有差异。鳗鲡为逃避盐水区中较大的鳗鲡或评估栖息地质量而迁移到淡水的假设也没有得到大小和年龄分布的支持。温带鳗鲡的洄游增加了被占领栖息地的多样性，从而降低了环境波动引起的适应度方差。温带鳗鲡的洄游可能是由于对最大几何平均适应度的自然选择，这对适应度方差很敏感。温带洄游鳗鲡也可能是适应不良的，至少在局部地区，因为随着时间的推移，选择压力发生变化，或者由于混合遗传系统无法适应当地条件。
