Morphological evolution reduces downstream displacement in juvenile landlocked salmon

Severe flooding often leads to downstream displacement of aquatic animals. Despite this, many salmonid populations persist in habitats located upstream of tall barriers, such as artificial check dams and/or natural waterfalls, that completely block fishes from returning to the upstream areas after flooding. The evolution of such populations may be affected by spatial sorting due to differential rates of downstream displacement. This study examined whether a morphological trait (increased body depth) that allows individuals to better maintain their position during flooding has evolved in juvenile amago salmon Oncorhynchus masou ishikawae inhabiting above-barrier habitats in two rivers. In both rivers, juveniles collected at the stations with multiple downstream barriers had deeper bodies than those collected at other stations. Similar differences were found in juveniles reared in a common-garden experiment. Field experiments with natural flooding also indicated that deep bodies help juveniles resist downstream displacement. These results consistently suggest that juveniles in some above-barrier habitats have evolved deep bodies to resist downstream displacement due to flooding. Our study is the first to show the evolutionary outcomes of passive spatial sorting during severe climate events.

严重洪涝常会导致水生动物被冲至下游。尽管如此，仍有许多鲑科（salmonid）鱼类种群存活于高大拦挡物上游的栖息环境中——这类拦挡物包括人工谷坊坝与天然瀑布，可完全阻断鱼类在洪涝后返回上游区域。这类种群的演化可能受空间分选作用的影响，该作用源于下游位移速率的差异。本研究针对两条河流中栖息于拦挡物上游生境的幼年樱鳟石川亚种（Oncorhynchus masou ishikawae）展开调查，旨在探究是否演化出了一种可帮助个体在洪涝期间更好维持位置的形态性状——体躯深度增加。在两条河流中，于存在多处下游拦挡物的采样点采集到的幼鱼，其体躯深度均大于其他采样点的个体。同质饲养实验（common-garden experiment）中也观测到了类似的性状差异。结合自然洪涝条件下的野外实验结果，同样证实了较深的体躯可帮助幼鱼抵御下游位移。上述各项结果一致表明，部分拦挡物上游生境中的幼鱼已演化出较深的体躯，以抵御洪涝引发的下游位移。本研究首次揭示了极端气候事件期间被动空间分选作用所带来的演化结果。