Water velocity shapes juvenile salmonids

Phenotypic plasticity in morphology is often considered adaptive. Stream-li ving fish encounter considerable spatial and temporal environmental variati on in their native habitats, and the ability to adapt to this variation is of utmost importance. We studied experimentally whether water velocity affe cts the body shape of juvenile Atlantic salmon (Salmo salar m. sebago Girar d) and brown trout (Salmo trutta m. lacustris L.). The fish were reared in slow and fast water flow, and their morphology was studied by measuring a n umber of morphometric characters. We studied which characters differed betw een the environments in each species, and found that water velocity caused morphological differentiation in both salmon and brown trout. The differenc es occurred especially in body height as well as in fin sizes, characters t hat are very likely to be of functional importance for life in the stream e nvironment. Salmon in fast flow became more robust, whereas brown trout in fast flow became slightly more streamlined. The observed variation in body morphology of salmon and brown trout indicates phenotypic plasticity, but t he species differed in their response to environmental variation, which may be due to different energetics and cost reduction strategies. Morphologica l differentiation caused by water flow occurred very rapidly, within 1-mont h exposure to the different water flows.