Phenotypic plasticity in brook charr: changes in caudal fin induced by water flow

In the field, juvenile brook charr Salvelinus fontinalis inhabiting high‐velocity water were found to have larger caudal fins and more slender bodies than those inhabiting low‐velocity water. Young‐of‐the‐year S. fontinalis were reared in either a high‐ or low‐velocity treatment for 16 weeks and their morphology was measured bi‐weekly. From the second to fourth weeks of the experiment onwards, fish reared in the high‐velocity treatment had larger maximum caudal fin heights and deeper caudal peduncles than fish reared in the low‐velocity treatment. This study demonstrated that the morphological variation in caudal area exhibited by wild juvenile brook charr from microhabitats differing in water velocity could be a consequence of phenotypic plasticity in response to hydrological conditions.