Effects of Isolation By Dams and Waterfalls on Fish Morphology

It has been demonstrated that large dams with upstream reservoirs can cause divergent selection in fish morphology; however, the effect of isolation from small dams without upstream reservoirs is largely unknown. The current study investigated the effect of isolation on fish morphology and also the effect of isolation time on morphological divergence by comparing fish associated with older barriers (waterfalls) to very new barriers (dams) in five cool-water stream species along the southern Lake Superior basin. Geometric morphometrics were used to quantify differences in fish body shape. Significant intraspecific differences between stream populations and between above-barrier and below-barrier populations were found in three species. The majority of body shape variation was associated with deeper body depths (Salvelinus fontinalis and Rhinichthys atratulus) and position of pre-opercular spine and head length (Cottus bairdii). There was greater morphological divergence between Cottus bairdii separated by waterfalls than by dams. Nonparametric multiplicative regression revealed that the best environmental predictor variable for body shape was wetted width (Salvelinus fontinalis and Rhinichthys obtusus) and stream population (Cottus bairdii). These results suggest that, for some species, isolation from dams and waterfalls is associated with divergent intraspecific morphology and the amount of divergence is increased by isolation time.