Modification of river systems through reservoir construction has given rise to altered ecosystems, characterised as slow flowing or still-water habitats. Fish populations isolated in reservoirs can exhibit adaptive divergence from the source population and flow velocity may be an important driver of this divergence, leading over time to functional and morphological trait changes. Several studies have demonstrated that fish from populations living in still and flowing habitats are substantially different in average body shape. It is widely assumed that selection favours body shapes that confer optimum swimming ability in a particular environment, thereby enhancing a population’s fitness and persistence.
The aim of this study was to test the hypothesis that variation in body shape predicts divergence in swimming performance between river and isolated reservoir populations of Australian smelt (Retropinna semoni). Fish were sampled from 6 rivers and 6 reservoirs across South Eastern Australia.
Geometric morphometric analysis of 16 homologous landmarks was used to compare average body shape among these populations, while swimming performance was assessed using the critical swimming speed (U-crit) test to determine if body shape was a predictor of swimming performance.
River and reservoir populations of Australian smelt had significant differences in swimming performance. However, geometric morphometric analysis revealed that body shape of fish did not consistently differ between river and reservoir populations. These results indicate that morphology does not necessarily predict swimming performance and other physiological factors may be involved in determining swimming performance.
This study provides insights into the relationship between morphology and swimming performance of Australian smelt living under different extremes of flow velocity. The role of swimming performance in life-sustaining activities and the potential micro-evolutionary consequences of human-induced habitat modification are discussed.