The Impacts of River Impoundments on the Biology of the Three-Spined Stickleback, Gasterosteus Aculeatus

River impoundments cause a dramatic shift in the environment from a fluvial habitat to a static one. As such, they are likely to cause considerable changes to the biology of species living in those rivers, including the three-spined stickleback, Gasterosteus aculeatus. Sticklebacks from streams were wider and deeper bodied than those from reservoirs, consistent with differences observed between lake-stream pairs (e.g. Hendry et al., 2002; Berner et al., 2010). Variation in armour morphology was not as associated with habitat type and was more likely affected by variation in the predation regime. Although stream fish had wider mouths and shorter gill rakers than those in reservoirs, differences expected based on the likely changes in prey type availability as a result of impoundments (Berner et al., 2009), there were no associated changes in prey preferences or feeding efficiency. Overall, feeding efficiency was best predicted by an individual’s standard length. The degree of morphological divergence was not correlated with levels of neutral genetic differentiation, suggesting that divergence has been the result of natural selection acting on traits differently in diverse habitats. However, the extent to which traits were heritable was population specific and highlights the importance of assessing and estimating the strength of selection for populations separately. There were also differences in the timing of the onset of the breeding season. Although reservoir sticklebacks began breeding up to 2 months before stream sticklebacks, stream fish showed compensatory growth so that there were no differences in size at the onset of winter and subsequent end of the growing season. This thesis demonstrates how reservoir-stream systems can be used as a proxy for studying divergence in lake-stream systems but with additional information about the age of the system. A recurrent theme is that of variation between systems and highlights the importance of studying multiple systems simultaneously.