The effect of temperature on the dynamics of common bream Abramis brama migrations between the reservoir and its tributary

An active preference for higher temperatures within a physiological optimum is beneficial for animal movement. For example, ascending temperatures induce an increase in cyprinid fish metabolism and swimming ability. Spring upstream migrations driven by the search for resources may be related to these increases. Conversely, downstream migrations in autumn follow a decrease in temperature. When fish migrations are driven by a search for resources, for example, food availability and reproduction, or to avoid predators, then the temperature effect can be reduced to approximately the threshold temperatures that induce up- and/or downstream movement. To test this assumption, we tracked the seasonal migrations of the common bream Abramis brama between a reservoir and its tributary using radio tags with temperature sensors during a 5-year period. Upstream migrations of the species into the tributary were not motivated by seeking temperatures different from those in the reservoir, that is, fish body temperatures in both environments were comparable across seasons. However, for long-distance migrations, increasing temperature did support upstream migrations. Temperature did not determine the direction or intensity of short-distance migration of the species between the reservoir and the tributary. No significant influence of temperature was recorded for the downstream migrations according to the results of the generalised additive mixed model (GAMM1), which related movement distance as the explanatory variable to the signed fish body temperature as the response. The second model (GAMM2) relating fish body temperature as the explanatory variable to the signed movement distance as the response obtained a threshold value of 19.1°C for the upstream migrations and 1.5°C for the downstream migrations of the common bream.