Seasonal mortality and the effect of body size: a review and an empirical test using individual data on brown trout

1. For organisms inhabiting strongly seasonal environments, over-winter mortality is thought to be severe and size-dependent, with larger individuals presumed to survive at a higher rate than smaller conspecifics. Despite the intuitive appeal and prevalence of these ideas in the literature, few studies have formally tested these hypotheses. 2. We here tested the support for these two hypotheses in stream-dwelling salmonids. In particular, we combined an empirical study in which we tracked the fate of individually-marked brown trout across multiple seasons and multiple years with a literature review in which we compiled the results of all previous pertinent research in stream-dwelling salmonids. 3. We report that over-winter mortality does not consistently exceed mortality during other seasons. This result emerged from both our own research as well as our review of previous research focusing on whether winter survival is lower than survival during other seasons. 4. We also report that bigger is not always better in terms of survival. Indeed, bigger is often worse. Again, this result emerged from both our own empirical work as well as the compilation of previous research focusing on the relationship between size and survival. 5. We suggest that these results are not entirely unexpected because self-sustaining populations are presumably adapted to the predictable seasonal variation in environmental conditions that they experience.