Freshwater fish personalities in the Anthropocene

Individual processes scale up: these are the unique phenotypes that undergo selection, constitute populations, interact with other species, and thus shape biological reality. Understanding how individual animals differ in behaviour, that is understanding how behavioural individualities (personalities) emerge and are selected, leads us to a better understanding of how higher level systems, such as food webs, communities, or ecosystems, function. As amidst the contemporary global environmental crisis, freshwater habitats and their fish inhabitants are disproportionately both biodiverse and threatened, it is of crucial importance to understand how individual fishes cope with the anthropogenic change. In the present work we first provide a snapshot view of what personalities in freshwater fish are, how they manifest in different species, are shaped under different selective pressures, emerge over ontogeny, and form complex traits. This includes the review of research on fish boldness, exploration, activity, aggressiveness and sociability, and on their consistency and correlations. We then focus on three major threats to freshwater fish, on how fish cope with them behaviourally, and on where personalities may have profound ecological and evolutionary outcomes. The chosen focus on habitat fragmentation by damming, climate change and chemical pollution provides us with an insight into the pervasive role fish personalities play in the contemporary world.