Aggregation in Riverine Fish: A Review from a Fish Passage Perspective

According to the most recent Living Planet Report, freshwater fish species are among the most threatened species on Earth, with many of them showing a decline in population due to altered river connectivity caused by barriers. Fish passages are conservation measures aimed at mitigating the impact of such obstructions for migrating freshwater fish, providing corridors that should not harm, kill, stress, or excessively delay fish movement. Fish passage efficiency, however, is highly variable and often unknown, particularly for non-salmonids. Despite many species being gregarious, research aiming at assessing and improving passage efficiency has focused almost solely on the behaviour and swimming performance of individual fish. Collective behaviour can, in fact, affect the way fish approach, enter, and pass a fishway. The mechanisms for which group behaviour affects fish movement in hydrodynamically complex environments, such as those occurring within fish passages, are multiple and not limited to: reduced energy expenditure, better navigation, reduced stress levels, increased exploratory behaviour, and change of predation dynamics. In this work, we review current research to illustrate how collective behaviour can be relevant for fish passage research. Our aim is to provide an overview of how collective behaviour might affect fish passage efficiency and how future research could improve the fish passage design.