The impact of increasing turbidity on the predator-prey interactions of freshwater fishes

Human activities are exacerbating environmental change globally. Turbidity in freshwater systems is increasing due to extreme weather events and intensification of activities such as deforestation, urbanisation, agriculture and altering water flow with dams and other structures. Prey species may benefit from turbid habitats as turbidity creates a visual barrier to predators; such predators thus face reduced foraging success as water clarity decreases. However, the literature on the effects of water turbidity on predator-prey interactions in fish is often contrasting. Environmentally driven changes in predator and prey behaviour, foraging and survival ultimately have the potential to affect community structure and ecosystem functioning, thus understanding the impact of changes in turbidity on predator-prey interactions should be a priority. Given the contrasting results in the primary literature, the aim of this review is to provide a broad overview of the current knowledge on the impacts of increased turbidity on predator-prey interactions in freshwater fishes, summarise the published literature and identify knowledge gaps on this topic. We collated 281 studies from the peer-reviewed literature that tested the impact of water turbidity on the foraging behaviour of predatory fish and anti-predator behaviour of prey fish. We recorded changes in predator and prey behaviours in response to changes in turbidity levels. Furthermore, we reported whether these behavioural changes were considered to have a positive (e.g. enhanced foraging success) or negative (e.g. higher mortality) impact on the focal species, or whether the impact was unspecified (e.g. changes in anti-predator behaviour without quantifying actual predation risk). The literature has some strong taxonomic, geographical and environmental biases, where most research focused on commercially important, temperate species. A large proportion of studies (47%) reported a negative (versus a positive or unspecified) impact of turbidity on the focal species (either predator or prey freshwater fish). Negative impacts were reported more often for predators (58%) compared to prey (19%) and while changes in behaviour were frequently reported in prey species, if these changes are adaptive or not is unclear, as actual predation risk was not assessed in the majority of relevant studies (55%). Increases in turbidity have complex effects on predator-prey interactions, with negative impacts more often reported for adult piscivore predators, while prey species may gain a relative advantage compared to visual predators. However, whether prey behavioural responses to increased turbidity are adaptive is generally unclear. We suggest that a more holistic and ecologically relevant experimental approach is needed to disentangle the underlying mechanisms and determine the adaptive nature of prey behavioural responses to increased turbidity. Our review serves as a broad and comprehensive summary of the empirical research conducted on impacts of increasing water turbidity on predator-prey interactions in freshwater fishes. It highlights knowledge gaps and provides a reference point to guide future experimentation and research synthesis in this crucial and growing area of research.