The Effects of Suspended Sediments, Dye, and Their Variation on Fish Behavior

As human activities intensify, many freshwater environments are becoming darker. Suspended sediments released by human-mediated erosion and manufactured dyes discharged into waterways act as pollutants that alter the visual environment. These substances absorb and refract light, reducing visibility levels and potentially interfering with animal behaviors such as predator avoidance, foraging, and social organization. Predicting the effects of such visual pollutants in the environment is complicated by the temporal variability of pollutant levels due to factors such as rainfall and wastewater flow. Here, we compared how an exposure to stable versus fluctuating levels of suspended sediments and dye respectively altered the social, movement, and anxiety-like behaviors of captive zebrafish (Danio rerio). We found that fish moved more when exposed to fluctuating dye concentrations but decreased their movement when exposed to a stable suspended sediment concentration. Aggression decreased under stable exposures to both pollutants, but fluctuating conditions counteracted these effects. In contrast, shoaling behavior increased when fish were exposed to either pollutant regardless of fluctuation. This suggests that zebrafish may have different movement strategies for moving through turbid and dyed environments and that low visibility likely increases zebrafish shoaling. We also performed an optomotor response (OMR) assay to assess zebrafish visual perception under the different experimental conditions. While our OMR assays did not reveal any clear effects of either pollutant on visual perception, the fish performed fewer anxiety-like behaviors during the low visibility OMR assays. This suggests that low visibility may reduce stress during stressful events. Overall, our results reveal that visual pollution influences the behavior of fish, and that both the variability and type of pollutant influence behavioral responses.