Effects of variable visual environments on fish movement and social behaviour

Abstract

Freshwater habitats are some of the most imperilled environments due in part to pollutants such as suspended sediments released by human-mediated erosion and manufactured dyes discharged in wastewater. Suspended sediments and dyes reduce visibility by absorbing, refracting, and diffracting light, and can interfere with vision mediated behaviours such as foraging or predator avoidance. In the wild, the concentrations of these visual pollutants frequently vary through time, and these fluctuations may themself influence animal behaviour. Unfortunately, most existing research fails to evaluate the dynamic patterns of suspended sediments, and almost no studies have evaluated the effects of dye on animal behaviour. Here, I assessed how kaolin clay and black pond dye independently affected the movement, social behaviour, and visual perception of zebrafish (Danio rerio), a visually-oriented, well-studied fish species. First, I showed that long-term exposure to suspended sediments interfered with movement and social hierarchy stability in ways that acute exposures did not (Chapter 2). Second, I discovered that suspended sediments and dye have contrasting effects on fish movement, with fish swimming less under suspended sediments but swimming more under dye exposure, despite both pollutants similarly decreasing aggression and increasing shoaling behaviour (Chapters 3 and 4). Third, I established that environmental fluctuation had a complicated interaction with the effects of low visibility, dampening habituation, decreasing the effects on aggression, and having contrasting effects on movement depending on the visual pollutant (Chapters 2, 3, and 4). My research reveals that fish may respond to visual pollution by forming tighter shoals, and by either moving more cautiously or avoiding polluted areas entirely. Taken together, my results show that suspended sediments and dye have similar but not identical effects on fish behaviour, and that both fluctuation and the duration of exposure play important roles on the overall in the impacts of visual pollutants on behaviour.