Effects of reduced visibility on fish behavior: low light is not equivalent to suspended sediments

Human development has increased soil erosion, elevating suspended sediment levels across aquatic habitats. Poor visibility caused by suspended sediments can influence the antipredator responses, social preferences, foraging, and other behaviors of fishes, and is therefore a major contributor to human impacts on waters. Studying the impacts of suspended sediments in the laboratory requires regular water turbulence to prevent sediments from settling out of suspension, so some researchers instead alter lighting to mimic the impacts of suspended sediments. Here, we compared the social and movement behavior of zebrafish exposed to suspended sediments versus black pond dye, a potential easier-to-use alternative to suspended sediments. We also evaluated whether fluctuation in the visual environment further alters any potential behavioral impacts. We found that the effects of pond dye differed from suspended sediments. Zebrafish moved more in pond dye than in suspended sediments, with the fish being on average 26% faster and 8% more active. Fluctuating visibility further amplified this effect. Fish also performed 126% more chases and 116% more follows in suspended sediments than dye. Our results suggest that black pond dye is not a substitute for suspended sediments and, further, that fluctuating visibility levels affect how fish respond to visual conditions.