Cascading effects of predation risk under high organic and inorganic turbidity: impacts on individuals and shoals of a mesopredator fish

Abstract

We tested whether changes in water transparency caused either by precipitation-mediated sedimentation (inorganic turbidity) or eutrophication (organic turbidity) differently interact with top predator presence to determine predation rates, and individual decisions of mesopredators between foraging and avoiding predators. We tested the hypothesis that fear-induced cascade effects are more pronounced in organic turbid water through an experiment in which we manipulated turbidity (clear water and organic/inorganic turbid water) and predation risk (presence/absence of a top predator) in a 3 × 2 factorial design. We assessed prey consumption, prey selection (benthic and planktonic invertebrates) and food partitioning among individuals within shoals of an invertivorous fish, Moenkhausia forestii. The overall prey consumption by mesopredators was similar with and without top predator in clear water, but the presence of a top predator resulted in decreased invertebrate consumption in turbid waters, with no difference between organic and inorganic turbidity. Also, fear-induced cascade effects caused a strong preference of planktonic prey over benthic in inorganic turbidity and decreased evenness in prey consumption among individuals. Our findings suggest that the interactive effects of turbidity and top predator presence on mesopredator foraging depend on prey type and highlight individual differences in foraging behaviours among shoal members. Increased anthropogenic impacts on aquatic environments could have lasting long-term population impacts for fishes in terms of foraging and predator avoidance behaviour.