Prey group cohesion mediates the outcome of predator–prey interactions under artificial lighting

The highest rates of piscivorous predation in the field have been recorded during crepuscular light levels associated with sunrise and sunset or artificial lighting at night (ALAN). Hypothetically, this occurs because, under low‐light conditions, prey lose the ability to visually sense and evade approaching predators, while predators maintain the ability to visually sense and attack prey. However, many prey fishes form social groups, where visually sensing and responding to one another is a key component of anti‐predator behaviours. Thus, for social prey, higher rates of predation under crepuscular conditions could additionally involve the deterioration of prey group behaviours with diminishing light.

We conducted a laboratory study exposing groups of predator‐naïve, hatchery‐raised juvenile rainbow trout ( Oncorhynchus mykiss ) to natural‐origin piscivorous largemouth bass ( Micropterus salmoides ) under one of three light treatments: ‘high’—representative of brighter crepuscular periods or direct ALAN illumination, ‘medium’—dimmer crepuscular periods or sky glow from ALAN, or ‘low’—night or no ALAN. We then statistically evaluated potential associations between light treatment, prey group cohesion and predator activity.

Under the high treatment, prey groups were most cohesive, predators attacked prey most frequently, and attacks were least likely to be successful. Under the medium treatment, prey group cohesion began to deteriorate, and attacks were most likely to be successful. Under the low treatment, prey groups were least cohesive and predators were least active. Across all light treatments, the less cohesive the prey groups were, the higher the probability that interested predators would attack, and that such attacks would result in prey being consumed.

Our results suggest that outcomes from interactions between piscivorous predators and social prey could depend more on prey group cohesion, and how this changes with light level, than on the light level itself. This could be relevant for current mechanistic models of foraging, where a predator's ability to successfully consume prey depends more on physical conditions, such as light level, than on biotic conditions, such as prey density. Incorporating the effects of social behaviour into mechanistic models could therefore help resolve the impacts of ALAN on imperilled fishes and prioritize relevant restoration efforts.