Importance of visual and chemical cues in infection detection and avoidance in freshwater fish

The characteristics of infected animals, including their odour, appearance, behaviour and sound, greatly differ from those of uninfected conspecifics. These differences can serve as cues to recognize and avoid infected individuals and minimize the risk of infection. Avoidance of infected conspecifics is a risk-sensitive behaviour that can be influenced by various factors, including sensory cues, which are poorly understood in fish. We investigated the ability of two populations of wild-caught pumpkinseed sunfish, Lepomis gibbosus , to distinguish between conspecifics infected with parasitic worms and uninfected individuals in two-choice experiments using visual and chemical cues separately. One population was from a lake without parasites (parasite-naïve), whereas the second population originated from a lake with a high prevalence of trematode and cestode worms (parasite-experienced). Both populations preferred to be with conspecifics, regardless of their infection level, over being alone when given visual cues but avoided conspecifics and preferred to be alone when given chemical cues, suggesting that visual and chemical cues are not redundant. Fish from neither population showed a preference between infected and uninfected conspecifics when given visual cues. However, there was a large interindividual variation in preference for chemical cues: some fish preferred uninfected conspecifics, whereas others preferred infected fish. On average, naïve fish avoided infected conspecifics, whereas experienced fish did not show any preferences, suggesting that fish from lakes with high prevalence of infection are habituated to infection cues. We suggest that pumpkinseeds use chemical rather than visual cues to discriminate between infected and uninfected conspecifics during shoaling decisions. Our study highlighted the importance of considering different sensory cues as well as environmental parasite abundance when studying avoidance and shoaling behaviours, especially considering the growing impacts of global changes on the sensory landscape and parasite abundance in freshwater ecosystems. • We conducted two-choice experiments on wild-caught pumpkinseed sunfish. • Visual and chemical cues were not redundant for conspecific avoidance. • Parasite-naive fish avoided chemical infection cues, but experienced ones did not. • Fish used chemical but not visual cues to discern infected and healthy conspecifics. • Indirectly transmitted parasites can affect avoidance behaviour.