Grouping facilitates avoidance of parasites by fish

Abstract

BackgroundParasite distribution is often highly heterogeneous, and intensity of infection depends, among other things, on how well hosts can avoid areas with a high concentration of parasites. We studied the role of fish behaviour in avoiding microhabitats with a high infection risk using Oncorhynchus mykiss and cercariae of Diplostomum pseudospathaceum as a model. Spatial distribution of parasites in experimental tanks was highly heterogeneous. We hypothesized that fish in groups are better at recognizing a parasitized area and avoiding it than solitary fish.MethodsNumber of fish, either solitary or in groups of 5, was recorded in different compartments of a shuttle tank where fish could make a choice between areas with different risk of being infected. Intensity of infection was assessed and compared with the number of fish recorded in the compartment with parasites and level of fish motility.ResultsBoth solitary fish and fish in groups avoided parasitized areas, but fish in groups avoided it more strongly and thus acquired significantly fewer parasites than solitary fish. Prevalence of infection among grouped and solitary fish was 66 and 92 %, respectively, with the mean abundance two times higher in the solitary fish. Between-individual variation in the number of parasites per fish was higher in the “groups” treatment (across all individuals) than in the “solitary” treatment. Avoidance behaviour was more efficient when fish were allowed to explore the experimental arena prior to parasite exposure. High motility of fish was shown to increase the acquisition of D. pseudospathaceum.ConclusionFish in groups better avoided parasitized habitat, and acquired significantly fewer parasites than solitary fish. We suggest that fish in groups benefit from information about parasites gained from other members of a group. Grouping behaviour may be an efficient mechanism of parasite avoidance, together with individual behaviour and immune responses of fishes. Avoidance of habitats with a high parasite risk can be an important factor contributing to the evolution and maintenance of grouping behaviour in fish.

Vertebrate defense against parasites: Interactions between avoidance, resistance, and tolerance

“…Here, trout clearly avoided areas containing parasites and this pattern was similar in the two genetically distinct populations. Overall, this is consistent with the results of earlier behavioral studies on rainbow trout and D. pseudospathaceum (Karvonen, Seppälä, & Valtonen, ; Mikheev et al., ). Salmon, on the other hand, preferred certain areas of the experimental tank irrespective of parasite occurrence.…”

“…Ten fish from the same group were tested simultaneously. One fish was placed into the middle compartment of each tank and allowed to explore for 1.5 hr to facilitate avoidance behavior (Mikheev, Pasternak, Taskinen, & Valtonen, 2013). All fish visited all compartments of the tank during the habituation.…”

Behaviour in the toolbox to outsmart parasites and improve fish welfare in aquaculture

“…Behavioural resistance in wild fish largely comes in the form of spatial and temporal avoidance of parasites. Individuals can detect and avoid parasite-risky habitats, infected conspecifics or sick prey (Barber 2005), and groups can make better avoid parasites, indicating the benefit of social interaction and group behaviour (Mikheev et al 2013). Several major fish parasites have specific depthrelated infection patterns.…”

Shoaling with infected conspecifics does not improve resistance to trematode infection

“…For example, social interactions between group members increase the risk of contracting contagious parasites (Alexander, 1974;Côté & Poulin, 1995;Kappeler et al, 2015;Patterson & Ruckstuhl, 2013;Rifkin, Nunn, & Garamszegi, 2012). On the other hand, the risk of infection with non-contagious parasites acquired from the environment can be lower in groups due to a decreased per capita attack rate with increasing group size (dilution effect, Poulin & FitzGerald, 1989;Mooring & Hart, 1992), or due to improved parasite avoidance, possibly through increased vigilance and information sharing (Mikheev, Pasternak, Taskinen, & Valtonen, 2013;Stumbo, James, Goater, & Wisenden, 2012).…”