Combined effects of flow condition and parasitism on shoaling behaviour of female guppies Poecilia reticulata

Abstract

Group living in fish can provide benefits of protection from predators and some parasites, more efficient foraging for food, increased mating opportunities and enhanced energetic benefit when swimming. For riverine species, shoaling behaviour can be influenced by various environmental stressors, yet little is known how flow rate might influence the shoaling of diseased fish shoals. In view of the increasingly unpredictable flow rates in streams and rivers, this study aimed to assess the combined effect of flow condition and parasitism on the shoaling behaviour of a model fish species. Shoal size, shoal cohesion and time spent shoaling of female guppies Poecilia reticulata were compared when infected with the directly transmitted ectoparasite Gyrodactylus turnbulli under flow and static conditions. Flow condition was an important factor in influencing shoaling behaviour of guppies with the fish forming larger shoals in the absence of flow. When a shoal member was infected with G. turnbulli, shoal cohesion was reduced, but the magnitude of this effect was dependent on flow condition. In both flow and static conditions, bigger fish formed larger shoals than smaller counterparts. Future changes to stream hydrology with more frequent flooding and drought events will affect the shoaling tendency of fish. During high-flow events, diseased fish may not be able to keep up with shoal mates and therefore have a higher risk of predation. Additionally, these findings may be important for aquaria and farmed species where an increase in flow rate may reduce aggregation in fish.

Keywords: Cohesion; Flow rate; Flume; Nearest neighbour distance; Shoal size; Social groups.

Fig. 1

Size of guppy Poecilia reticulata shoals in flow (closed circles) and static (open circles) conditions. The predicted data trends generated by the top-scoring generalised linear mixed model are shown for flow (solid line) and static (dashed line) conditions

Fig. 2

Mean shoal cohesion (nearest neighbour distance) in guppy Poecilia reticulata shoals uninfected (open circles) and infected (closed circles) with Gyrodactylus turnbulli in flow and static conditions. Error bars show 95 % confidence intervals. Plots in a show overall changes in shoal behaviour in response to member being infected and b show change in shoaling behaviour for individual infected fish