Risk taking in parasitized sticklebacks under threat of predation: effects of energetic need and food availability

Abstract

Threespine sticklebacks (Gasterosteus aculeatus L.) parasitized by the cestode Schistocephalus solidus (Miiller) have a greater need for energy than uninfected fish, and therefore should be hungrier and more willing to compromise safety from predation for foraging gains. We hypothesized that the magnitude of this trade-off is directly related to the fish's parasite load (i.e., energy requirement) and food abundance. After being frightened by a model heron, sticklebacks fled shorter distances, remained motionless and cryptic for shorter periods, returned sooner to forage in a food patch near the predator, and remained active longer in the patch with increasing parasite load. The correlations of these responses with fish parasite load were significant most consistently at the highest food density tested, and suggest that the fish were taking a greater risk of predation with an increasing level of parasitic infection, particularly when the potential energetic gain from foraging was high. On average, the fish stayed longer in the food patch near predation hazard and consumed more food during this period with increasing prey density in the patch. However, individual feeding rate was independent of parasite load and food density, suggesting that the parasite places a constraint on the fish's foraging effort.