Structure of Aggressive Behavior in Underyearlings of the Rainbow Trout Oncorhynchus mykiss (Salmonidae) Changes under the Influence of Diplostomum pseudospathaceum (Trematoda) Parasites

The dynamics of aggressive behavior of the underyearlings of the rainbow trout Oncorhynchus mykiss infected with metacercariae of the trematodes Diplostomum pseudospathaceum has been studied in an experimental heterogeneous environment for 48 days. The fish competed for the dark area of the bottom by charges, biting, chasing, and frontal and lateral displays. The sharply increased aggressiveness of fish when the parasites became infective (1.0–1.5 months after infection), i.e., capable of infecting the final host (piscivorous birds), manifested itself in an increased frequency of bites (more than 20 times compared to the control) and charges (almost three times) and was accompanied by fast movements, sharp maneuvers, and low ability to compete for shelters. The share of comparatively slow displays and chasing was much smaller. The strategy of D. pseudospathaceum manipulation of fish behavior consists of two stages: (1) protecting the host from predators; (2) increasing the vulnerability of the host. At the first stage, the general level of aggression decreases sharply; at the second, on the contrary, aggressiveness rapidly increases, but the ratio of different categories of aggression changes. The share of categories that make the host more vulnerable to predators increases. High activity and low competitiveness of infected fish contribute to their rapid extermination by predators or moving to another habitat. Such migration of fish can increase the fitness of the parasite, facilitating its dispersal in the water body.