Hafen, K. and P. Budy. 2015 Agonistic behavior among three stocked trout species. North American Journal of Fisheries Management, Management Brief: 35:3, 551-556. USGS FSP: IP-058150.
Among salmonids, agonistic behavior is a means to increase an individual’s fitness by obtaining resources, defending resources, and establishing dominance over other individuals. Such behavior may also allow one species to gain growth or competitive advantage over others at a community level. In Scofield Reservoir, Utah, Bear Lake cutthroat trout (Oncorhynchus clarkii utah) and tiger trout (Salmo trutta, female X Salvelinus fontinalis, male) were added to the rainbow trout (O. mykiss) stocking regimen as potential biological controls for Utah chub (Gila atraria), creating an unusual complex of predatory species. A study evaluating the feeding ecology of the three trout species in the reservoir indicated substantial niche overlap between tiger trout and cutthroat trout at top tropic positions, and overlap between rainbow trout and Utah chub at lower positions, raising concern that community interactions may result in an overall decrease of trout condition. To evaluate the potential for negative interactions among these three species, we designed laboratory experiments to assess agonistic behavior. We determined differences in agonistic behavior between rainbow trout, cutthroat trout, and tiger trout with replicated, pair-wise treatments of one fish of each species together in a tank at different densities. We then observed the number of chases and contacts occurring between species, and considered density and relative length differences in our analysis. In all treatments rainbow trout initiated significantly more aggressive interactions than tiger trout or cutthroat trout (p<0.001). Feeding did not have a significant effect on agonistic behavior for any species; however differences in density and relative fish size appear to influence the degree of aggressive behavior. Though agonistic behavior has benefits, its tradeoffs may decrease rainbow trout condition in natural environments.