Pacific Salmon (Oncoryhnchus spp.) and Brown Trout (Salmo trutta) are non-native species actively stocked in the Great Lakes. Since the introduction of these species, native Brook Trout (Salvelinus fontinalis) have declined in abundance in tributary streams for unknown reasons. Potamodromous species, like salmon, accumulate nutrients and energy while maturing in lakes, and transfer this material to lake tributaries where they spawn and die. Within the native range of salmon, the provision of salmon-derived resources can increase growth and change the isotopic composition of resident salmonids. However, the influence of salmon-derived material on resident fish outside their native range is unclear. We addressed whether salmon tissue has differential effects on the growth and isotopic composition of co-occurring Brook and Brown Trout. Hatchery age-0 Brook and Brown Trout were reared for 7 weeks in flow-through mesocosms with regular rations of salmon tissue augmented by bloodworms. Fish mass and length were measured weekly, while carbon and nitrogen isotope ratios were determined for all food sources and fish at conclusion of the experiment. Randomized-block ANOVA was used to assess factors of salmon tissue (present/absent) and Brown Trout (present/absent). Response variables were growth using length (mm/day) and mass (g/day), and δ15N and δ13C (‰).
Results/Conclusions
Growth rate of Brook Trout was not altered by presence of salmon tissue or Brown Trout (p>0.05). Growth rate of Brook Trout was significantly higher than Brown Trout (p<0.001), but was not modulated by salmon tissue presence. Brook and Brown Trout were both significantly 15N-enriched and 13C-depleted in salmon tissue presence (p<0.001), but isotopic signatures did not differ between species. Strong differentiation in isotopes but not growth between treatments suggests salmon tissue was readily incorporated into bodies of both species, but did not provide a differential resource subsidy for either species. Thus, observed growth responses of resident fish in salmon native range are likely not a result of direct consumption of salmon tissue. Rather, the biochemical makeup of salmon material may influence growth responses (e.g., lipid-rich eggs versus protein-rich tissue). Higher growth rates of Brook Trout compared to Brown Trout may reflect a generalist feeding strategy, which favors rapid growth but reduced lifespan. Alternatively, cooler water temperatures (11.0±1.8°C) in mesocosms may have favored Brook Trout growth. Overall, our results have implications for the influence of introduced salmon on stream-resident fish, the role of the environment in modifying those effects, and interactions between non-native and native fish in Great Lakes tributaries.