Stocking hatchery-raised rainbow trout (RBT) in lakes is a popular practice in the United States and elsewhere, but it has the potential to cause top-down effects that promote eutrophic conditions. As visually-oriented predators, RBT preferentially consume large-bodied zooplankton grazers (e.g., Daphnia pulicaria) and when populations of these grazers are suppressed, algae concentrations increase and water clarity decreases. This study uses a paleoecological approach to examine ecological changes in Square Lake (Minnesota, USA), a lake that has become more eutrophic over the past several decades during which RBT have been annually stocked. To evaluate the impact of the RBT stocking program we compared zooplankton remains (exoskeletons and resting eggs) of three crustacean zooplankton species (Bosmina, D. mendotae, and D. pulicaria) in the sediment records of Square Lake and a reference lake that has never been stocked with RBT. Cores from the two lakes were Pb210-dated and analyzed for the zooplankton remains. We hypothesized that the sizes and relative abundance of the remains of the large-bodied Daphnia (D. pulicaria) in Square Lake sediments would decrease since the initiation of the RBT stocking program (1981), while we did not expect to see these patterns in the sediment samples from the reference lake.
Results/Conclusions
Results of this study largely support our expectations regarding the effects of RBT predation in Square Lake since 1981. We found that 1) D. pulicaria body size (inferred from sizes of post-abdominal claws and ephippia) decreased in sediments from Square Lake after 1981, but not in the reference lake; 2) the abundance of D. pulicaria relative to the smaller-bodied D. mendotae (inferred from post-abdominal claw data) decreased in the Square Lake record, but not in the reference lake. Interestingly though, when ephippia were used as the proxy for Daphnia abundance the expected shift in species composition in Square Lake sediments since 1981 was not observed. In deep lakes (like those in this study) that support perennial populations of Daphnia, sexual reproduction events in which ephippia are produced are known to be episodic. Therefore, ephippia abundance in sediments may not be a valid proxy for assessment of population size. Instead, exoskeletal remains (e.g., post-abdominal claws) generated by these animals irrespective of reproductive mode are likely to be more accurate indicators of past population densities. Findings from this research are consistent with a recent monitoring study of Square Lake and will inform fisheries managers about the ecological impacts of RBT stocking programs.