Autumn-to-Spring Energetic and Diet Changes among Kokanee from North Idaho Lakes with and without Mysis relicta

Abstract

Changes in the Pend Oreille Lake, Idaho, zooplankton community during spring and autumn have been attributed to planktivory by the opossum shrimp Mysis relicta. To examine how the altered zooplankton community affects kokanee Oncorhynchus nerka, we compared seasonal body lipids and feeding patterns of age-0 and age-1 individuals from Pend Oreille Lake with those of kokanee from Coeur d'Alene and Spirit lakes, Idaho, which did not contain M. relicta. We also contrasted our feeding pattern analysis with results of a similar study conducted in Pend Oreille Lake before M. relicta introduction. Age-0 Pend Oreille Lake kokanee showed no evidence of malnourishment, as body lipids and feeding patterns were generally similar among the three lakes. Age-1 Pend Oreille Lake kokanee had lower body lipids and stomach fullness than age-1 kokanee in the other lakes, suggesting that zooplankton community changes might affect older kokanee. We saw no evidence of direct starvation mortality in either Pend Oreille Lake year-class, since body lipids were higher than prior studies have reported for starving individuals. Seasonal kokanee feeding patterns were similar between age-classes but differed among lakes, Pend Oreille Lake individuals consuming a lower proportion of Daphnia during autumn compared with Coeur d'Alene Lake and Spirit Lake fish. Kokanee fed upon small copepod species during spring in all three lakes. Before M. relicta establishment in Pend Oreille Lake, kokanee in autumn ate a higher proportion of cladocerans, consumed a greater total prey biomass, and had fewer empty stomachs, but spring feeding patterns were more similar to those we observed. Our results show that seasonal changes in zooplankton availability in Pend Oreille Lake have probably changed age-1 kokanee foraging and energy accumulation patterns, particularly in autumn, although we cannot exclude the possibility that recent changes in predation intensity may also explain our results.