Abiotic and biotic factors related to growth of nonnative Walleyes in Lake Pend Oreille, Idaho

Abstract Objective Understanding factors influencing growth is vital to effective management of fish populations. This study investigated potential factors influencing growth of an established nonnative population of Walleyes Sander vitreus in the Lake Pend Oreille system in northern Idaho. Methods We used relative growth index to describe growth of Walleyes relative to populations across North America. Mixed-effects modeling was used to relate growth to abiotic (i.e., mean summer water temperature, river inflow) and biotic (i.e., kokanee Oncorhynchus nerka abundance and biomass; opossum shrimp Mysis diluviana density) variables. Models were ranked using Akaike's information criterion corrected for small sample size. Individual variability in growth was related to diet represented by stable isotopes (i.e., δ15N, δ13C) using linear regression for age-1, age-2, age-3, and age-5 individuals. Subsequently, for each age-class, we evaluated differences in δ15N and δ13C between fast-growing (i.e., 75th and higher percentiles of growth) and slow-growing (i.e., 25th and lower percentiles of growth) individuals. Result Walleyes grew fast relative to other populations. Growth of Walleyes was positively associated with temperature as well as abundance and biomass of kokanee; growth was unrelated to river inflow and opposum shrimp density. The top model indicated that growth of Walleyes was related to temperature and kokanee abundance. An equally plausible model explaining growth consisted of only temperature. Generally, fast-growing Walleyes exhibited higher δ15N than slow-growing individuals. Similarly, δ13C was more depleted in the fast-growing individuals for all ages except age 1. This suggests that fast-growing individuals likely consumed prey items at higher trophic positions and fast-growing Walleyes (except age 1) used higher proportions of pelagic prey items. Conclusion This study showed that kokanee abundance and temperature likely influenced growth of Walleyes in the Lake Pend Oreille system. Additionally, variability in growth was apparently related to variability in diet, particularly for age-1 Walleyes.