The role of water clarity in structuring niche dimensions and overlap between smallmouth bass and walleye

Abstract

Smallmouth bass (Micropterus dolomieu) have experienced substantial range expansions in northern Ontario over the past century, with adverse consequences for native salmonid predators. It is unclear how climate-induced water clarity shifts will affect interactions between native dark-adapted walleye (Sander virteus) and invading smallmouth bass in northern Ontario. This study used stable isotopes of nitrogen (15N/14N) and carbon (13C/12C) in fish muscle tissue to investigate how resource partitioning between walleye and smallmouth bass is related to water clarity in 34 small (100-200 ha) Boreal Shield lakes (ranging from 1 to 8.5 m Secchi depth, and from 3.2 to 13.1 mg/L DOC). Quantitative metrics of trophic niche dimensions (based on the size, position, and dispersion of multivariate ellipses drawn around sampled individuals in δ15N vs. δ13C biplot space) were calculated for each individual population and used to determine trophic interactions and niche overlap between sympatric walleye and smallmouth bass. Linear and multiple regressions were then used to explore potential relationships between trophic interactions and water clarity. Available habitat and fish assemblage data was also explored for potential influences on isotopic niche dimensions and trophic interactions of walleye and smallmouth bass. Total occupied niche space decreased significantly with increasing water clarity for smallmouth bass as populations occupied a narrower range of trophic levels and made greater use of pelagic resources. In contrast, walleye trophic niche dimensions did not respond significantly to water clarity. Isotopic niche overlap ranged from 0 to 65%, but no metric of trophic overlap was significantly related to water clarity. Other abiotic and biotic variables, however, did have a significant influence on the similarity between some aspects of walleye and smallmouth iv bass isotopic niche dimensions (but not interaction). Both species fed on more isotopically similar carbon sources as lakes became shallower, which may be a result of a lack of distinct littoral and limnetic habitats. Walleye and smallmouth bass niche size also became more similar with increasing DOC but this may be a result of lower prey diversity. Both species also occupied more similar average food web positions as yellow perch relative abundance increased, indicating that the exploitation of yellow perch by both species increased with perch abundance. Together with other studies that have found little evidence for an impact of smallmouth bass on walleye fitness and abundance, this research suggests that, unlike salmonid and cyprinid species, walleye may be resilient against smallmouth bass invasions regardless of water clarity conditions in oligotrophic boreal lakes.