Early life history and community structure of large river fishes

Independent investigations of early life history, habitat selection, trophic status, and community stability were used in an effort to integrate community-level perspectives into studies of fishes. Three studies are presented that collectively address the major phases of fish ontogeny: eggs and larvae, young-of-the-year, and adults. A fourth study describes a methodological advancement in the use of stable isotopes to discern trophic relationships within a fish community. Whether egg quality (nutrient content) affects subsequent larval performance (chapter 2) was examined to better understand ramifications of differences in the timing and distribution of fish spawning. Egg quality in northern pike increased with egg mass, but no relationship was found with egg diameter or female body size. Larger larvae were produced from eggs of greater mass (r2 = 0.58). The second study (chapter 3) examined the habitat utilization of native young-of-the-year (YOY) muskellunge ( Esox masquinongy) relative to the fish community and vegetative structure in the Upper St. Lawrence River. Muskellunge YOY CPUE was positively correlated with prey fish abundance and vegetation density, but negatively associated with yellow perch (Perca flavescens) abundance and Chara. The third study (chapter 4) analyzed the stability of the adult fish community (n = 19 species) in terms of its size-structure and species rank abundance indexed in the St. Lawrence River. While community size-structure was highly stable over time, relative abundance rankings showed large interannual variability. Multiple ecological perturbations, primarily double-crested cormorant (Phalacrocorax auritis) population expansion and decreased phosphorus loading were significantly correlated with declines in total fish abundances. The perturbations had greatest influence on fishes 200--400 mm in total length, but perturbation effects radiated throughout the entire fish assemblage. The final chapter (chapter 5) evaluated the effect of lipid extraction on the delta13C and delta15N isotopic values used to interpret food web relationships. Paired isotopically derived food webs for the St. Lawrence River were analyzed with lipid extracted and non-extracted samples. Extraction shifted the entire food web positively along both axes (by on average delta13C: 1.07‰ and delta 15N: 1.59‰), but did not influence relative trophic position among species.