IMPACTS OF ZOOPLANKTON ABUNDANCE AND COMMUNITY COMPOSITION ON LARVAL FISH HABITAT QUALITY AND LARVAL COREGONID DIETS
Offshore spring zooplankton biomass in northern Lake Michigan is currently dominated by calanoid copepods and lacking in cyclopoid copepod biomass, which is a preferred prey type for larval coregonids in the Great Lakes. As such, we survey nearshore beach zooplankton to determine if nearshore environments are following offshore trends and describe larval coregonid diets and prey selectivity during spring 2015-2019. Copepod nauplii and calanoid copepods were the most abundant prey types in the nearshore beach environments, and although larvae did not consume copepod nauplii, larvae did incorporate substantial later life stages of calanoid copepods into their diet. Additionally, larvae exhibited positive selectivity for both calanoid and cyclopoid copepods. High consumption of and selectivity for calanoid copepods in small larval coregonids is not a common observation in the Great Lakes, as previous diets of small larvae have been comprised of mainly cyclopoid copepods and cladocerans. Future research into the foraging costs and nutritional benefits of larval diets with differing ratios of cyclopoid to calanoid biomass should be investigated to understand the potential impacts of differing diets on larval growth and survival.
Many species of larval fish in Lakes Michigan and Huron experience a prolonged offshore pelagic stage as during early life, but environmental conditions in offshore environments have changed in the past quarter century. Under current conditions, offshore habitats may be unsuitable for larval fish, given recent increases in offshore water clarity and decreases in offshore primary production and zooplankton biomass in both lakes. To evaluate habitat suitability for larval fish, we characterized ambient environmental conditions using data streams from multiple high frequency sensors to develop growth rate potential models as an index of habitat quality of nearshore-offshore transects in central Lake Michigan and southern Lake Huron. Since temperature regimes differ throughout the summer, we compare habitat quality throughout the summer for larval yellow perch and smelt, two species that have demonstrated offshore pelagic stages as larvae. Early in the summer, high habitat quality was concentrated nearshore, while later in the summer high habitat quality was concentrated near the thermocline, at depths where larvae are unlikely to inhabit due to limited swimming ability. The offshore (15-60 m depth contour) surface waters of both lakes provide poor habitat quality in May and July, although the surface waters of transects collected during June in southern Lake Huron provided high quality habitat at all distances from shore. These results suggest that offshore advection and prolonged offshore pelagic stage duration during early life of fish could contribute to decreased growth and survival of larval fish in lakes Michigan and Huron.