Riverine drift communities during larval fish dispersal over multiple recruitment years

Predation during the downstream dispersal of larval stages represents a major recruitment bottleneck for fish populations. The co-occurrence of other organisms in river drift may reduce predation, but our knowledge of the factors influencing the composition and abundance of drift communities during post-hatch dispersal of larval fish remain limited. A multi-year (2011-2018) study was conducted to investigate abiotic factors influencing drift communities during larval lake sturgeon (Acipenser fulvescens) dispersal in the Upper Black River (Cheboygan, MI, USA). Cumulative water temperature was a strong predictor of drift for larval lake sturgeon, suckers (Catostomidae) and macroinvertebrates, with reduced macroinvertebrate drift during lunar phases with higher lunar illumination. Nights with a new moon had on average, three times the drifting macroinvertebrate biomass as nights during a full moon, with Heptageniidae and Isonychiidae displaying higher abundances during lower light conditions. Favorable conditions for other taxa to reduce larval lake sturgeon predation through predator swamping were common, though variability in timing (overlap between drifting taxa), biomass, and abundances likely alters the strength of such effects among years. A better understanding of the conditions influencing drifting communities during larval fish dispersal may assist in predicting larval mortality and year-class strength for managed fish populations.