Harmful algal bloom effects on fish habitat use and community structure in western Lake Erie

Harmful algal blooms dominated by cyanobacteria have been increasing in frequency, duration, extent, and intensity in freshwater ecosystems worldwide, yet our understanding of their impact on aquatic foodwebs remains limited. This study aims to generate ecological insights that could benefit fisheries management in freshwater ecosystems experiencing harmful algal blooms, we conducted daytime and nighttime trawling (bottom and midwater) and hydroacoustic surveys inside and outside of blooms in western Lake Erie during July-August 2019. We evaluated if prey fishes use harmful algal blooms during the day as a refuge from predation and avoid them at night due to darkness and potential cyanotoxins. We also examined if visually feeding piscivorous fishes avoid these blooms due to low water clarity and high cyanobacteria concentration that could impair foraging. We did not find differences in total fish catch (in both types of trawls) or total fish density (estimated by down-looking acoustics) between harmful algal blooms and non-harmful algal blooms sites. Similarly, we found few differences in the use of harmful algal blooms among feeding/habitat guilds. Walleye, Sander vitreus, an important piscivore, and abundant benthivores (Yellow Perch Perca flavescens, White Perch Morone americana, and Trout-perch Percopsis omiscomaycus) did not vary between harmful- and non-harmful algal bloom sites, regardless of when sampling occurred or trawl type (bottom or midwater). While the catch of pelagic fishes preferred as prey by Walleye (i.e. Rainbow Smelt Osmerus mordax, Gizzard Shad Dorosoma cepedianum, and Notropis spp.) was higher inside of HABs relative to outside, this difference was only observed in bottom trawls (not midwater trawls). Our acoustics data did show, however, that harmful algal blooms modified the vertical distribution of fish. Specifically, fish were located deeper in the water column in non-harmful algal bloom sites during the day relative to night, with this difference disappearing inside of harmful algal blooms. In addition to explaining the likely mechanisms responsible for the extensive use of harmful algal blooms by Lake Erie fishes, we discuss implications for fisheries management during the harmful algal blooms season.