Effects of crayfish density, body size and substrate on consumption of lake sturgeon (Acipenser fulvescens Rafinesque, 1817) eggs by invasive rusty crayfish [(Orconectes rusticus (Girard, 1852)]

The density and size of benthic predators such as crayfish and the physical characteristics of aquatic habitats including benthic substrate size have been hypothesized to significantly affect levels of lake sturgeon (Acipenser fulvescens) egg consumption. The invasive rusty crayfish (Orconectes rusticus) was used to quantify consumption and apportion variation in egg loss in an experimental setting. Experiments consisted of large flow-through rectangular tanks (0.8 m wide, 2.96 m(2) total bottom surface area) receiving river water to mimic natural diel temperature regimes. Four crayfish densities were tested (5, 10, 15, 30 individuals or 1.7, 3.4, 5.1, 10.1 individuals/m(2)) that span the range of densities observed in natural settings. Crayfish were allowed to freely move among three substrates commonly found at spawning locations where lake sturgeon eggs are deposited (cobble, gravel and sand). After an acclimation period (15min), 200 lake sturgeon eggs were randomly scattered over each substrate category (600 eggs per tank). Mean (+/- SE) egg consumption over 48hr was 80 +/- 3% (range 19-100% across 16 replicate trials). Egg consumption increased significantly as a function of increasing crayfish density. The effect of substrate size, however, depended significantly on crayfish density (substrate sizexpredator density interaction). Egg consumption significantly increased as substrate size decreased (e.g. from gravel to sand), at both high and low crayfish densities. Egg consumption also increased with increasing mean crayfish body size. Our results revealed that levels of lake sturgeon egg consumption by rusty crayfish are high in some settings. Further work is needed to determine if egg consumption in natural river settings is similarly high and whether egg consumption by crayfish and other egg predators could negatively affect population levels of recruitment.