Fine-Scale Dynamics of Lake Erie's Hypoxic Zone and Associated Fish Responses

In systems that are seasonally affected by hypoxic bottom waters, such as Lake Erie, population assessments may be influenced by anomalous high catch rates of particular species.  Although water quality data usually indicates normoxic bottom conditions, we hypothesized that such large catches were caused by aggregation of fish in marginal habitats due to avoidance of low oxygen.  To address this hypothesis we quantified epi- and hypo-limnetic spatial distribution of fishes across a depth gradient and associated ecotone of hypoxia in the central basin of Lake Erie.  We used a combination of hydroacoustic surveys, bottom trawls, and mid-water trawls to characterize spatial patterns for individual species, fish assemblage structure, and total fish biomass.  We examined diel migration effects with paired daytime-nighttime surveys in both August (during hypoxic bottom conditions) and September (during normoxic bottom conditions).  Our findings demonstrated that the hypoxic zone is even more dynamic than anticipated (varying from hypoxic to normoxic on sub-daily time scales) and that fish respond to hypoxia by moving both vertically and laterally.  Results from this work help refine statistical sampling strata thresholds based on dissolved oxygen, catch rate variance, and the probability of large catches that are important to ongoing juvenile fish assessment surveys.