Hypoxia's Impact on Fish Distributions and Population Estimates

Hypoxia (< 2 O₂ mg/L) is a growing issue of concern in both freshwater and marine ecosystems worldwide, owing to its ability to potentially impact food web and fishery production. Lake Erie is no exception, as stratification that results in hypolimnetic hypoxia in the offshore waters (>15 m depth contour) of its central basin during summer and early fall has been shown to elicit avoidance behavior in fishes that can aggregate individuals near the edges of the hypoxic zone. In turn, these resulting patchy distributions hold the potential to bias estimates of population size that are determined from fishery-independent sampling and which are pivotal to determination of commercial and recreational harvest quota allocations in this system.  Toward better understanding the potential of hypoxia to bias estimates of population size, we first quantified dissolved oxygen thresholds for species of ecological or economic importance in Lake Erie, including yellow perch (Perca flavescens), walleye (Sander vitreus), rainbow smelt (Osmerus mordax), white perch (Morone americana), round goby (Neogobius melanostomus), and trout-perch (Percopsis omiscomaycush).  Next, using yellow perch as a model species, we quantified how hypoxia could lead to bias in routine fishery-independent assessments, and in turn, population estimates.  Preliminary results indicate that 1) dissolved oxygen thresholds exist for most species of Lake Erie fishes examined, but that these thresholds were species-specific (ranging between 0.7 and 5.7 O₂ mg/L), and 2) biased sampling due to hypoxia can result in elevated estimates of yellow perch population size.  We ultimately provide recommendations for fishery-independent monitoring and harvest allocation in systems that experience hypoxia.