Dynamic Hypoxic Zones in Lake Erie Compress Fish Habitat Altering Vulnerability to Fishing Gears

Development and persistence of a large (~5,000-10,000 km²) hypoxic zone in the central basin of Lake Erie typically occurs below the thermocline during August and September.  Recent observations indicate that significant hypoxic zones may also develop in adjacent areas.  Further, anomalous high fish catches in survey trawls and the commercial fishery appeared to be related to spatial variation in hypoxia.   To assist managers with population modeling and evaluation of decision rules for stock assessment data, we examined fine-scale spatial and temporal variability in hypoxia as it relates to fish distribution and catches.  Temperature and dissolved oxygen loggers placed along depth gradients at 4 transects in central Lake Erie revealed much higher than expected variability.  Hypoxic episodes were highly variable and frequently punctuated by periods of normoxia, with little coherency across transects.   High-resolution snap-shots of water quality and hydroacoustics in focused study areas supported the notion that fish habitat is compressed, resulting in higher densities near hypoxia.  Finally, proximity to hypoxia explained significant variation in trawl survey and commercial catches.  Our results provide insights into how hypoxia affects the statistical properties of fishery data via compression of habitat.