Sampling low dissolved oxygen waters can change population estimates

Lake stratification, occurring predominantly from July through September, often results in hypoxic conditions in the offshore waters (>15 m) of the central basin of Lake Erie. Below certain thresholds, fish exhibit avoidance behavior and may aggregate on the edges of the hypoxic zone. Monthly bottom trawl surveys are used to estimate yellow perch young-of-the-year and yearling indices. Sampling in the hypoxic zone with bottom trawls can result in atypically low catch rates.  Adhering to sample design, where habitat becomes unsuitable, may invoke bias. Our first objective is to assess how hypoxic conditions affect the spatial distribution of yellow perch.  We will determine the dissolved oxygen thresholds for yellow perch and if fish are aggregating at hypoxic edges.  Secondly, we will assess how hypoxic conditions affect the yellow perch population estimates.  Some preliminary findings have shown that not sampling low dissolved oxygen areas could result in elevated estimates of indices and yellow perch populations.