Estimating Catchability of North American and Fall Walleye Index Netting Gillnetting Standards for Walleye

The Provincial Government of Ontario has implemented a large scale-study to measure the specific catchability (i.e., relationship between catch-per-unit-effort (CPUE) and fish density (kg/ha)) of the North American (NA) and Fall Walleye Index Netting (FWIN) gillnetting standards for walleye (Sander vitreus). The study design acknowledged factors that may influence catchability (e.g. seasonality and climate), and paired mark-recapture estimates of fish density/biomass with multiple across season NA surveys and a single FWIN survey. Catch-per-unit-effort was adjusted for gear selectivity, and both CPUE and density adjusted to a common size range (350mm+ total length).  Results indicated that walleye CPUE was correlated with estimates of biomass, with a catchability coefficient of 0.43 for NA and 1.0 for the FWIN standard. Although there was significant seasonal variability in NA CPUE within lakes, no consistent trend among the lakes was observed. Study of the catchability of NA gillnets for walleye in warmer and colder climates requires more collaborative inter-agency research, and Ontario is eager to develop partnerships with other agencies for this purpose.