Gill-Net Saturation in Lake Erie: Effects of Soak Time and Fish Accumulation on Catch per Unit Effort of Walleye and Yellow Perch

Abstract

Gill-net saturation was analyzed through a delta model (i.e., two-stage model) by examining the effects of soak time and fish accumulation (number of fish of all species enmeshed per square meter of a given gill net, including the species of interest) on catch per unit effort (CPUE) of walleyes Sander vitreus and yellow perch Perca flavescens in Lake Erie. The analysis was based on fishery-independent survey data for 1989–2003. In the delta model, the positive values of CPUE were estimated by a generalized additive model (GAM) assuming a log-gamma distribution, and the probability of obtaining nonzero values of CPUE was estimated by a GAM assuming a binomial distribution. Soak time and fish accumulation had significant effects on CPUE. The CPUE of walleyes decreased in gill nets soaked for 10 h and started to decline when fish accumulation was around 2 fish/m². We did not observe a substantial decline in the CPUE of yellow perch within the soak time interval we examined, but we did observe a decline when fish accumulation was 6–8 fish/m². The decline in CPUE with increasing soak time for walleyes and with increasing fish accumulation levels for both walleyes and yellow perch indicates that gill-net saturation did exist in Lake Erie gill-net surveys for these two species and that the gill nets were saturated faster by walleyes than by yellow perch. We suggest that gill-net saturation be considered when applying CPUE from gill-net surveys to stock assessment and that the generalized linear additive-based modeling approach be considered as an alternative in gill-net saturation analyses.

Received March 11, 2010; accepted January 6, 2011

ACKNOWLEDGMENTS

This research was supported by grants from Virginia Polytechnic Institute and State University; the U.S. Department of Agriculture Cooperative State Research, Education, and Extension Service; Hatch Project 0210510; and the Ontario Commercial Fisheries' Association to Y.J.

Notes

^aNumber of fish of all species enmeshed per square meter of a given gill net, including the species of interest.

^bWater depth at which the gill net was set.

^cWater temperature at the depth at which the gill net was set.

^dWater temperature at the surface.

^eCanned nets are suspended in the water, bottomed nets on the bottom.