ABSTRACT
Water clarity increased in nearshore areas of western Lake Erie by the early-1990s mainly as a result of the ltering activities of dreissenid mussels (Dreissena spp.), which invaded in the mid-1980s. We hypothesized that increased water clarity would result in greater trawl avoidance and thus reduced ability to capture sh in bottom trawls during daytime compared to nighttime. We examined this hypothesis by summarizing three analyses on sh data collected in western Lake Erie. First, we used a two-tiered modeling approach on the ratio (R) of catch per hour (CPH) of age-0 yellow perch (Perca flavescens Mitchill) at night to CPH during daytime in 1961-2005. The best a priori and a posteriori models indicated a shift to higher CPH at night (R > 1) between 1990 and 1991, which corresponded to 3 years after the dreissenid invasion and when water clarity noticeably increased at
nearshore sites. Secondly, we examined effects of nighttime sampling on estimates of abundance of age-2 and older yellow perch, which form the basis for recommended allowable harvest (RAH). When data from night sampling were included in models that predict abundance of age-2 yellow perch from indices of abundance of age-0 and age-1 yellow perch, predicted abundance was lower and model precision, as measured by r-squared, was higher compared to models that excluded data collected at night. Furthermore, the use of only CPH data collected at night typically resulted in lower estimates of abundance and more precise models compared to models that included CPH data collected during both daytime and nighttime. Thirdly, we used presence/ absence data from paired bottom trawl samples to calculate an index of capture probability (or catchability) to determine if our ability to capture the four most common benthic species in western Lake Erie was affected by dreissenid-caused
Martin A. Stapanian and Patrick M. Kocovsky
CONTENTS
Abstract .................................................................................................................................................................................681 Introduction .......................................................................................................................................................................... 682 Methods ............................................................................................................................................................................... 682
Field Collections and Study Area ................................................................................................................................... 682 Data Analysis .................................................................................................................................................................. 683
Diel Shift in CPH of Age-0 Yellow Perch .................................................................................................................. 683 Effects of Diel Shift on Management of Yellow Perch .............................................................................................. 684 Diel Shifts in Catch Probability Indices of Other Benthic Fishes ............................................................................. 684
Results .................................................................................................................................................................................. 685 Diel Shift in CPH of Age-0 Yellow Perch ...................................................................................................................... 685 Effects of Diel Shift on Management of Yellow Perch ................................................................................................... 686 Diel Shifts of Catch Probability Indices of Benthic Fishes ............................................................................................ 687
Discussion ............................................................................................................................................................................ 688 Acknowledgments ................................................................................................................................................................ 690 References .............................................................................................................................................................................691
