Improving fishery-independent indices of abundance for a migratory walleye population

Abstract

The primary goal of many biological surveys is to provide an unbiased representation of trends of population abundance. However, there are often factors other than abundance that vary over time and influence catch rates and thus inferences about population trends from surveys. This is particularly true for highly mobile species because of interannual variation in the timing, extent, and duration of movements, and for surveys that are not randomized with respect to space and time. We developed general and generalized linear mixed models to standardize Canadian and United States fishery-independent surveys that provide an index of basin-level walleye (Sander vitreus) population abundance trends in Lake Erie (1983–2008). In Canadian waters, the probability of a non-zero catch was associated with the type of gill net set (canned>bottom sets), the presence of hypoxia (negative trend; −), and secchi depth (−). Positive catch rates were associated with the set type (canned>bottom) and water depth (+). In United States waters, survey catch rates were associated with secchi depth (−) and surface water temperature (+). For each case, the best model included random effects (interactions between year, week, basin, sub-basin) which accounted for a modest amount of the total variation. General abundance trends were similar between the standardized and nominal indices, but substantial annual variation in the direction and magnitude of the difference between indices was observed. We recommend the use of standardized indices for walleye population assessments because these account for factors influencing catch rates other than changes in abundance.