Trends in abundance and mean size of fish captured in gill nets from Minnesota lakes, 1983-1997

We investigated seasonal and 15-year trends in catch per unit effort (CPUE) and mean weight of black crappies Pomoxis nigromaculatus, ciscoes Coregonus artedi, northern pike Esox lucius, walleyes Sander vitreus (formerly Stizostedion vitreum), white suckers Catostomus commersoni, and yellow perch Perca flavescens captured in gill nets from 433 Minnesota lakes between 1983 and 1997. Seasonal trends were modeled using least-squares linear and polynomial regressions, and interannual trends were investigated with a linear mixed-effects model and an augmented serially alternating design. Four of the six species exhibited significant seasonal trends. Walleye CPUE increased linearly, whereas walleye and cisco mean weight and northern pike CPUE decreased linearly from spring through fall. Cisco CPUE and yellow perch mean weight exhibited curvilinear relationships, decreasing from spring through summer and then increasing from summer through fall. Two species exhibited significant 15-year trends across Minnesota from 1983 to 1997: Walleyes experienced significant positive trends in both CPUE and mean weight, probably benefiting from more efficient stocking or warmer June temperatures, and black crappies experienced a significant decreasing trend in CPUE, possibly due to replacement by walleyes, loss of spawning habitat, or increased harvest by recreational anglers. Catches of ciscoes, northern pike, white suckers, and yellow perch in gill nets were unchanged across Minnesota between 1983 and 1997, indicating population stability. The results of this study suggest that lakes should be sampled with gill nets at the same time of year or that catches should be corrected to a common date to allow direct comparisons among lakes. The monitoring of trends in fish populations and habitat variables using standardized sampling on broad spatial scales should be more widely used to detect changes, generate hypotheses regarding these changes, and focus research and management efforts.