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Catch rates of Yellow Perch Perca flavescensin standard gill‐net surveys conducted by the Minnesota Department of Natural Resources have declined since 1970, but it is unclear whether this trend was due to reduced abundance or to changes in size structure affecting catchability. Because the minimum capture length of standard gill nets may limit effective sampling of some populations, the objectives of this study were to determine if length‐based metrics could be used to characterize populations and their susceptibility to standard gill nets, if these metrics were correlated with a suite of abiotic and biotic variables, and if temporal trends in these metrics were evident. Data were collected from a broad size distribution of Yellow Perch sampled with standard gill nets, boat electrofishing, and small‐mesh gill nets in 17 lakes. Estimated length at 50% maturity (L50) for females varied up to 100 mm across lakes, with values as small as 70 mm documented in some populations. Female L50 from supplemental gears was significantly positively correlated with the average length of the 15 largest fish (Lmax) from all gears, and both of these metrics were positively correlated with standard gill‐net catch. Exploratory analysis of female L50 indicated that this metric was positively correlated with latitude and lake size and negatively correlated with water clarity. Additionally, a statewide analysis of Lmaxconducted with standard gill‐net data from over 1,000 lakes found significant declines during the past 25 years, indicating that populations in Minnesota have likely shifted towards a smaller size structure, reducing catchability in standard gill nets. These results indicate that standard gill nets were ineffective for sampling some Yellow Perch populations and that length‐based metrics such as L50 and Lmaxcan be used to infer catchability in standard gear and to monitor populations through time.

Large-scale ecological stressors such as climate change, habitat degradation, and invasive species increasingly affect fisheries across watershed or political boundaries within which they are typically managed. Though broad-scale analyses of trends in fish populations and changes in aquatic communities can be useful for detecting the effects of these stressors, they are rarely possible due to a lack of long-term data sets. The Minnesota Department of Natural Resources has used gill nets and trap nets to monitor fish communities in lakes since 1940, and the techniques have been standardized since 1970. We applied a random-coefficient mixed-effects model to mean CPUE data to describe statewide and local population trends of eight key fish species (Black Crappie Pomoxis nigromaculatus, Bluegill Lepomis macrochirus, Largemouth Bass Micropterus salmoides, Northern Pike Esox lucius, Smallmouth Bass M. dolomieu, Walleye Sander vitreus, White Sucker Catostomus commersonii, and Yellow Perch Perca flavescens) sampled from 1970 to 2013 in 119–1,499 lakes. We found increasing statewide trends in gill-net CPUE of Black Crappie, Bluegill, Largemouth Bass, Smallmouth Bass, Northern Pike, and Walleye and decreasing trends for Yellow Perch and White Sucker. Statewide trends in average trap-net CPUE were positive for Bluegill, negative for Black Crappie, Walleye, White Sucker, and Yellow Perch, and stable for Largemouth Bass, Northern Pike, and Smallmouth Bass. We compared the CPUE trends across lake types developed for Minnesota lakes and found some differences, especially in medium and large mesotrophic lakes, suggesting that some species display changes in abundance more in those lakes types than in others. This study quantifies temporal changes in Minnesota's fish community structure and can help managers focus future research and management activities in the state and in similar waters across central North America. Received May 14, 2014; accepted September 9, 2014

AbstractLarge-scale ecological stressors such as climate change, habitat degradation, and invasive species increasingly affect fisheries across watershed or political boundaries within which they are typically managed. Though broad-scale analyses of trends in fish populations and changes in aquatic communities can be useful for detecting the effects of these stressors, they are rarely possible due to a lack of long-term data sets. The Minnesota Department of Natural Resources has used gill nets and trap nets to monitor fish communities in lakes since 1940, and the techniques have been standardized since 1970. We applied a random-coefficient mixed-effects model to mean CPUE data to describe statewide and local population trends of eight key fish species (Black Crappie Pomoxis nigromaculatus, Bluegill Lepomis macrochirus, Largemouth Bass Micropterus salmoides, Northern Pike Esox lucius, Smallmouth Bass M. dolomieu, Walleye Sander vitreus, White Sucker Catostomus commersonii, and Yellow Perch Perca flavescens) sampled from 1970 to 2013 in 119–1,499 lakes. We found increasing statewide trends in gill-net CPUE of Black Crappie, Bluegill, Largemouth Bass, Smallmouth Bass, Northern Pike, and Walleye and decreasing trends for Yellow Perch and White Sucker. Statewide trends in average trap-net CPUE were positive for Bluegill, negative for Black Crappie, Walleye, White Sucker, and Yellow Perch, and stable for Largemouth Bass, Northern Pike, and Smallmouth Bass. We compared the CPUE trends across lake types developed for Minnesota lakes and found some differences, especially in medium and large mesotrophic lakes, suggesting that some species display changes in abundance more in those lakes types than in others. This study quantifies temporal changes in Minnesota's fish community structure and can help managers focus future research and management activities in the state and in similar waters across central North America.Received May 14, 2014; accepted September 9, 2014

AbstractWhite Perch Morone americanahave been introduced into many inland systems throughout the United States. To determine factors affecting White Perch abundance and size structure, we compared White Perch growth, timing of maturity, and trophic level; the abundance of a predator (Largemouth Bass Micropterus salmoides); the abundance of an ecologically significant mid-level omnivore (Gizzard Shad Dorosoma cepedianum); prey availability (chironomid and zooplankton abundances); and environmental variables (specific conductivity, Secchi depth, dissolved oxygen concentration, and temperature) among four reservoirs (two with high White Perch abundance and two with low abundance). White Perch size structure was closely tied to abundance, with truncated size structure as abundance increased. Among the other variables we tested, only Largemouth Bass abundance had a significant (negative) relationship with White Perch abundance. White Perch size structure appeared to be highly density dependent, and variables that commonly explain variation in abundance of introduced fishes did not explain differences in the four White Perch populations we studied. Further study of the competitive and predatory interactions of White Perch and Largemouth Bass over ontogeny could shed light on the mechanism(s) potentially shaping population structure of the two species where they coexist.Received June 28, 2013; accepted July 24, 2013

White Perch Morone americanahave been introduced into many inland systems throughout the United States. To determine factors affecting White Perch abundance and size structure, we compared White Perch growth, timing of maturity, and trophic level; the abundance of a predator (Largemouth Bass Micropterus salmoides); the abundance of an ecologically significant mid-level omnivore (Gizzard Shad Dorosoma cepedianum); prey availability (chironomid and zooplankton abundances); and environmental variables (specific conductivity, Secchi depth, dissolved oxygen concentration, and temperature) among four reservoirs (two with high White Perch abundance and two with low abundance). White Perch size structure was closely tied to abundance, with truncated size structure as abundance increased. Among the other variables we tested, only Largemouth Bass abundance had a significant (negative) relationship with White Perch abundance. White Perch size structure appeared to be highly density dependent, and variables that commonly explain variation in abundance of introduced fishes did not explain differences in the four White Perch populations we studied. Further study of the competitive and predatory interactions of White Perch and Largemouth Bass over ontogeny could shed light on the mechanism(s) potentially shaping population structure of the two species where they coexist.