Seasonal Influences on Fish Sampling Data for Standing Waters: A Case Study for Bluegill Population Assessments in Iowa Lakes and Impoundments

Seasonal fluctuations in relative abundance, size structure, and condition require the development of standard sampling methods for freshwater fish.  However, standard sampling methods often vary because of species targeted, region (e.g., climate), and water body size.  We evaluated the influence of season (i.e., spring, summer, fall) on estimates of catch-per-unit-effort (CPUE), proportional size distribution (PSD), and relative weight (W_r) for two standard sampling methods (i.e., boat electrofisher, fyke net) commonly used to target bluegill Lepomis macrochirus in six Iowa natural lakes and impoundments in 2008.  Gear bias was observed for CPUE and PSD, but not W_r.  Differences in CPUE were likely the result of habitat use (e.g., fyke nets effectively sampled deeper habitats in the summer compared to electrofishing).   As such, CPUE tended to be greatest in the summer with fyke nets, whereas electrofishing CPUE was generally greater in the spring and fall samples.  Seasonal trends in PSD were consistent between gears, but were commonly greater with fyke nets and varied by lake.  Decreased PSD estimates with electrofishing suggested selectivity for smaller fish compared to fyke net estimates.  Overall, results of this study suggest high variability between lakes for seasonal patterns in estimates of population characteristics, but consistent biases between sampling methods for bluegill.  Therefore, sampling objectives are essential in determining methods (e.g., gear, combination of gears) and timing (i.e., season) of sampling in standing water bodies.