Characterizing fish assemblages in lentic ecosystems is difficult due to high habitat heterogeneity and the diversity of methods available to sample fish. Therefore, multiple sampling methods are almost always necessary for gaining reliable estimates of species richness and for sampling different sizes of a particular species. However, most research on the use of multiple sampling methods has targeted recreationally-important species. As such, little information is available regarding the influence of multiple methods and timing (i.e., temporal variation) on characterization of lentic fish assemblages. We sampled six lakes and impoundments seasonally (i.e., spring, summer, fall) with beach seines, benthic trawls, boat electrofishing (i.e., day and night), modified fyke nets, mini-fyke nets, and gill nets (fall only) to evaluate the combined influence of sampling methods and timing on the number of species and individuals sampled. Probabilities of detection for species indicated strong selectivities and seasonal trends. Additionally, patterns in seasonal probabilities of detection and species accumulation curves provided guidance on optimal seasons to use gears when targeting multiple species. The evaluation of species richness and number of individuals sampled using multiple gear combinations demonstrated that appreciable benefits over relatively few gears (e.g., three to four) used in optimal seasons were not present. Our results indicated that the characterization of lentic fish assemblages was highly influenced by the selection of sampling gears and seasons, but did not appear to be influenced by waterbody type (i.e., natural lake, impoundment). The standardization of data collected with multiple methods and seasons to account for bias will allow fisheries scientists to have greater reliability in their interpretations and decisions made using information on lentic fish assemblages.

All freshwater fish sampling methods are biased towards particular species, sizes, and sexes and are further influenced by season, habitat, and fish behavior. Thus, the development of standard sampling methods is important for repeatability of assessments and comparisons of data collected across various temporal and spatial scales. We sampled six lakes and impoundments representing a diversity of trophic conditions in Iowa, USA, using multiple gears (i.e., modified fyke-nets, experimental gill nets, beach seine, benthic trawl, boat electrofishing) to determine the influence of sampling methodology and season on freshwater fisheries assessments. Specifically, we were interested in describing the influence of season on catch-per-unit-effort (CPUE), size structure (proportional size distribution [PSD]), and the number of samples required to obtain 125 stock-length individuals for 12 species of recreational and ecological importance (e.g., invasive). Gear selectivity varied from those species sampled with a variety of methods (e.g., bluegill Lepomis macrochirus, black crappie Pomoxis nigromaculatus, freshwater drum Aplodinotus grunniens) to species predominately sampled with a single gear (e.g., largemouth bass Micropterus salmoides, electrofishing). Mean CPUE generally peaked in the spring and fall as a result of sampling method effectiveness (e.g., shallow littoral oriented) and habitat use of species (i.e., movement offshore during summer). Using modified-fyke nets, mean PSD was consistent among seasons for black bullhead Ameiurus melas and yellow bullhead Ameiurus natalis, but decreased from spring to fall for white bass Morone chrysops, bluegill, and black crappie. The mean number of samples required to sample 125 stock-length individuals was minimized with fall gill nets for 5 of the 12 species evaluated (walleye Sander vitreus, yellow perch Perca flavescens, channel catfish Ictalurus punctatus, freshwater drum, and white bass). Our results provide fisheries scientists with relative comparisons between several recommended standard sampling methods and illustrate the effects of seasonal variation on estimates of population indices.

Reservoirs are often managed similarly to natural lakes because they are assumed to be functionally comparable. However, direct comparisons of fish assemblage-environment relationships between these two ecosystems are rare and this assumption has not been adequately evaluated. We investigated associations of fish assemblage structure from 45 natural lakes and reservoirs in Iowa, USA. Fish sampling was conducted with benthic trawls, modified-fyke nets, and night electrofishing. Increased species diversity in reservoirs was most strongly related to morphometric characteristics (i.e., larger surface area, increased depth); whereas, fewer species were observed in natural lakes with low water clarity and high suspended solids. Fish assemblage structure between natural lakes and reservoirs was consistently dissimilar for all sampling methods. Structuring of species composition in reservoirs was correlated with a variety of limnological and physical characteristics, but was largely dependent on the sampling method. In contrast, trophic structure of fishes in reservoirs was weakly associated with the environmental factors evaluated and was similar to fish species structure of natural lakes. Fish trophic composition of natural lakes was related to waterbody size, but was consistent among sampling methods. Overall, distinct differences in fish assemblage structure were observed between natural and artificial lentic ecosystems. Our results emphasize the need to consider waterbody origin (i.e., natural or artificial) on fish assemblage characterization and subsequent inferences made from environmental correlations.