Posthodiplostomum minimum utilizes a three-host life cycle with multiple developmental stages. The metacercarial stage, commonly known as 'white grub', infects the visceral organs of many freshwater fishes and was historically considered a host generalist due to its limited morphological variation among a wide range of hosts. In this study, infection data and molecular techniques were used to evaluate the host and tissue specificity of Posthodiplostomum metacercariae in centrarchid fishes. Eleven centrarchid species from three genera were collected from the Illinois portion of the Ohio River drainage and necropsied. Posthodiplostomum infection levels differed significantly by host age, host genera and infection locality. Three Posthodiplostomum spp. were identified by DNA sequencing, two of which were relatively common within centrarchid hosts. Both common species were host specialists at the genus level, with one species restricted to Micropterus hosts and the other preferentially infecting Lepomis. Host specificity is likely dictated by physiological compatibility and deviations from Lepomis host specificity may be related to host hybridization. Posthodiplostomum species also differed in their utilization of host tissues. Neither common species displayed strong genetic structure over the scale of this study, likely due to their utilization of bird definitive hosts.

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Characterizing the structure of riverine fish populations that are patchily distributed and at low densities is often difficult. Due to the wide distribution but low abundance of Spotted Bass, Micropterus punctulatus, throughout the lower Wabash River (LWR), captures from fixed and random sampling designs were compared to assess their performance in determining relative abundance and size structure. The LWR is monitored using both sampling regimes-the Long Term Electrofishing Program monitors community assemblages uses a stratified-random sampling, whereas the Illinois Department of Natural Resources monitors the fish community using a fixed-sampling design. Mean CPUE was significantly higher for fixed site sampling compared to random sites. However, the size structure of Spotted Bass sampled by each design were similar. These results suggest random-sampling designs should be used when monitoring low density populations of fishes in large riverine systems because this method provides the greatest spatial coverage of habitats, evaluates heterogeneity in distribution, and provides a representative measure of abundance throughout an entire aquatic system. In situations where size distribution or temporal trends are the only goal, fixed site sampling would be sufficient.