Estimation and Interpretation of Proportional Stock Density for Fish Populations in Ohio Impoundments

Abstract

We used fish population and food consumption data from five Ohio impoundments plus results from computer simulations to (1) examine sources of sampling error in estimating Proportional Stock Density (PSD), an index of fish population structure, (2) evaluate the relative effects of growth, mortality, and recruitment on PSD, and (3) assess use of PSD to predict outcomes of predator-prey interactions when the largemouth bass (Micropterus salmoides) is the major piscivore in medium to large impoundments (>50 hectares) with complex fish communities. When largemouth bass were collected with electrofishing gear, estimates of PSD varied considerably. During the spring, PSD was positively related to water temperature. PSD was lowest during summer when large bass were not vulnerable to electrofishing gear and increased in autumn when water temperatures declined. The PSD of a largemouth bass population in one impoundment ranged from 11 to 36% over a 4-year period as growth, exploitation, natural mortality, and recruitment changed. We used a computer simulation model to show that PSD increased linearly with survival and curvilinearly with growth. Random variations in recruitment caused large fluctuations in PSD and we concluded that, in medium to large impoundments, recruitment was likely to have more influence on PSD than either growth or survival. We also examined the effectiveness of largemouth bass as predators in Ohio impoundments and the likelihood of observing changes in the PSD of prey populations in response to changes in size structure of largemouth bass populations. In only one of five impoundments were bluegills (Lepomis macrochirus) and crappies (Pomoxis spp.) a major part of largemouth bass diets. When present, the gizzard shad (Dorosoma cepedianum) was the major prey. We used a bioenergetics model to estimate annual food consumption by a dense population of largemouth bass. Largemouth bass could consume most of the gizzard shad production in years when shad produced small year classes; however, largemouth bass would not control bluegill or crappie populations when gizzard shad were present.