Factors Affecting Largemouth Bass Size Structure at Wheeler and Guntersville Reservoirs, Alabama
Type of DegreeMaster's Thesis
Fisheries and Allied Aquacultures
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Understanding the characteristics of a fish population is vital to the assessment and management of the stock. Size structure and abundance are two such characteristics of a population and they have the potential to vary spatially and temporally. Causes for this variation between and within stocks are not well understood. Comparing largemouth bass (Micropterus salmoides) growth, mortality, and year-class strength in Wheeler and Guntersville reservoirs, two Alabama impoundments with observed size structure differences, provides an opportunity to investigate factors that affect population size structure. The study was executed by 1) evaluating between-lake differences in size structure, growth, maturity, condition, and diet, 2) quantifying associations between natural mortality and early-growth rates within and between the two populations using a catch curve analysis, 3) evaluating the sensitivity of between-lake differences in relative abundance of 500-mm and larger largemouth bass to changes in growth parameters, natural mortality, fishing mortality and fishing regulations within the estimated parameter ranges for these lakes using a per-recruit age structure model, and 4) fitting a statistical catch-at-age analysis (SCAA) to reconstruct year-class strength and fishing mortality time series’ using age composition and relative abundance data from spring electrofishing surveys. Results of the between-lake comparison suggests that Wheeler largemouth bass grew faster, which may have constrained the population to a higher mortality and shorter life expectancy, and thus a lower proportion of large fish. The catch curve analysis demonstrated that differences in early growth could explain the variation in natural mortality of largemouth bass and the sensitivity analysis of the relative abundance of 500 mm largemouth bass between lakes suggested that natural mortality likely influences size structure more than growth alone. Results of the SCAA indicated that recent year-class strength estimates were robust to changes in model assumptions and correlations among lake were weak. The data suggest that variation in early growth could be attributed to differences in diet, possibly due to habitat availability differences; however, this was not confirmed with bioenergetics modelling. This study reiterates the interrelated nature of life history traits growth, mortality and age-at-maturity where reproductive fitness is optimized by adjusting age-at-maturity given a trade-off between growth and mortality.