Doctor of Philosophy (Ph.D.)
Virginia Institute of Marine Science
John M. Hoenig
The index-removal method estimates abundance, exploitation and catchability coefficient, given surveys conducted before and after a known removal. The method assumes a closed population between surveys. Index-removal has seldom been applied due to its strong assumption of constant survey catchabilities. This work generalizes the method to allow multiple years of data to be incorporated, and the assumptions of the original model to be relaxed. If catchability is constant across years, precision can be improved by analyzing multi-year data simultaneously. Two multiple-year models were developed: the first, 1qIR, assumes constant catchability within and among years; the second, 2qIR, allows catchability to change between surveys within years, but assumes survey-specific catchability constant across years. The new models were tested by Monte Carlo simulation then applied to data from two southern rock lobster (Jasus edwardsii) populations. The 1qIR model produced reasonable estimates in one application, but the 2qIR model was required to produce reasonable estimates for the second population. A likelihood ratio test found 1qIR to be the most parsimonious model, even when, the assumption of constant survey catchability appeared to be violated. In that case, diagnostic plots suggested that the 2qIR model provided the most reliable estimates. However, when the constant catchability assumption is tenable, the 1qIR model offers the greatest precision for parameter estimates. Size- and sex-specific heterogeneity of catchability introduces bias in model estimates. Field experiments were performed to test whether the catchability of small lobster was constant for southern rock lobster during two seasons when fishing occurs. No evidence of heterogeneous catchability was observed during the spring. However, significantly more small lobster were caught in control traps and traps seeded with one large adult male lobster than were caught in traps seeded with one large adult female during the summer, when females are preparing to molt and reproduce in Tasmania. Because heterogeneous catchability occurred during the summer, but not the spring, an index of recruitment based on the catch of lobsters one molt size below legal size might be developed for the spring, however, more sampling is needed to resolve the annual timing of sex- and size-specific catchability changes.
© The Author
Ihde, Thomas F., "Development of generalized index-removal models, with particular attention to catchability issues" (2006). Dissertations, Theses, and Masters Projects. William & Mary. Paper 1539616707.