Modelling an abalone fishery system - implications for management
Creators: name: Bedford, Ray
Publisher: name: RMIT University
Year: 2024

description: Australia currently has the largest commercial wild abalone fisheries in the world. However this industry is coming under increasing pressure due to declining profitability, mainly attributable to a sustained appreciation in the Australian dollar and rising operational costs (e.g. fuel, labor). Reduced harvests have also contributed to dwindling profit levels. Therefore it has become imperative to examine all management strategies to ensure that the wild abalone industry remains both profitable and sustainable. In particular management have expressed concern about meeting key management objectives in relation to i) biomass recovery following a high mortality event ii) alternative spatial scales of management and iii) the fishery effects of diver behaviour. These are the foci of this dissertation research. To address these issues a detailed abalone population dynamics model adapted from biological research data was developed. The model, which categorizes individuals by shell length, was initially used to compare the efficacy of two newly proposed harvest control rules. The objective was to find which rule adapts best to changing fishery conditions. It was found that the proposed harvest threshold mechanism (HTM) rule was ineffective in regard to recovering stock following a high mortality event. Alternatively the proposed mean length of commercial catch (MLCC) control rule was able to respond to a large downward shift in biomass through setting appropriate harvest adjustments. To investigate system behaviour at alternative spatial scales of management the population dynamics model was extended to include spatial differentiation.<br><br>The main aim of this work was to measure the effect of preferential harvesting within a system by seperating the system into two reefs of equal size where one reef was preferred by divers for harvesting. The finding was that the cost and effort associated with implementing management at finer spatial scales would not be justified, in general. However in the case of fisheries with reefs that are strongly preferred by divers, the adverse effects upon management objectives may necessitate such measures. There is evidence to suggest that divers move among reefs in response to changing profit at each location. To investigate the effect of this behaviour on the whole system a bioeconomic model was constructed that explicitly included this diver movement. The findings of this model indicated that fisheries that operate at low biomass density and contain a substantial range of locational harvesting costs are susceptible to rapid biomass loss at some locations. The models developed in this thesis and the findings achieved have yielded insight into an abalone system. This in turn, has led to a better understanding of some management strategies under consideration. It is hoped this will contribute towards improved management practices and hence, sustainability of Australian abalone fisheries. Furthermore, the findings may extend to certain other renewable resource systems. <br><br>