Toward assessing the South African abalone Haliotis midae stock using an age-structured production model

摘要

A deterministic age-structured model is developed for use in estimating resource dynamics parameters and projecting biomass trends for the South African abalone Haliotis midae. This constitutes the first quantitative approach applied to the management of this commercially valuable resource. The model focuses on one of seven commercial fishing zones, zone C, because particular concerns have been expressed about the status of the resource in this region, inter alia because of the considerable catchestaken by the illegal fishing sector. The modeling approach is summarized, and some preliminary results from a single application of the model are given. Model parameters including the pre-exploitation spawning biomass B_0~(sp), the present annual poaching take CP_(max) and the natural mortality M, are estimated by fitting the model to CPUE data for the period 1980 to 1998, Fishery-Independent Abalone Survey (FIAS) abundance data as well as several years of catch-at-age data for each of various components of the fishery. The CPUE data are standardized using a general linear model, and zone C is split into two "linked" areas, a "poached" and "nonpoached" subarea with common resource dynamics parameters but separate fishing histories. This was necessarybecause of substantial differences in both the relative abundance trends and catch-at-age information for these two areas. Model results suggest that the resource in the "poached" and "nonpoached" areas of zone C is presently at ca. 1 percent and 85 percent, respectively of its pre-exploitation level (B_0~(sp) = 3,360 and 1,580 tons, respectively). The model-estimated annual poaching catch for zone C from 1995 to 1998 (240 tons) exceeds the annual commercial catch in zone C over this period. The model indicates that the maximum sustainable yield (MSY) is reduced by more than one-third if catches include the proportion of animals substantially smaller than the minimum legal-size limit thought to be taken by poachers. Model predictions should be interpreted with caution, because more work needs to be done to improve and evaluate the confidence that can be attached to model predictions and to refine the Structure and assumptions of the model. A number of associated suggestions are presented.