Susan J Johnston

Pretesting the Likely Efficacy of Suggested Management Approaches to Data-Poor Fisheries

Abstract The thrust of this paper is that decision rules for the management of data-poor fisheries cannot be based on expert judgment alone. Such rules need to specifically link management responses to the values of the indicators available for the fishery and their trends. Prior simulation testing is needed to confirm that the application of any rules suggested is likely to achieve the objectives sought for the fishery. The management procedure (MP) approach (also called management strategy evaluation), which provides a framework for such testing, is summarized briefly. How this approach could be used to develop a decision rule for a fishery for which the only indicator available is the mean length of the catch is presented as an example. The extent to which the ability to meet management objectives could be improved if an unbiased index of relative abundance were available, and an MP based on a fitted population model applied, is illustrated. An MP developed for the fishery for Patagonian toothfish Dissotichus eleginoides off the sub-Antarctic Prince Edward Islands is summarized. This illustrates how the MP testing framework can be used in circumstances in which the available indicators conflict, leading to considerable uncertainty about the present resource status. The information content of indicators is closely related to the extent to which they vary about trends in the underlying resource attributes (e.g., catch per unit effort and underlying abundance). The compilation of lists of the statistical properties (such as the coefficients of variation and autocorrelations) of the residuals about detrended time series of the indicators, together with their likely relationships to the underlying attribute, for fisheries worldwide is suggested. This would provide a sound basis for specifying error structure in the simulation tests advocated for both generic and case-specific decision rules for data-poor fisheries.

A review of the assessments of the major fisheries resources in South Africa

The waters off South Africas coastline boast a rich mix of commercially fished species. Quantitative assessments of these marine resources have developed from simple methods first applied in the 1970s, to models that encompass a wide range of methodologies. The more valuable resources have undergone regular assessments in recent decades, with frequencies closely related to the management approach employed for each fishery. Many of these assessments form the operating models used to simulation-test candidate management procedures. This paper provides a comprehensive review of the assessments of 11 of the most important fisheries resources in South Africa. Some assessments use simple biomass dynamics models, whereas others are a hybrid of age- and length-based models, each designed to model the specific characteristics of the resource and fishery concerned. Many of the assessments have been disaggregated by species/stock and/or area as related multispecies/stock/ distribution hypotheses have arisen. This paper explores the similarities and differences in the data available and the methods applied. The review indicates that, whereas the status of three of these resources cannot be estimated reliably at present, the status of six resources is considered to be reasonable to good, whereas that of abalone Haliotis midae and West Coast rock lobster Jasus lalandii remains poor.

Size-based estimates of survivorship for the South African rock lobster Jasus lalandii

Abstract Natural survivorship estimates ( s ) for the South African west coast rock lobster, Jasus lalandii, are obtained by regressing a size-based population model to pristine population size structure information. The model assumes that juvenile settlement and somatic growth rates are invariant from one year to the next. The pristine size structure information that is used is obtained from unfished protected populations (sanctuary populations), and from catches made at the beginning of the fishery. A total of ten different size structure datasets are investigated. The age of the sanctuary at the time the data were collected is shown to be critical when estimating the natural survivorship. A dynamic analysis is carried out to adjust for the bias due to the age of the sanctuary. Once the age of sanctuary effect is taken into account, the estimates of survivorship vary from 0.88 to 0.95. The precision of the natural survivorship estimates is calculated, and the sensitivity of the estimates to somatic growth levels, as well as to other parameters and model assumptions is determined.