Sound estimates of natural and fishing mortality form the basis of fishery stock assessment and modelling. Without these data, the extent to which the spawning biomass has been reduced or the effectiveness of management measures in sustaining wild fish stocks cannot be determined. These estimates of mortality are crucial if the commonly-used reference points for fishing mortality and biomass are to be used in managing fisheries. Considerable advances have been made in obtaining estimates of mortality when data on the age composition of the annual catch are available. However, when there are only data on the length composition of the catches, analyses become more complex and the algorithms are not as well defined. Although information is usually available within the biological and fishery data that allows estimation of total mortality, the information on natural mortality must usually be obtained from empirical models that relate natural mortality to characteristics of the life history for other fish species or by estimating the relationship between total mortality and fishing effort, thereby obtaining an estimate of natural mortality by subtraction. Furthermore, there is a need to understand how natural mortality varies with size. Although the study of mortality deserves to be a primary focus of fishery research, it is difficult and thus often set aside. For many of Australia’s finfish stocks, there is a need to develop length-based methods to estimate mortality, which can be used to monitor the stock status of the recreational fisheries, and to obtain improved estimates of natural mortality that can be used in assessing sustainability. The project falls within FRDC’s Natural Resources Sustainability Program and is intended to increase and apply knowledge of stock assessment methods by developing methods of estimating mortality using length composition data which, compared with age composition data, are relatively inexpensive to collect.

Models are urgently required that will allow stock assessment for fisheries in which a significant component of the catch is taken by recreational fishers, where these models will rely on abundance indices from the commercial fishery, occasional length or age composition samples from the total catch and occasional estimates of total catch. Given the expense associated with recreational surveys, there is a need for the development of an approach that would allow determination of an appropriate frequency for such creel censuses in order that they might provide the data necessary to achieve a specified level of precision from the resulting stock assessment. A method is required that will allow an assessment of the value of data derived from commercial fisheries statistics for use in assessing the stocks that are shared by recreational and commercial fishers, prior to making final management decisions on catch re-allocation from the commercial to the recreational fishing sector.

Through the research being undertaken in FRDC Project 2000/311, “Development of research methodology and quantitative skills for integrated fisheries management in WA”, it has been recognised that the dietary data available for Western Australian fish are limited and may result in an imprecise specification of the food web. An urgent need to collate the existing dietary data in order to assess their adequacy in developing information on the extent to which fish species predate upon fish and invertebrates, or ingest plant material, has been identified. Ecosystem modelling and monitoring of the “health” of the non-landed prey species will require the development and maintenance of a database containing consistent and comprehensive details of the relative proportions of the prey species in the diets of their predators. Gaps within the resulting database need to be determined and the research methods, that are required to fill these gaps, need to be identified. Without such data on dietary compositions, it will be impossible to ascertain accurately the relationships between species, thereby hampering the development of accurate ecosystem models. The utility of the existing data sets will be considerably enhanced by their collation and synthesis, and the resulting data are considered to be essential for the development of ecosystem models capable of providing the information required to manage fisheries in accordance with the principles of ESD.