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.