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.

Until recently management of prawns in the Joseph Bonaparte Gulf (JBG) has been a “byproduct” of measures designed for other NPF stocks. In recent years, the JBG has been fished with the two seasonal openings (April to June, August to November), however in 2007 this arrangement changed, and fishing has only been allowed August to November. The rationale of this change was to see if average size of prawns, and total value of the catch could be increased. However, no clear objectives, performance measures or monitoring system were established to evaluate whether this was achieved.

Little targeted research has been carried out on red-legged banana prawns (Penaeus indicus), the main JBG species. In recent years only rudimentary catch and effort, and commercial grading data have been collected. Even this has not been subject to systematic analysis. In these circumstances anecdotal accounts of recent trends can be argued to support five plausible explanations with differing management implications:
1. The late season has optimized value of production.
2. Value is being forfeited because the season is too late and peak annual biomass is being missed.
3. Value is being forfeited because poor weather late in the year and other fishing commitments prevent limit boats days in the JBG.
4. The late season intensifies fishing on the aggregated spawning biomass risking recruitment over-fishing.
5. Natural variability has produced lower catches than normal.

The NPF Industry Pty Ltd needs its discussion of management in the JBG, and other localized stocks, to be based on sound scientific data. The immediate need is to provide a scientific basis for discussions about the 2010 season, which will occur in late 2009. The medium term need is for NPF Industry Pty Ltd develop its capacity to conduct cost-effective data collection programs.