It is unknown whether the current apparent failure of the stocks to recover in the NP Tiger fishery is related to limited management options, serial depletion of stocks or to the use of the now somewhat discredited MSY and EMSY management targets (see, for example, Larkin, 1977 and summaries in Pitcher & Hart, 1982).

In two recently completed FRDC projects (FRDC 95/014 and 98/109), a preliminary attempt at stock-based assessments was undertaken. These show that some stocks are much more depleted than the single-stock model would suggest. There is a need to clarify which areas are most affected and why these are performing so poorly. There is also a need to develop a multi-stock operating model to open a new direction for modelling in the NPF. This technically complex model would have the potential to benefit the management of benthic crustacean species worldwide. (It should be noted that no operating model, particularly not a spatially explicit one, has been developed for any prawn fisheries in Australia.)

In species, such as prawns, whose dynamics are dominated by yearly recruitment variation, the MSY may well give a false expectation of stability. Management targets that relate to present conditions rather than to equilibrium conditions (e.g. a target fishing mortality rate) may better serve intrinsically variable fisheries, such as prawns. However, reference points developed worldwide have concentrated on output controlled management systems. Given AFMA’s requirement to satisfy its ESD objective, there is therefore a need to consider uncertainty explicitly and to identify performance indicators and harvest strategies that are as robust as possible to incorrect assumptions and estimation errors deriving from limited data. Most importantly, these should be developed in the context of spatially explicit stock assessment models and an input controlled management system.

An international review of the NPF tiger prawn assessment agreed with the conclusions of the 2001 assessment that tiger prawn stock levels were critically low, especially for brown tiger prawns. The 2002 assessment further concluded that brown tiger prawn levels were too low and also emphasized the critical need for an independent monitoring program given the confounding and complexities of the catch rate data used as the sole index of abundance in the NPF assessments. The 2003 assessment suggests that brown tiger prawn stocks are recovering but, given the high level of uncertainty in the assessment, this recovery needs to be independently tested.

The survey data used to determine the initial design for this project (see Background) was more than a decade old and did not cover the full study area. Since the first survey, changes have been made to the survey design to improve the accuracy of the abundance estimates obtained from the surveys. This design needs to be further developed and tested. Work has also begun on developing methods for incorporating the results of the surveys into stock assessments, but more research is required to overcome several technical difficulties encountered.

In this proposal, the CSIRO salaries associated with testing the survey design and with developing new methods of incorporating the results into stock assessments are seen as research. We are therefore requesting about $47,000 from FRDC’s MOU funds. For this reason, CSIRO is also supporting the project to the scale of about $86,000. The remainder of the project, some $520,000, will be underwritten by the industry as agreed in NORMAC, June 2003. The industry and NORMAC have also re-affirmed the long-term need for regular industry-funded monitoring surveys based on the output of this project.

There is a need to provide an updated design for the NPF that would work in the long-term to provide indices of abundance for key species and enhance a difficult-to-use commercial catch rate series. Furthermore, this design needs to address target, byproduct and possibly some effects-of-trawling issues to make the best use of the surveys.

To assess whether the ESD and MEE objectives are met there is a need to determine the status of prawns stocks in the NPF and to develop guidelines to define whether the present status of the stocks may require management actions. It is important to precisely define what population parameters should be monitored and what biological reference points should these parameters be compared in order to determine whether management action is required. The Northern Prawn Fishery Assessment Group (NPFAG) has identified that spawning stock biomass and standardised fishing effort are the two most important indicators for target stocks in the NPF. The NPFAG has also established that targets and limits for these indicators need to be set and reassessed as new information is collected. The NPFAG has also recommended that future advice provided to them by researchers should include an explicit measurement of the probabilities that each of these targets may be exceeded. Calculation of such probabilities requires formal risk analysis to be carried out as part of the stock assessment.

Additionally, there are a number of future management options that have been recently considered by NORMAC. These include reductions of pool of the licensing units used in the NPF, (A-units, representing vessel length and engine power), gear restrictions as well as further seasonal and spatial closures. Although the operational implications of adopting some of these options have been the subject of NORMAC discussions, the scientific evaluation of options is not carried out in a structured framework but rather as individual assessments as different options are proposed by NORMAC. There is a need to establish a structured framework for management strategy evaluation so that the NPFAG and NORMAC can compare different options in a consistent way. This framework for management strategy evaluation should allow for the integration of risk analysis into the evaluation of management options. The consequences of each management strategy should be quantified and evaluated against the indicator of performance established by the NPFAG. The evaluation should include the estimation of the probability that, in the future, certain undesirable or desirable states of the stock are reached.