Recent assessments of the NPF have identified a need for the fishery to be managed at a finer spatial scale than that of the NPF managed area (AFMA 2003, DEH 2003). The stocks of some prawn species appear to comprise regional subpopulations that, although not genetically isolated, mix little enough to be manageable as separate stocks. This view is consistent with the experience that depleted stocks in some regions (e.g. north of Mornington Island and Weipa) have not recovered when stocks elsewhere in the NPF were healthy.
The assessments also identify a need to broaden the scope of management of the NPF beyond prawn stocks, iconic species and bycatch to include benthic habitats and species. Prawn fishing has a number of impacts on the ecosystem, including: removal of target species; removal of bycatch and byproduct; removal of benthic plants and animals; removal of habitat-forming species; disruption of sediment structure; suspension of sediment; and feeding of dolphins, sharks, seabirds, fish and benthic invertebrates with discards (Poiner et al. 1998). Some impacts, such as removal of seagrass in nursery habitats, are known to negatively affect prawn stocks. Other impacts are likely to affect stocks in unknown ways, positively or negatively, and in some habitats may affect the sustainability of the stocks.
Broadening management of the NPF to include impacts on benthic ecosystems is therefore prudent from both an environmental and industry viewpoint. It is also consistent with the recommendations of the NPF strategic assessment (DEH 2003), and will prepare the industry for the increasingly sophisticated environmental awareness of export markets.
Management of the NPF is currently based on sound stock assessment and population monitoring procedures, and uses maximum sustainable yield as the management limit reference point. Spatial stock assessment has been investigated with mixed success, but is not used operationally. Current environmental management focuses on fragile habitats (mainly seagrass), prawn spawning areas, iconic species (e.g. turtles) and bycatch. Recently, ABARE suggested a move towards economic efficiency targets, such as maximum economic yield. (Rose and Kompass 2004).
To do this the FRDC project 2004/022 will integrate the existing stock and economic assessments into an MSE process.
To enable stock, economic and environmental objectives to be effectively pursued in a spatial context and with minimal conflict, the stock, economic, bycatch and ecosystem components of NPF management must be integrated into a single, spatially explicit management framework. This project will contribute with major missing elements for that integration and will develop this needed spatial management framework. Equally, the timing of this proposal is opportune given that stock assessment and monitoring are already mature, interactions with iconic species and bycatch are becoming well understood, and the integration of stock assessment with economics is currently underway. To achieve highly effective technical communication and integration we will involve PIs from past and present projects and a Steering Committee with members from CSIRO AFMA staff and NORMAC-REC and NPF-RAG members.

Management arrangements for the NPF tiger prawn fishery are currently chosen so that the spawning stock biomass should recover to the level at which Maximum Sustainable Yield, MSY, is achieved with more than 70% certainty by 2006. MSY is a management reference point that is based solely on biological considerations. From an economic point of view (one that seeks to maximize economic efficiency and fishery returns) this target is inappropriate. The current rebuilding strategy implies that the NORMAC may, over time, be able to reassess the present harsh management measures instituted in the NORMAC agreed effort reduction program in 2001. However, in recent years it has become clear that the fishery is unable to wait for prawn recovery without addressing economic efficiency. NORMAC has identified an urgent need for a further fishery restructure (without compromising biological recovery) so as to maintain economic sustainability and profitability.

An immediate need is for this project to quantify the size of the fleet and length of the season given the biology of ALL the prawn resources. At present it is only broadly possible to answer this question if prawns other than tiger prawns and within year dynamics are ignored (which is unacceptable).

Additionally, this prawn fishery still needs to keep track of its fishing impacts. The main method in the past, with much success, has been through bycatch reduction by TEDs and BRDs. It is now possible, also to include fishing impacts in the modelling mechanisms to allow broader and better informed decision making.

Management advice provided by the NPF Assessment Group needs to take account of the impacts on the stock, economic efficiency and the ecosystem. Input controls are such that several different combinations of fleet size, gear size and season length can produce the same biological outcome but these options would not be equally economically efficient. It is not only overall effort levels that matter for economic efficiency but the manner in which vessels combine inputs in harvest that matters for economic efficiency. This decision over input combinations is sensitive to management decisions and as yet there is no clear economic evaluation of the fishery efficiency under current management practices in a combined biological and economic study.

Limit and target reference points, such as the MSY, in this fishery have only been investigated from the point of view of tiger prawn sustainability. It has been shown in other parts of the world that choosing management arrangements so that fishing effort corresponds to MSY does not necessarily lead to the highest profits and, in fact, lower effort levels generally lead to larger profits and more efficient outcomes. Furthermore, fishing below MSY may also benefit bycatch and byproduct species.

There is therefore a need that future stock assessment undertakes a holistic management view of this prawn fishery. Reference points and management advice should be aimed towards maximising economic return, while ensuring long term target species sustainability and minimising the impact of this fishery on other species wherever possible.

This can only be achieved by:
a) joining the databases held separately by AFMA, CSIRO and ABARE/ANU, and
b) combining the Management Strategy Evaluation frameworks produced by CSIRO on tiger prawns (FRDC 2001/002), ANU/ABARE on economic efficiency (FRRF) and CSIRO investigating effects of trawling on the seabed (FRDC 2002/102).

The spatial extent of the NPF has changed over time and now concentrates on relatively small hotspots. This means that the only index of biomass, CPUE (derived from fishery log book returns), is providing little information on the areas no longer fished (including inshore areas that generally have been closed to fishing). However, stock assessment estimates for banana and tiger prawns need to take into account the abundance of prawns in all areas, including those not fished.

Although recent research has suggested that inshore waters are probably the most important spawning areas in the NPF, this conclusion is based on laboratory research on the behaviour of postlarval prawns, combined with models of the currents in different regions of the NPF. To validate this conclusion we need to confirm that substantial populations of spawners do occur in the inshore waters at the appropriate times of year by targeted field sampling.

The movement patterns of prawn populations over the season and between inshore and offshore areas are highly relevant as they all have a general offshore migration as they increase in size from pre-recruits to recruits and spawners and an inshore migration as larvae. These issues need to be more explicitly investigated with regard to the assumption of the relationship between catch rates and biomass over time.

This project will fill essential gaps in our knowledge and also develop a scientific basis for long-term investments in fieldwork. Although this project will concentrate on the less assessed white banana prawn (Penaeus merguiensis) and the tiger prawns (P. semisulcatus, P. esculentus), the fieldwork, design and research concepts would probably also apply to endeavour prawns (Metapenaeus endeavouri, M. ensis).

Related projects
Die D, Loneragan N, Haywood M, Vance D, Manson F, Taylor B, Bishop J. (2001). Indices for recruitment and effective spawning for tiger prawns stocks in the Northern Prawn Fishery. Final Report toFRDC for Project 1995/014. 82 pages with 8 appendices.

NORMAC 45 agreed that as a high priority a pro-active approach to assessing the potential areas for designation as MPAs, and the impact of their potential designation on the fishery, would benefit the NPF. NORMAC also suggested that the NPF be offered as a trial fishery to test and refine the Environment Australia criteria, to demonstrate that a fishery is ecologically sustainable. The Commonwealth government is committed to setting up a National Representative System of Marine Protected Areas throughout Australia's entire marine environment that will protect areas representative of all major ecological regions and the communities of plants and animals found there. It is in the interest of the NPF that this system is used as an opportunity to achieve sustained, and possibly increased returns from tiger prawn fishing and also to conserve biodiversity.

The outcomes of this project will assist NORMAC in meeting their responsibility to demonstrate that the NPF is an ecologically sustainable commercial fishery, under the Commonwealth Fisheries Management Act (1991), the new Environmental Protection and Biodiversity Conservation (EPBC) Act (effective July 2000) and the Wildlife Protection (regulation of Exports and Imports) Acts 1982. Fisheries will be assessed to ensure that they are conducted in accordance with the EPBC Act and if they are not, their ability to export product could be threatened. It is, therefore, critical for NORMAC to evaluate its management strategies to ensure that they meet the requirements of the legislation. This proposal also addresses key research priorities of the benthic communities/physical impacts research area of the FRDC Effects of Trawling Sub-Program.

The results from the proposal contribute to the strategic directions of Environment Australia. In the short term, EA are looking at developing a proposal for a Marine Protected Area in the southern Gulf of Carpentaria. However, in the longer term, the results from this project on identifying different ecosystems, and modelling different management scenarios, will provide information for the selection process of MPA's in the Northern Prawn Fishery and on potential management strategies for conserving biodiversity values. This project will also provide information on methods for assessing the effectiveness of MPAs for conserving biodiversity.

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.