Crustacean fisheries (such as those targeting rock losters, crabs and prawns) contribute more than a billion dollars to the Australian economy annually. Population models for stocks that support these fisheries often rely on indirect measures of age (e.g. size-modal analysis) which can confound estimates of other life history parameters and introduce errors into the modelling process.

A method to directly age commercially exploited crustaceans (by counting growth bands in calcified regions of the eyestalk and/or the gastric mill) has been developed by Dr Raouf Kalida at the University of New Brunswick, Canada and published last year. His technique has the potential to substantially improve the precision of stock assessments on crustacean fisheries around the world.

There is a need to transfer this technology to fisheries research agencies in Australia so that we can better understand growth patterns in crustaceans, reduce uncertainty in crustacean population models and provide better advice to fishery managers. This in turn should lead to improved management practices and greater stability in the national crustacean harvest, noting that production by several crustacean fisheries is also strongly linked to environmental factors.

The most effective means to address this need is to host Dr Kalida in Australia for a period of time to share his knowledge of crustacean ageing and provide guidance to domestic fisheries scientists working in the field. The proposed project aligns with a number of FRDC Strategic Priority Areas including: Theme 4 (Ecologically Sustainable Development), Theme 13 (Innovation Skills) and Theme 14 (Extension and Adoption).

The indigenous development scholarship is part of the suite of professional development opportunities developed and managed through the FRDC people development program. This particular scholarship addresses the following objectives of the people development program 2008-2013:

Objective 1 Enhance industry leadership within all sectors
and; Objective 3 Provide opportunities for knowledge transfer and R&D adoption

The significant drivers identified in the FRDC people development program, addressed by this project
are:
- There is a shortage of industry leaders in all sectors of the fishing industry.
- There is an urgent need to develop people within all sectors who have the skills to effectively contribute to debate and policy development for significant challenges, including access to fish resources.
-Early career researchers and emerging industry leaders lack opportunities for formal mentoring and
professional development.
-There is a shortage of opportunities for people in industry to develop skills that are going to directly
improve business profitability and sustainability.
-The industry is geographically dispersed and fragmented, and needs opportunities to learn within and
across sectors.

Mud crabs (Scylla serrata) are a highly prized natural resource, with the volume and value of the 2004 (national) commercial harvest exceeded 1500 tonnes and $20 million, respectively. The recreational and indigenous take of this species is also significant, with estimates for the 12 month period starting May 2000 in the order of one million crabs.

There have been widespread fluctuations in mud crab landings in recent years. For example, the commercial harvest in the NT increased from ~600 tonnes in 1997 to 1100 tonnes in 2001, then dropped to 300 tonnes in 2005. This variability is probably due to a combination of fishing activity and the environment, but the extent to which each factor influences the mud crab catch is unknown. Identification of the drivers of recruitment and stock abundance would enable resource managers to implement harvest strategies based on predicted yields.

The ability to predict mud crab yield depends on the collection of appropriate biological and environmental data, one of which being the magnitude of recruitment. This requirement corresponds to Phase 3 of the National Strategy for Research on Mud Crabs, which recommends the 'development of a fishery independent index of stock abundance based on a juvenile pre-recruit index'. It also ranks as a high priority for the Northern Territory Strategic Plan for Fisheries Research and Development 2007-2011.

To ensure that future mud crab recruitment models are widely accepted and robust, it is essential that research and management agencies across northern Australia first agree on the best approach to implement Phase 3 of the National Strategy. This will be achieved through a national mud crab research network and workshop.

The 1999-2003 Northern Territory Strategic R&D Plan in considering catch rates, current levels of exploitation and management requirements, ranked NT Coastal Line Fishery research as one of highest priority. The most recent revision of this plan further clarifies these concerns and clearly identifies that "Harvesting of fish and other aquatic resources can only be sustainable if the long-term rate of harvest does not exceed the long-term rate of replacement to the stock(s). Increasing catches and target fishing evident for black jewfish and high exploitation of spawning aggregations may render the stocks unsustainable. In view of the above, re-commencement of research on coastal fisheries is considered a high priority for the next 5 years" (NT Strategic Plan for R&D 2002-06). Worldwide experience in managing aggregating species indicates that they are extremely vulnerable to overfishing.

The Coastal Line Fishery (CLF) logbook data and anecdotal information from fishers provides evidence of target fishing on black jewfish aggregations in NT waters. Significant changes in the CLF catch composition have occurred over a short period and the increase in total catch can be directly attributed to the increased catch of black jewfish. Over the period 1990-2001 the percent contribution of black jewfish to the total catch has increased from 11% to 77%. Once a multi-species fishery the CLF is now primarily a black jewfish fishery with a reef fish bycatch. In 1999 the CLF black jewfish catch reached 58 tonnes increasing 93% in just twelve months to 101 tonnes in 2000. Catch per unit effort (CPUE) for the same period increased just 27%. Black jewfish catch continues to increase, reaching 136 tonnes in 2002 and 167 tonnes in 2003, while CPUE has remained stable.

Of great concern is that these changes in black jewfish catch have occurred while considerable latent effort exists in this fishery. In 2002 only 27 of the 58 (46%) CLF licences issued, actively fished and of these just seven were responsible for 80% of the total catch. Increasing returns to a small group of fishers has created much interest in this fishery with new vessels and fishers from other fisheries preparing to fish for jewfish. The potential for increased effort in this fishery is high.

The 1995 the NT recreational harvest of Black Jewfish was estimated at 232 tonnes (FISHCOUNT) approximately six times the 1995 commercial catch (32 tonnes). The 2000 National Recreational and Indigenous Fishing Survey indicates that the NT recreational catch of black jewfish has reduced by approximately 25% to around 140 tonnes in 2000 (Henry and Lyle 2003).

Anecdotal reports from both commercial and recreational fishers suggesting a decline in the average size of black jewfish in the NT. Re-commencement of on-board monitoring of the commercial fishery is a key component of this project as such data is considered critical in assessing the impact of fishing on these aggregations.

Analysis of commercial catch and effort data suggests that the commercial fleet is moving from one aggregation to another, as aggregations are depleted and the same pattern of a more localised nature is highly likely for the recreational fishery.

The Aquatic Resource Management Section of the NT Fisheries Group has an urgent need for information on the make up of black jewfish aggregations in the NT. Information such as: Why do the aggregations occur? Why do they occur at particular sites? What is the extent and timing of movements between aggregations? Where do the fish go when not in aggregations? This project will address these questions directly. It will also provide leads for broader and more complex questions such as: what proportion of the stock is vulnerable in aggregations; to what extent does protection of one site affect fish at other sites; how mobile might sites be etc.

The current paucity of information is a critical impediment to the ability of the Fisheries Group to provide adequate protection of the Northern Territory’s black jewfish stocks and associated fisheries.

The recreational fishery for barramundi in the NT is regulated by seasonal closures, a minimum size and possession limits, and the increasing practice of catch and release fishing. The effectiveness of such regulations and practices is to a great degree dependent on the rate of mortality of the released fish. Released fish may die directly from the stress of capture, or from injuries received while being landed, or they may be at a higher risk of predation as they recover. Post-release mortality (also called cryptic mortality) has been described for many species worldwide but there is no estimate available for barramundi. As recreational effort increases in the NT, in line with projected population increases, there may be a need to tighten management of barramundi by output controls. Alternatively, other arrangements could be considered such as the designation of some more popular areas for catch and release fishing only. To properly assess the effectiveness of such restrictions it will be necessary to be able to estimate cryptic mortality. In situations where large catches of sub-legal size barramundi are seasonally common (such as at Shady Camp barrage, Roper Bar and Daly River Crossing) even a low rate of post-release mortality can lead to a significant increase in actual fishing mortality. At Shady Camp in 1989 and 1990 it was estimated that catch and release of small barramundi was so high that a cryptic mortality of only 10% could have caused an increase in total deaths due to fishing of almost 70% (Griffin, 1987). A study of recreational fishing in the NT in 1995 (Coleman, 1998) found that barramundi anglers caught a total of 229000 fish but retained only 94000 (41%) of them. If 20% of the released fish had died the extra fishing induced deaths would amount to 27000, an increase of 29% on the recorded fish mortality.

A recent study of line caught striped bass (Morone saxatilis) in the southern USA (Bettoli and Osborne, 1998) estimated cryptic mortality rates to be as high as 67%. Observations during the annual Barramundi Classic fishing tournament at Daly River (White, 1998) have shown a reasonably high number of tagged fish collected after being fatally injured (ie bitten in half) by sharks. Others may well have been completely consumed by the sharks or other predators such as crocodiles or birds. In some years of the tournament around 1% of the tagged fish were recovered dead during the tournament. While this does not suggest a high rate of cryptic mortality the actual number of deaths in those instances was likely to be substantially higher given the high abundance of predators capable of disposing of the evidence.

Information on stress physiology will also be of use to the aquaculture industry in understanding and managing stress in barramundi farm stock, but is of secondary consideration in this project.