The effective and sustainable management of the Eastern Tuna and Billfish Fishery (ETBF) is dependent on having an understanding of the impact of the ETBF on the fish resources which occur off eastern Australia. While changes in catch-per-unit-effort are widely used as an indicator of changes in resource availability, changes in operational practices and prevailing oceanographic conditions makes it difficult to determine the effective effort directed at particular species. This in turn makes it difficult to interpret changes in catch rates as changes in resource availability. However, as the ETBF has expanded, catch rates in some of the earlier regions to be fished have declined significantly leading to increased concern as to whether recent catch levels can be sustained.

For the ETBF, the following four issues highlight the need to better understand the relationship between catch rates and resource availability. By addressing these needs, this research directly addresses aspects of research priorities 1 and 2 identified for the ETBF in 2004.

1. The need to develop indicators of resource availability off eastern Australia
Current stock assessments for the principal tuna species in the WCPO still remain uncertain, and uncertainties in the spatial distribution of both the resource and recruitment patterns makes it difficult to infer from these assessments the status of the resource in a limited region such as off eastern Australia. In order to provide an understanding of the impact of the ETBF on the fish resources which occur off eastern Australia, the Fisheries Assessment Group for the ETBF has identified as a high priority the need to develop a number of performance indicators for monitoring the status of these resources. These indicators are to be based on the monitoring of temporal and spatial changes in catch rates (and sizes) which, in turn, will require gaining a better understanding of the factors, apart from resource availability, which influence catch rates.

2. The need to improve the data and methods used to standardise catch rates.
To improve our understanding of those factors which influence catch rates, information needs to be collected on a range of operational factors which influence the effectiveness of longline fishery gears. These factors include targeting and gear setting practices, resulting hook depths, depth preferences of the target species, time-of-capture, and prevailing oceanographic conditions. Furthermore, an understanding of these relationships is crucial if one is to make use of the new habitat-based models which have been developed to standardise longline catch rates.

3. The need to improve indicators of stock status in the WCPO.
Improvements in regional stock assessments are needed to assist managers of the ETBF gain a better understanding of the status of the stocks on which the ETBF depends. While several factors contribute to the uncertainties in the WCPO assessments, improvements in the construction of indices of stock biomass based on the analysis of longline catch-per-unit-effort have been identified by the Standing Committee on Tuna and Billfish as a critical factor and a high priority for further research (SCTB 2003). The availability of accurate indices of stock biomass will also a critical input for the development of assessment models for those pelagic resources (such as swordfish and striped marlin) which have a more regional SW Pacific stock structure.

4. The need to avoid the incidental capture of important bycatch species.
Information on the fishing characteristics of longline gears in the ETBF is also needed to help address another major concern to the management of the ETBF - the real or perceived threat that longlining has to threatened and endangered species. An improved understanding of the factors influencing the configuration of longline fishing gears and resulting catch rates will help identify fishing practices which may be used to avoid the incidental capture of important bycatch species, such as turtles and other threatened and endangered species (Polavina et al 2003). This will be similar to the observer-based research carried out in the mid-1990s in the Coral Sea to help identify methods to avoid the capture of black marlin. This work will also assist in the identification of fishing practices which selectively avoid the capture of individual target and by-product species (eg. swordfish, sharks) if required for management purposes.

Finally, by gaining a better understanding of the data requirements needed to develop performance indicators for monitoring resource status in the ETBF, this project will provide guidance to AFMA on the types of data which will be needed to be collected for management purposes. This will also assist in deciding which data is best sourced from logbooks and which may be best provided through an observer program.

Implementation of ecosystem-based fishery management requires sound knowledge of food webs if the indirect effects of fishing on trophic structure are to be understood. There is an urgent need to construct a quantitative food web so that we can understand the ways in which the marked declines in the abundances of important fish species off the lower west coast of Australia, which led to the recent closure of the metropolitan commercial fishery, are likely to have impacted on the ecosystem. While there are sound dietary data for most of the more abundant and important species in this region, no such data are available for Snapper (Pagrus auratus) and Silver Trevally (Pseudocaranx dentex). Snapper was a primary target species for commercial fishers, and Snapper and Silver Trevally both continue to be of great importance to recreational fishers. Sound dietary data for these species, which take into account the ways in which their diets change with body size and season, are essential for the construction of a reliable food web. There is a need to combine the new dietary data for these species with those that were collated in the database developed in FRDC 2002/016 and with the results from more recent studies, converting them into common dietary categories, size categories and a common format before they can be used to construct a reliable food web. The opportunity exists, while Ian Potter remains actively involved in research, to apply the experience and knowledge that he and Margaret Platell possess (noting that these two biologists have been involved in collecting much of the available dietary data for the demersal fish species of south-western Australia) to bring together the results of approximately 15 years of research to create a food web for this region that will be invaluable for future research and management.

1. Need to thoroughly assess the developmental fishery
Logistical difficulties encountered during the first year of Queensland's developmental fishery have recently been overcome and levels of effort during the final year seem likely to approach the capacity of the vessel and crew. Extending the research project to include this final year of fishing will facilitate the acquisition of catch-at-age data from the first year of large-scale commercial utilisation of a virtually virgin stock. This will enhance assessment of the fishery’s potential as these data are essential for the application of age-based simulation models (C. Dichmont, pers. comm) that will be used to obtain a preliminary estimate of biomass for comparison with estimates obtained using the DEPM and that may be used for future assessments of the fishery.

2. Need to assess inter-annual variations in stock size
Studies in South Australia and Victoria indicate the potential for large inter-annual variations in the spatial and temporal patterns of distribution and abundance of clupeoid eggs/larvae. These fluctuations, in conjunction with variations in adult spawning parameters and age-specific mortality rates, can greatly affect estimates of spawning biomass obtained using the DEPM and have significant implications for agencies responsible for the management of pilchard stocks. Several scientists (e.g. Dr D Williams, AIMS) have suggested that interannual variations in abundance may be particularly large in locations that are relatively close to the edge of the geographical distribution of pilchards, such as southern Queensland. Quantification of the magnitude of inter-annual fluctuations requires egg surveys to be conducted over several years. Conducting egg surveys during a year of intensive fishing will facilitate the comparison of estimates of spawning biomass obtained using the DEPM with those calculated using fishery-dependent methods (Hilborn and Walters 1992).

3. Need to develop a fishery-independent sampling method
Researchers throughout Australia urgently require an efficient fishery-independent method for collecting the samples of adult pilchards required to monitor seasonal changes in reproductive status and estimate parameters required for application of the DEPM (e.g. K. Jones, M. Kinloch (SARDI), D. Gaughan (WADF) - pers. comm.). The use of a multi-mesh gill net to sample fish attracted to surface and underwater lights was tested from the RV Warrego in October 1997. This method appears to provide a cost and time efficient means of catching adult pilchards but needs to be refined and tested.

4. Need to develop and assess methods to minimise impacts on dolphins
Predatory fishes, seabirds and marine mammals that occur in the waters of southern Queensland appear to feed opportunistically and seem unlikely to be seriously effected by an ecologically sustainable fishery for pilchards. However, the common bottle-nosed dolphin, Tursiops truncatus, is sometimes encircled and occasionally drowned in purse seine fisheries throughout Australia. The occasional capture of such animals in fishing gear must be given due consideration by fisheries managers as conservation groups have recently used this issue as a reason for closing down sustainable fisheries, even when capture rates are so low that impacts on population levels are minimal. The incidental capture of dolphins in purse seine nets in southern Queensland can only be quantified and the need for and/or success of mitigating procedures for use in Australian fisheries can only be properly assessed during an extended period of intensive fishing such as will occur during 1998/9.

Lack of knowledge of Western Australia’s fisheries oceanography fundamentally limits understanding of the recruitment of Western Rock Lobster, Panulirus cygnus, in a fishery worth $200-300 million/year to Australia. The life cycle of P. cygnus includes a planktonic “phyllosoma” larval stage that can be transported ~1500 km offshore, via ocean currents. Development then continues at sea for up to 11 months, before juveniles (“puerulus”) cross the shelf to recruit to coastal reef areas. Critical to improving management of this fishery is an understanding of oceanographic mechanisms driving coastal recruitment. The last three years of puerulus settlement have been low, with the latest (2008/09) the lowest in 40 years. This recent low settlement remains unexplained by environmental drivers previously identified as important, so its cause represents a key unknown for managers assessing the fishery’s sustainability. We will test the hypothesis that variation in settlement is driven by variation in food availability during the open-ocean stage of the phyllosoma larvae. We suggest that ocean productivity, particularly the nitrate-driven classic food chain supporting diatoms, and copepods, (as the ultimate prey of phyllosoma), limits phyllosoma growth and survival in their oceanic phase. Our work targets the peak autumn/winter plankton bloom in the Leeuwin Current, quantifying oceanographic parameters crucial to modelling rock lobster larval dynamics and attempting to link these directly to the food chain on which the phyllosoma as active predators, rely very heavily upon for survival.