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.
