The SBT resource is estimated to be at historically low levels and biological concerns exist about the status of the stock (Anon. 1998, 2001). There is also large uncertainty about the sustainability of current catches. The advice from the CCSBT Scientific Committee’s stock assessment in 2001 was that under current catch levels there was a ~50% chance that the stock would continue to decline or increase. The 2002 CCSBT Scientific Committee concluded that there was no basis for changing its previous advice on catch levels. The current management practice in the CCSBT is based on a global total allowable catch (TAC) with no consideration or restriction on where the catch is taken. Genetic studies, along with the fact that there is only one known SBT spawning ground, have led to the conclusion that SBT comprises a single reproductive stock. However, tag return and catch distribution data suggest that there may be substantial spatial structuring and incomplete mixing of SBT among the major feeding areas. Spatial structuring of the stock would have large implications for SBT assessments and for managing the rebuilding of the SBT resource. The combined results from the SRP conventional tagging program and this proposed archival tagging project would provide the basis evaluating the need for and, if necessary for developing, spatially-explicit population assessment and management response.
There is a critical need for direct and improved estimates of juvenile fishing mortality rates (or equivalently recruitment) to reducing uncertainty in the stock assessments and to provide a robust evaluation of the sustainability of recent catch levels. The SRP conventional tagging program is intended to provide this information. However, in order to achieve this objective, it is essential that sufficient information is available to account for incomplete mixing and the spatial dynamics of SBT in the analyses of the results from the conventional tagging data.
Catch-per-unit-effort (CPUE) indices are used to provide stock indicators of trends in the SBT stock size and form an essential input into the analytical stock assessment models. Interpretation of CPUE data is complicated by spatial and temporal variation in the availability and catchability of SBT in relationship to fishing effort. Interpretation of catch rates has been and continues to be a major source of uncertainty in the SBT stock assessment. The CCSBT Scientific Committee have repeatedly identified the need to develop alternative approaches for modeling and interpreting the SBT catch and effort data and this need has been incorporated into the agreed CCSBT SRP. Both availability and catchability are expected to vary with environmental conditions that modify the habitat suitability for SBT. Information for habitat-specific CPUE standardization was recognized as an important alternative approach for modeling catch rate data at the last CCSBT Scientific Committee meeting. This standardization approach takes into account changes in environmental conditions so that CPUE indices reflect the actual SBT habitat. This should allow the indices to more clearly reflect the actual changes in the abundance of SBT. This is especially critical as CPUE indices are seen as providing one of the key inputs in the decision-rule-based management procedure under development by the CCSBT. Thus, it is essential that to the extent possible that the CPUE indices provide a reliable indication of shorter term trends and that the CPUE signal is not confounded by short-term environmental fluctuations. Archival tags provide a unique tool for collecting the required habitat-specific requirements of SBT. Without such data, these habitat based standardization approaches are intractable. For example the archival tag data on vertical and horizontal distribution allow habitat preferences to be estimated, and CPUE standardization is possible.
In summary, this project aims to provide information to provide a substantial improvement in our current understanding of SBT movements and spatial dynamics. In particular, the proposal has been developed in response to three specific needs for an improved understanding of SBT spatial dynamics:
1. Estimation of mixing rates for the estimation of mortality rates from conventional tagging (particularly in the context of the large scale juvenile tagging program which is a high priority component of the CCSBT Scientific Research Program);
2. Habitat definition to allow the standardization of CPUE indices for use in the CCSBT stock assessment process; and
3. Requirements within the guidelines under the strategic assessment provisions of the Environmental Protection and Biodiversity Conservation Act 1999 that “the distribution and spatial structure of the stock(s) has been established and factored into the management response”.
In addition to these three specific needs, there is a general need to ensure that the current stock assessment models are robust to their implicit assumptions about spatial homogeneity.
Final report
Results have increased our confidence in the recruitment index based on the aerial survey in the Great Australian Bight (GAB) by confirming that the timing and duration are ideal, that the majority of juvenile SBT are likely to return to the GAB each summer, and that based on current evidence it is unlikely that a large proportion of juvenile SBT remain off South Africa over summer. This is of benefit to all stakeholders and management bodies, including the CCSBT. The extension of a tag based assessment model for SBT to include a spatial component, and the incorporation of archival tag data into the model, is a significant achievement and of major interest to the international community. Methods developed to study migration patterns and habitat preferences of SBT suggest that habitat-based CPUE standardization is unlikely to be as useful for SBT as first anticipated, but these methods may be useful for dealing with unfished areas when standardising CPUE.
By using information in this report, the fishery can now also address requirements within the guidelines under the strategic assessment provisions of the Environmental Protection and Biodiversity Conservation Act 1999 that “the distribution and spatial structure of the stock(s) has been established and factored into the management response.
Spatial structure of fish populations has long been recognised as a potentially critical factor in a population’s overall dynamics, and hence of importance to stock assessments and management. Neither the population model used for southern bluefin tuna (SBT; Thunnus maccoyii) assessment and evaluation of management procedures, nor the management of SBT by a global total allowable catch (TAC) take spatial structure into account. In this ambitious project we used archival tags to provide the necessary data to start integrating the spatial dimension into the population ecology and assessment of SBT.
Keywords: Southern bluefin tuna, Thunnus maccoyii, SBT, archival tagging, spatial, dynamics, habitat use, spatial mark-recapture model, CCSBT, Commission for the Conservation of Southern Bluefin Tuna.
