Spatial interactions among juvenile southern bluefin tuna at the global scale: a large scale archival tag experiment

Funded Activity Summary

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.

Objectives:
1. Tag 150-200 juvenile SBT/year for 3 years with archival tags throughout the full range of spatial habitats in order to provide a comprehensive understanding of their spatial dynamics;
2. For each tag returned (expected to be ~ 20-30%) estimate daily positions based on the stored light and temperature data and develop a database for the storage and analysis of all relevant location, temperature and depth data;
3. Provide a comprehensive analysis of the evidence for temporal changes in the spatial dynamics of juvenile SBT and analyses of the implication of the information provided on mixing rate between themajor SBT fishing and their changes over time for the use of combined archival and conventional tagging data to provide fishery independent estimates of fishing mortaility for monitoring the SBT fishery.
4. Provide critical information and contribute to developing a framework for incorporating the archival tag and conventional tagging data within the SBT stock assessment model;
5. Integrate the position, temperature and depth data provided by the tags with oceanographic data to develop a seasonal model of residence times and habitat use for regions with consistent temporal patterns across the years;
6. Evaluate the implication from a seasonal habit model for the interpretation of future catch and effort data and monitoring strategies.
7. Evaluate implications of the spatial dynamics of juvenile SBT for the management of the SBT resource (e.g. the potential consequences and benefits of either ignoring or using spatially explicit management actions).

Funded Activity Details

Start Date: 19-06-2003

End Date: 01-05-2011

Funding Scheme: Funding Scheme not available

Funding Amount: $1,206,055.00

Funder: Fisheries Research and Development Corporation

Research Topics

ANZSRC Field of Research (FoR)

There are no FoR codes available for this funding activity

ANZSRC Socio-Economic Objective (SEO)

There are no SEO codes available for this funding activity

Other Keywords

Animal Welfare | Behaviour | Fishing Effort | GIS | Habitat | Harvest Strategy | Migration | Survey | Tagging | Wild Catch

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