The parental biomass of SBT remains at historically low levels, there is evidence from CPUE and VPA analyses that recruitment has continued to fall throughout the 1990’s, and there is significant disagreement within the CCSBT Scientific Committee on the prediction of population levels into the future.

It is therefore essential to know more about the recruitment dynamics of SBT, and in particular to reduce the uncertainty in the aerial survey estimates of surface abundance of juveniles in the GAB. These remain the only fishery independent source of abundance data on SBT, and as such their importance cannot be overstated.

To reduce the uncertainty in the current aerial survey estimates we need to investigate how environmental factors and surfacing behaviour influences what is seen during the surveys. If the current levels of uncertainty in the indices are reduced by incorporating these sources of variation into the indices, the value of the indices to the CCSBT will be substantially increased.

There is large variation in estimated surface abundance between survey replicates within a season and between seasons but there are also large differences between years in environmental conditions in the GAB (eg sea surface temperature), which confound the interpretation of changes in the index between years. It is possible that the apparent decline in surface abundance over the last 6 years is due to environmental and behavioural variation rather than indicating a true decline in the global abundance of juvenile SBT.

To adequately understand how environmental variation and the resulting behavioural responses of SBT affect the recruitment indices we require thorough analyses of:
1 the surfacing behaviour in SBT and its relationship with environmental variables, migration patterns and possibly also feeding behaviour,
2 the relationship between surface abundance (ie. what the aerial survey detects) and environmental variables,
3 the spatial variation in abundance of SBT in the GAB (incorporating both data on surface abundance from the aerial survey and data from archival and conventional tagging experiments).

The proposed project will use all existing data, collected over almost a decade, with funding from industry, FRDC, CSIRO and Japan. These data are an invaluable resource. The integration of behavioural, environmental and abundance data into an improved estimate of the surface abundance of SBT is listed under Priority 2 and 3 of STBMAC Research Priorities.