Two key inputs are used to calculate the TAC. The first is the catch per unit of effort, which is an indicator of the current abundance of older fish. This is measured by the number of fish caught per 1000 hooks. The data for this indicator is taken from the Japanese longline fishery, which covers the widest area of all the fisheries and represents a continuous time-series dating back to the late 1950s. However, catch per unit effort rates are not always a direct indicator of the umber of fish – they can vary dramatically according to the fishing location, the season and the ships collecting data. The second input is the data collected during an annual aerial survey of juveniles in the Great Australian Bight, where most juveniles spend their first two to three summers before dispersing across the southern oceans. This is an indicator of how many juvenile fish are coming up through the system to replenish the adult population in the future.
Different management procedures using these two inputs’ data series are tested using an operating model, or ‘virtual reality’, of the SBT stock and fishery. This allows for a range of scenarios to be tested to select the best performing strategy – that is, the one that still achieves the rebuilding of the stock despite the worst-case scenarios
From 2011, the quotas are based on the scientific Management Procedure (MP). An MP is a mathematical formula, which fixes the global quota at the level required to ensure the stock, is sustainable. This means the quota can increase or decrease – depending on the trends in the two stock Indicators used in the MP (i.e. the Australian scientific aerial survey, and Japan’s longline catch rate). The minimum TAC change is 100 tonnes, the maximum TAC changes is 3000 tonnes.
The MP is tuned to a 70% probability of rebuiliding the stock to 20% of its original spwaning stock bioass by 2035. Under the MP, it was decided that the TAC would be set for three-year periods.
The national allocation of the TAC within each three-year period is apportioned according to the Resolution on the Allocation of the Global Total Allowable Catch