Recovery plan for an exploited species, southern bluefin tuna

Abstract

Southern bluefin tuna (SBT) were heavily depleted in the mid-1980s, and the fishing quota has been restricted since 1985. As a result of this restriction and protection of immature individuals, spawning stock biomass (SSB) recently has shown a slight increase. The Commission for the Conservation of Southern Bluefin Tuna (CCSBT) has the target of recovering SSB to the 1980 level by 2020. We investigated whether SBT populations will recover and reach the target level set by CCSBT. Our projection shows that the SSB of the SBT will temporarily decrease again after 1999. This temporary decrease of SSB does not always mean failure of the recovery plan, because the SSB trend is highly vulnerable to age-composition dynamics. The SBT is an example of this. The number of mature SBT was small during the 1980s because of overfishing. Thus, the number of eggs that were spawned by these mature SBT was small in these years, and when these small numbers of immature fish become mature, the SSB will decrease again. We call this effect the inverse baby-boom effect. The inverse baby-boom effect may be common for managed bioresources that have once been overexploited. We also examine the use of spawning potential (SP) and SSB as an index of stock recovery.