Analysis of Close Kin Relationships as a Fishery Independent Approach to Estimate Spawning Stock Biomass in Southern Bluefin Tuna

Several methods are available for estimating standing stock biomass, a critical measure for management of fisheries stocks and setting total allowable catch (TAC) quotas.  For southern bluefin tuna (SBT), conventional stock assessment methods can give a reasonably precise estimate of relative abundance of SBT today versus near the start of exploitation.  However, estimates of absolute abundance are much less precise, and quite sensitive to different assumptions about data reliability (eg. unreliable reporting of tag returns and changes in fish targeting practices).  In turn, predictions of the effects of different TACs on future stock size are difficult to estimate.  We describe a method for estimating the absolute spawning stock size of SBT, based on genetic identification of parent-offspring matches in adults sampled from the spawning grounds in Indonesia and juvenile fish from the Great Australian Bight. The method is related to mark-recapture, and provides an estimate of true adult numbers (not the unrelated genetic concept of Ne or effective population size). No catch or CPUE data is used, so the estimate is not subject to biases and interpretational problems associated with recent SBT assessments. We explain the statistical basis of the method, comment on its robustness, and describe progress on analysis of DNA from 14,000 fish profiled using 24 microsatellite loci.