Feasibility Study on Applying Close-Kin Mark-Recapture Abundance Estimates to Indian Ocean Tuna Commission Shark Species

The Close-Kin mark recapture (CKMR) method is an innovative approach that allows estimating abundance and other important parameters by finding pairs of related individuals in a population based on their genetic make-up. The method that has been demonstrated suitable for application to fish and elasmobranch species and has been applied or is under consideration for application to about a dozen species. A revision of the studies performed or ongoing has revealed that the CKMR method can be applied to species spanning a large variety of life-histories, for which diverse levels of background biological knowledge is available, or with limited or extensive sample accessibility, as long as the model is adapted for each particularity. The compilation of the technical considerations associated with a CKMR study design, the evaluation of alternatives to overcome potential complications, and the review of available biological knowledge, catch data and tissue sampling programs has allowed to perform a preliminary assessment of the potential feasibility of CKMR for IOTC sharks. We have identified the blue shark, the shortfin mako and the silky shark as the most suitable species for application of CKMR. The blue shark has the advantage of having a well-known biology and potential for appropriate sampling logistics, but its large abundance suggests that a potentially impracticable number of samples will be needed to obtain the required number of pairs of related individuals. The shortfin mako has sufficient biological knowledge, potential for appropriate sampling logistics and the advantage of having a previous evaluation done for the South Atlantic suggesting that about 5,000-10,000 samples would be sufficient for finding the required number of pairs of related individuals. The silky shark has sufficient biological background but has the disadvantage of having less catch at size data, which might result in a more uncertain estimation of the samples needed. The next steps for assessing if CKMR is considered feasible for these species is establishing more formal sampling designs which incorporate the relevant biology and available information on likely population size to estimate samples sizes required for varying levels of precision in the abundance estimates.