Feasibility of a Close-Kin Mark-Recapture for Stock Assessment of Indian Ocean Swordfish (Xiphias gladius)

The Indian Ocean Tuna Commission (IOTC) manages swordfish, Xiphias gladius, a species of high economic importance in the Indian Ocean. Current stock assessments rely on catch per unit effort indices, which can be biased, complicating reliable abundance estimates. We explored alternative approaches by assessing the feasibility of the close-kin mark– recapture (CKMR) method, a powerful genetic-based approach. This pilot study aimed to validate protocols and guide future CKMR implementation at the IOTC scale. CKMR simulations were conducted to estimate the sample sizes required to detect sufficient kin pairs. Kinship analyses assumed a single panmictic population across the Indian Ocean, consistent with current IOTC management. A total of 2068 individuals were genotyped using SNP markers, identifying one parent-offspring pair (POP) and at least two half-sibling pairs (HSPs). As predicted by simulations, this sampling scale precludes robust estimation. However, results indicate that robust CKMR-based estimates could be achieved by sampling at least 18,000 swordfish over three years, representing about 20% of the current sampling effort already undertaken by contracting parties. The annual cost of genomic data generation for CKMR represents less than 0.5% of the first-sale market of swordfish in the Indian Ocean. Overall, this study supports the feasibility of CKMR for swordfish and provides a foundation for scaling up future programs to improve Indian Ocean stock assessments.