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This paper provides (i) brief consideration of options to collect data for improving the IOTC yellowfin stock assessment, (ii) an introduction to the general concept of Close-Kin Mark Recapture (CKMR) - a reasonably new, but proven fisheries assessment tool (e.g. it has been successfully applied to southern bluefin tuna), and (iii) a rough evaluation of the logistical and economic feasibility of applying this tool to the IOTC yellowfin tuna (YFT) population.
CKMR is a form of mark-recapture experiment in which the spawning population size (and various other population parameters) can be estimated, based on the number of closely-related individuals identified (using modern DNA genotyping technologies) in a sample from the commercial catch. The population model estimator involves detailed demographic calculations (resembling typical age-structured assessment models, and with the potential to be integrated into such a model). However, the general concept is intuitively easy to understand, and illustrated with cartoon examples, e.g. given a random sample of individuals from a population, more closely-related pairs are expected to be observed in a smaller population.
CKMR avoids many of the problems that affected the IOTC RTTP tagging programme, including: i) there is no tag release phase that requires locating and handling live fish, ii) tagging mortality is irrelevant, because only the catch is sampled, iii) tag shedding is irrelevant because the “tag” is the DNA of the fish, and iv) tag recovery reporting rates are irrelevant because you know exactly which fish are “inspected for tags” (and there is effectively zero chance of missing the tag). CKMR also avoids many of the problems of commercial CPUE data, because the data are acquired from a systematic sampling design with the a priori intent of answering specific questions. In contrast, commercial fisheries are motivated to maximize profits and are expected to rapidly change fishing technology, species targeting, time and areas of operation, etc., without regard for the scientific requirements of proper experimental design (consistency, range coverage, orthogonal factor contrast, etc.). CKMR samples must be collected with forethought about spatial issues, but this should not be a problem for yellowfin tuna with IOTC member cooperation in commercial catch sampling.
Based on the population numbers estimated from the 2018 yellowfin assessment (and an alternative sensitivity case with numbers doubled), it appears that CKMR should be a viable assessment tool. A one-time application could provide information about absolute spawner abundance (and other population parameters) with a cost (of DNA extraction and genotyping) of 1-2 million Euro. This is around the cost of the current Indian Ocean stock structure project, or 10-20 % of the (inflation-adjusted) RTTP-IO tagging programme. An ongoing CKMR programme could be used to monitor population trends (for stock assessment and Management Procedures). This paper is only intended to introduce the CKMR concept to the IOTC WPM/WPTT and demonstrate that it is a viable option for raising the scientific rigour of IOTC yellowfin tuna stock assessments – it is not a full design study.
