Southern bluefin tuna consist of a single highly migratory stock.[1, 2] Juvenile fish spawn in one area in the Indian Ocean south of Java and the adults move far and wide, with those caught in Aotearoa New Zealand’s EEZ being the easternmost. The broad reach of this species requires a collective effort across the regions to ensure the sustainability of southern bluefin tuna. The Commission for the Conservation of Southern Bluefin Tuna (CCSBT) is an intergovernmental organisation responsible for managing this species, of which Aotearoa New Zealand is a member.
The Commission requires a robust understanding of the size and demographics of the population when setting the total allowable global catch. Traditionally, the Commission relied on physical tagging markers to track individuals and understand population dynamics. However, the loss of tags in the environment and non-reporting by fishers led to data and knowledge gaps and undermined efforts to sustainably manage southern bluefin tuna across the various fisheries.
CSIRO developed an alternative tagging method using genetic technologies to provide better understanding of the stock’s status. The genetic tag is essentially a DNA ‘fingerprint’– it establishes a unique genetic signature for each individual. The tag is lifelong (because the individual’s DNA won’t change) and invisible (because there is no physical tag attached to the fish). It does not rely on reporting from individual fisheries and therefore provides independently verifiable estimates of abundance. Sequencing can be performed quickly in large numbers with high-throughput technologies. However, there is still the need to tag and recapture thousands of individuals, which requires sea time and robust experimental design.
The genetic tag is essentially a DNA ‘fingerprint’– it establishes a unique genetic signature for each individual. The tag is lifelong (because the individual’s DNA won’t change) and invisible (because there is no physical tag attached to the fish). It does not rely on reporting from individual fisheries and therefore provides independently verifiable estimates of abundance.
The researchers tagged individual juvenile southern bluefin tuna and released them back to sea. A year later, samples were taken from the tuna caught by Australian commercial fishers. The researchers estimate how abundant the population is by comparing how many captured fish match the juvenile fish sequenced a year earlier. A high overlap indicates that a high proportion of fish were originally captured, suggesting a smaller overall population size. A low overlap shows that the population is bigger than the number captured earlier. After the first two years of data collection, the researchers estimated that there were approximately 2.3 million age two fish, based on finding 20 matching DNA fingerprints between 3,000 tagged and 15,000 harvested fish. These findings were similar to the median estimate in 2017 stock assessment models of 2.1 million, corroborating this approach. This sampling method is now happening every year so that an annual estimate of species abundance can inform management decisions about the global catch.
This sampling is now happening every year so that an annual estimate of species abundance can inform management decisions about the global catch.
Mark-recapture programmes can be used to understand stock recruitment, movement, growth and survivorship. A similar approach could be used for other fisheries to determine population size to inform TAC. It’s important for the biopsy sampling technique to be suited to the species of interest to reduce the possible harms from this invasive sampling approach. For example, Fisheries New Zealand have evaluated genetic tagging technologies for use with snapper in mark-recapture programmes. While not considered feasible in the short term, it was considered that investment in their development was worthwhile given potential efficiency and precision gains. Research into suitable markers, protocols, development of a biopsy hook and hook deployment protocols would be required before genetic tagging for snapper could be implemented.
References and footnotes
 Proctor, C. H. et al. (1995) Stock structure of the southern bluefin tuna (Thunnus maccoyii): An investigation based on probe microanalysis of otolith composition, Marine Biology, 122(4), pp. 511–526.
 Grewe, P. M. et al. (1997) Genetic population structure of southern bluefin tuna (Thunnus maccoyii), Marine Biology, 127(4), pp. 555–561.
 Bravington et al. (2016) Absolute abundance of southern bluefin tuna estimated by close-kin mark-recapture. Nature Communications, 7(13162).
 Mace, P. et al. (2020) Report of the workshop on the utility of genetic analyses for addressing New Zealand fisheries questions, New Zealand Fisheries Science Review 2020/01.
 McKenzie, J. R. et al. (2015) Evaluation of tagging programme designs for SNA 1 and SNA 8, New Zealand Fisheries Assessment Report 2015/35.