Tuna farming and fattening play an important role in Mediterranean aquaculture production and the tuna sushi sashimi industry. To preserve the tuna stock, the International Commission for the Conservation of Atlantic Tunas (ICCAT) establishes mandatory management measures to regulate fishing effort, catch, and farming capacity, and allocates annual state quotas (Fromentin, 2006). To better control Bluefin tuna farming/fattening practices, the Commission requested in 2018 an update on farmed fish growth rates, i.e., determining the growth of individual fish under farmed conditions. Therefore, the objective of this study is to present the novel tagging method, used for the first time in tuna farming, with the derived results of individual fish growth during the 18-month farming cycle.
Materials and methods
The tagging experiment targeted 2-3-year-old fish (> 8 kg) caught for commercial farming purposes and then kept under standard farming conditions for 18 months. In July 2019, a total of 206 tuna juveniles between 7.5 and 25 kg were PIT tagged in the head muscle. The fish were individually weighed, measured, and distributed in equal numbers in the two experimental cages for farming. During the farming cycle, fish growth was monitored seasonally for each cage using a stereo camera and image analysis system. All fish were harvested at the end of the season and subsequently measured individually.
Results and discussion
A total of 206 fish distributed between two cages were tagged with PIT tags, most of which were successfully captured at harvest, allowing 157 individual growth rates to be determined. PIT tagged fish were mostly evenly distributed across all size classes in the experimental cages. During the farming cycle, juvenile Bluefin tuna reached the overall harvested weight between 58 and 64 kg. The average 500% increase in body weight during the experimental period did not differ between two experimental cages or between tagged and untagged fish. Recorded mortality of tagged fish throughout the farming period was negligible (1%). The unrecorded proportion of tagged fish (22%) could be due to a combination of factors, such as detector and reader failure due to routine harvest procedures. The PIT tag appears to be biologically inert, as there is no evidence of tissue inflammation in the wound area. In summary, an experienced and trained team is capable of performing mass tagging of juvenile tuna for scientific research purposes with acceptable losses.
Financial support for this study has been provided by Atlantic-Wide Research Programme For Bluefin Tuna (ICCAT GBYP) (Phase 10).
Fromentin, J.M. Atlantic Bluefin Tuna. In ICCAT Manual. International Commission for the Conservation of Atlantic Tuna; International Commission for the Conservation of Atlantic Tuna: Madrid, Spain, 2006; pp. 93–111.