Bluefin tuna tagged by AZTI off Basque coast has been recaptured in Malta seven years after being released
It was just a year and a few months old, weighed just over five kilograms and measured 65 cm at the time of its release. When it was captured seven years later, it was two metres in length and weighed 160 kilograms. It had become an adult specimen, capable of reproducing. It was found in the ADJ Tuna marine farm located in Malta. The bluefin tuna had got there after being captured via purse seine off the southern coast of the Mediterranean country and was transferred to the farm for fattening. At the time of its recapture in the marine farm, an observer from the International Commission for the Conservation of Atlantic Tunas (ICCAT) was there, who proceeded to take down the recapture details and the electronic tag. The tag was internal, located in the belly. It had sensors that measured temperature, light intensity and depth. Participation of the ICCAT observer is decisive, as these tags can only be recovered if the person who finds them, passes them on to the corresponding research centre. Therefore both internal and external tags come with a reward to encourage their return and to ensure that the campaign work is not in vain. This is the first recovery of an Atlantic juvenile bluefin tuna that was at liberty for more than seven years with the tag inside. The probability of recovering tags placed in pre-adult specimens is very low, which reveals the huge importance of the AZTI tag that has been found, storing data for 1,593 days, or in other words 4.3 years. This period is a milestone in the research of pre-adult bluefin tuna. The previous record was for a young specimen captured five years after its release, whose tag contained data for a period of just over three years. Tuna migration estimation is deduced from the information taken from the tag. Initially, AZTI researchers have determined that until the age of two, the animal moved to the centre of the North Atlantic in winter and then returned to the Bay of Biscay in summer. At the age of three and four, the animal crossed the Atlantic Ocean to spend the winter on the North American coast, after which it returned to the European coast, without entering the Mediterranean. Research into young bluefin tuna is particularly important as it is the most unknown phase of the species. The Bay of Biscay is a migration area for young bluefin tuna. One year old specimens come from the Mediterranean and come for feeding. The aforementioned Bay is ideal for the study of pre-adult bluefin tuna and has been used to calculate an abundance index of juvenile tuna that has enabled the eastern stock of bluefin tuna to be evaluated and managed. The bluefin tuna is an emblematic species that stands out for the large size it reaches - over three metres long - with prehistoric evidence of it being fished and it is well appreciated today. This makes its appropriate management vitally important. Therefore, bearing in mind its great migration capacity, which changes with age, scientists resort to methods such as tagging in the evaluation of the two stocks in the North Atlantic. The specimen found in Malta was tagged by AZTI scientists aboard the live bait vessel, Nuevo Horizonte Abierto, as part of a project funded by the European Union as well as the Basque and Spanish Governments, within a data collection programme. The project was carried out in conjunction with researchers from the Spanish Institute of Oceanography from Santander. The precise definition of the population structure and migrations is important to minimise uncertainty in evaluation and subsequent management. Therefore, given the impossibility of carrying out direct campaigns, researchers need to complement the fishery information with a large variety of additional studies. Among them, and in order to know the population structure of bluefin tuna, studies using micro-chemistry, genetics, tagging, etc. are being conducted. This recapture is a vitally important piece to improve scientific knowledge of this valued species.
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