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Juvenile bluefin tunas can dive to depths of more than 1000 meters

A bluefin tuna, tagged by AZTI-Tecnalia in August 2008, 15 miles to the north of Donostia-San Sebastián, was caught on 15 August of this year by a pleasure craft 88 miles north of Bakio.

The animal had fitted electronic tag which enabled its migratory movements and the depth of these, amongst other data, to be obtained. This is a yet another success story in the tunafish tagging campaigns promoted by the Department of the Environment, Land Planning, Agriculture and Fisheries of the Basque Government and undertaken by AZTI-Tecnalia in collaboration with the Basque Federation of Sailing and Recreational Fishing Associations (FASNAPER) and the San Sebastián and Hondarribia Sailing Clubs.

The recovery of these fish enables the data on the migratory habits and the biological parameters of this species to be improved. The information gained provides an enhanced knowledge of the structure of bluefin tunas and, thus, helps to improve their management.

According to the AZTI-Tecnalia researchers, the first estimations of the geographical location of the recovered tag revealed that this fish had undertaken migrations between the Azores and Portugal during the winter, later to return to the Gulf of Bizkaia in spring and also that it had dropped, during the winter, to depths of more than 1000 metres.

The bluefin tuna was tagged 26 of August 2008 in the Gulf of Bizkaia at coordinates 43º34’N ND 2º00’O and that day it measured 89 cm. in length and its weight was 13.5 kilograms. When caught, it was at 44º44’N and 2º49’O, weighed 22.3 kilograms and its length was 104 centimetres. The electronic tag used in this case was ceded for the project by the University of New Hampshire in the United States, within the remit of the cooperation agreement for the tagging of juvenile bluefin tuna in the North Atlantic. The fishing was undertaken with the collaboration of the San Sebastián and Hondarribia Sailing Clubs.

Conventional tagging involves fixing a visible tag to the underbelly of the fish. Once its biological data - species and size – have been noted and the time, date and position of the catch, as well as the state of the sea, amongst other parameters, the animal is tagged and freed. When the fish is caught again- by another vessel in all likelihood – the information is sent to AZTI-Tecnalia, where it is analysed together with all other second-time catches. In this way, comparing initial and final information helps to determine variables such as growth and migration, data that is of great interest to the scientific community and the fishing sector.

However, in this case, the electronic tag used was surgically implanted in the peritoneal cavity of the fish and, besides storing the previously mentioned data, it also stored information about temperature, pressure and light intensity, which enables a detailed monitoring of the medium in which the fish is moving at any time and an estimate of its geographical position. In order to download the stored information the whole fish has to be sent to AZTI-Tecnalia.

It is not the first time that a tagged fish has been recovered. In fact, the crew members of the recreational craft, Tía Rosario, which tagged this fish, also found a bluefin tuna undertaking a transatlantic migration on 25 July 2005: the fish was subsequently found off the coast of Massachussets (USA) on 14 September 2007. It measured 68 centimetres and weighed 5.5 kilograms when it was tagged in the Gulf of Biscay; 775 days later it was found measuring 114 centimetres and weighing 30 kilograms. The fish had travelled 3,330 marine miles, some 6,000 kilometres (measured as the crow flies).

The fishing campaign of keeping tunafish alive for tagging purposes has been carried out in the Basque Country for seven years now, promoted by the Basque Government on signing the joint working agreement between the Basque Federation of Sailing and Recreational Fishing Associations (FASNAPER) and the AZTI-Tecnalia Technological Centre - experts in Marine and Food Research. Since then Basque pleasure boats have tagged 4,074 fish – long-finned tuna, bluefin tuna, bigeye tuna or skipjack, in the contest category of both tagging and releasing as with the usual activity of recreational boats. This collaboration with those who fish for pleasure is key to tagging and notifying catches of tagged animals. To date, 123 such recreational vessels have been involved in these campaigns.

Irati Kortabitarte | EurekAlert!
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