Tagging casts light on mysteries of bluefin tuna migrations
July 22 2010 Lewis Smith
Bluefin tuna being reeled in for tagging
Electronic tags attached to bluefin tuna have thrown up unexpected findings about the behaviour and migratory routes of the threatened fish.
By fixing recording tags to bluefin tuna researchers have been able to get unprecedented data about the movements of the creatures.
Among the most surprising findings was that none of the fish tagged in the exercise left the Mediterranean for the Atlantic Ocean.
Large numbers of adult bluefin tuna are believed to enter the Mediterranean in the spring to spawn and leave at the end of summer.
However, none of the 38 fish that were caught and tagged in the WWF project swam into the Atlantic which suggests many bluefin spend far more time in the Mediterranean than previously believed.
One of the tagged fish, a juvenile, spent 391 days swimming around before it was recovered, making it the first bluefin to have been recorded for an entire year’s life cycle in the Mediterranean.
Data provided from the tag revealed its route as it sought out food and roamed the sea. It was caught and tagged off the coast of Roses, in Spain, in August 2008 and was recovered in September 2009 close to its starting point having taken in the Balearic Islands, the South Tyrrhenian Sea, and going beyond the west coast of Sardinia before swimming southwards past Sicily and to the Tunisian coast. From here it returned to cruise the Balearics again until its recapture. It travelled 11,607 miles with the tag, travelling an average of 13.7 miles daily.
Researchers said the first results of the programme suggest the life of bluefin tuna, one of the world’s most overfished species, is far less well understood than had been believed. In particular, they suggest that the routes and timings of migrations, and the choice of foraging areas are much more complicated than previous observations have shown.
Dr Sergi Tudela, Head of Fisheries at WWF Mediterranean, said the ioneering research should act as "a wake-up call" to the politicians, officials, scientists and fishermen involved in managing fisheries and setting quota levels.
"These results of WWF's tuna tagging work in the Mediterranean Sea have provided surprising and valuable insights into bluefin behaviour," he said.
"WWF has distributed its new findings widely and is urging fisheries decision-makers to apply the precautionary principle in Atlantic bluefin tuna management: until impacts on the species are fully understood, extreme caution must be applied."
Of the 38 fish involved in the study, 25 were tagged in 2008 and a further 13 in 2009, with more planned this year. Useful data was gathered from 11 recovered tags – others are presumed lost, are still on the fish or came off too quickly to be of any use. Apart from the 391-day juvenile, the fish were recapured from 30 to 172 days after being fitted with tags.
The tags record a mixture of data including the fish’s speed, location, depth, times and the temperatures of both the animal and the water.
Analysis of the data provided by the tags revealed differences in bluefin behaviour in winter compared to summer. During the winter months the fish spent most of the day at depths of 150 to 300 metres but in the summer they spent much more time at the surface, while making frequent dives to 300m. At night time, they swam close to the surface in both seasons but would make deep dives more frequently during the winter.
In its report, 'On the Med Tuna Trail', on the initial results of the tagging programme the WWF said: "Results obtained from the 2008 and 2009 expeditions so far suggest that both adults and juveniles of the species in the western and central Mediterranean spend more time exploiting the habitats of the Mediterranean basin than originally thought.
"Results suggest that tuna use the waters around the Balearic Sea not only as a spawning ground, as has been believed to date, but also as a suitable foraging ground."
The lack of cross over between western and eastern Mediterranean fish also suggested that interaction between the two populations may be much more limited than had previously been supposed.
Two types of tags were fitted to the tuna. Adult fish had 'pop-up' tags which are designed to release and rise to the surface to transmit data. Juveniles had a tag inserted beneath their skin in their undersides which could be recovered when the animals were caught - a 300 Euro reward was offered to fishemen for each one recovered.
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