POP-UP SATELLITE TAGS APPLIED ON BLUEFIN TUNA (THUNNUS
THYNNUS L.): PROBLEMS CONCERNING THE SATELLITE DETECTION
RATES IN THE MEDITERRANEAN SEA
G. P. Arnold#, J.M. de la Serna, C. Yannopoulos, G. Sylos Labini, M. Deflorio, A.
Buckley, J.L. Cort, P. Megalofonou, M. Pappalepore, G. De Metrio.
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CEFAS Lowestoft Laboratory, Pakefield Road, Lowestoft, Suffolk NR33 0HT, UK.
Tel 44 0 1502 524355; Fax +44 0 1502 513865; email address: g.p.arnold@cefas.co.uk
Atlantic bluefin tuna (Thunnus thynnus L.) has been assessed on the hypothesis that
there are two separated stocks - the Eastern-Atlantic stock, including the Mediterranean,
and the Western-Atlantic stock - with limited rates of exchange. Since the hypothesis is
untested and exchange rates between the Western and Eastern Atlantic, as well as those
between the Western and Eastern Mediterranean, remain unknown, an EU-funded
research programme (TUNASAT) involving Italy, Spain, Greece and the UK, carried
out during the three-year period 1998-2001, aimed to help to solve this problem by
tagging bluefin tuna specimens with pop-up satellite tags. A total of 84 tags were
deployed in some places of the Mediterranean and just out the Strait of Gibraltar: 61
PTT-100 single-point and 23 PAT archival pop-up satellite tags. The Argos satellite
system successfully located 12 PTT-100 and 13 PAT tags in the Mediterranean and
eastern Atlantic. Since the rate of detection of the PTT-100 tags was much lower than
expected from previous experience with the same type of tag in the western and central
North Atlantic, it was conducted onshore a series of tests which allowed us to evaluate
the accuracy of locations provided by Argos in the Mediterranean area and eastern
North Atlantic and compare these with values in the eastern USA. The results of the
detection tests clearly indicated that there was a problem with detecting the PTT-100
tags, in soma areas of the Mediterranean Sea, where they were expected to pop up. The
cause of the problem was the relatively low power output of these tags, which were
masked at the Argos satellite receiver by more powerful signals from Argos PTTs on
larger platforms (e.g. oceanographic buoys and fishing vessels), that have resulted in the
non-detection of tags that may otherwise successfully surfaced when programmed to
detach themselves from the fish.