Dagorn et al.: Association of Thunnus olbacares with tracking vessels 
47 
joined by yellowfin tuna 1, and also helped to determine 
the size of the school that yellowfin tuna 3 was drawn 
away from. However, the use of a multibeam sonar that 
can observe the few meters below the surface could pro- 
vide complementary information, especially for such close 
observations. Similar sonar units have been successfully 
used to observe the structure and the behavior of small 
pelagic fish schools (Gerlotto et al., 1999) and would be 
particularly appropriate for the observation of both tuna 
schools and prey in the vicinity of a tracking vessel. A 
long-range multibeam omnidirectional sonar could also 
provide useful information on the horizontal distribution 
and spatial dynamics of tuna schools around the vessel. 
Rather than disregarding tuna aggregations around 
tracking vessels, we propose to continue collecting infor- 
mation on such events. The tracking vessel represents 
a useful and fully instrumented, mobile floating object 
adapted to conduct detailed ethological observations to 
improve our knowledge of the behavior of tuna aggregated 
around floating objects. 
Acknowledgments 
This work was done within the frame of the ECOTAP 
Program (studies of tuna behavior using acoustic and 
fishing experiments), which is a joint program between 
SRM (Service des Ressources Marines de Polynesie 
Frangaise), IFREMER (Institut frangais de recherche pour 
l’exploitation de la mer), IRD (Institut de Recherche pour 
le Developpement). The authors wish to thank all the sci- 
entists who participated in the cruises, as well as the offi- 
cers and crew of the IRD RV Alis for providing valuable 
help during all the cruises. We are also very grateful to 
David Itano and Dave McConaghay, who kindly improved 
the English text, and the three referees who made many 
constructive suggestions. 
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