Dagorn et ai.: Association of Thunnus albacares with tracking vessels 
41 
1 52 22 W 
Time (hours) 
12:00 16:00 20:00 0:00 4:00 8:00 12 00 
Figure 1 
Horizontal (upper graph) and vertical (lower graph) movements of yellowfin 
tuna 1, 60 cm FL. Tracking lasted 22 h (October 1995). The period of associa- 
tion between the fish and the tracking vessel is shown by the bold line. Arrows 
indicate the direction of horizontal movement in the upper graph. The light gray 
patch represents a prey patch observed by the echo sounder. 
tween 10 and 500 m depth with a SIMRAD (SIMRAD, 
Horten, Norway) EK500 scientific sounder connected to a 
hull-mounted SIMRAD ES38B split-beam transducer (fre- 
quency 38 kHz, beam angle 6.9°). Acoustic data, along 
with vessel position, were simultaneously logged on a per- 
sonal computer running SIMRAD EP 500 software (Sim- 
rad, 1994). Vessel speeds were estimated from straight-line 
calculations by using positions of the tracking vessel based 
on data from the Global Positioning System for the first 
two fish. Speeds were taken directly from the knot meter 
of the vessel for fish 3, which provided a greater volume of 
data on real-time movements of the tracking vessel. 
When the crew suspected the fish had become associ- 
ated with the tracking vessel, experiments were developed 
to test the association (complete turns, changes in vessel 
speed and direction, etc.). Fish that clearly followed the 
vessel during such tests were considered to be associated. 
Results 
Of the fourteen yellowfin tuna that were tagged and 
tracked in French Polynesia from 1985 to 1997, three indi- 
viduals clearly exhibited strong and lengthy associations 
with the research vessel. 
Tuna 1 (60 cm FL) was caught at a depth of 120 m at 
midday close to a FAD anchored near Maupiti Island, lo- 
cated within the Leeward Islands of the Society Archipel- 
ago. The fish was tracked for 22 h as indicated in Figure 1. 
This fish associated with the FAD immediately after 
release but shifted to a free-swimming (unassociated) 
phase directed offshore (eastward movement) until 17:14 h, 
crossing for the first time a patch of mid-water prey ob- 
served on the echo-sounder. After crossing the patch for 
the second time (beginning of the night), the tuna returned 
to the FAD but did not re-associate; rather it began a cir- 
