40 
Abstract — Three yellowfin tuna (Thun- 
nus albaca res) carrying ultrasonic depth- 
sensitive transmitters developed a 
strong association with the tracking 
vessel, following it at speeds up to 5 
knots (2.6 m/s). Two fish associated 
with the tracking vessel during day- 
time, and the other fish during day 
and night periods. Swimming behavior 
appeared to depend on the speed of the 
vessel. The tuna remained within a few 
meters of the surface when the vessel 
was traveling at high speeds but moved 
deeper when the vessel drifted. The 
behavior of these fish is compared to 
those of other yellowfin tuna tracked in 
other situations (associated with fish- 
aggregating devices or unassociated 
with devices). The reasons for these 
associations are not known but some 
hypotheses are advanced. 
Manuscript accepted 10 July 2000. 
Fish. Bull. 99:40-48 (2001). 
Association of yellowfin tuna ( Thunnus albacares ) 
with tracking vessels during ultrasonic telemetry 
experiments 
Laurent Dagorn 
Institut de Recherche pour ie Developpement (IRD) 
BP 5045 
34032 Montpellier 
Cedex 1, France 
E-mail address: dagorn@ird.fr 
Erwan Josse 
Institut de Recherche pour le Developpement (IRD) 
BP 70 
29280 Plouzane, France 
Pascal Bach 
Institut de Recherche pour le Developpement (IRD) 
BP 5045 
34032 Montpellier 
Cedex 1, France 
Tunas associate with floating objects, 
such as logs, anchored man-made fish- 
aggregating-devices (FADs) (see Freon 
and Misund, 1999, for a review), and 
fishing boats (Fonteneau and Diouf, 
1994). Numerous ultrasonic telemetry 
experiments have been conducted 
(Cayre and Chabanne, 1986; Holland 
et al., 1990; Cayre, 1991; Cayre and 
Marsac, 1993; Marsac et ah, 1996; Bach 
et ah, 1998; Josse et al., 1998; Marsac 
and Cayre, 1998; Brill et ah, 1999) 
to determine the behaviors of tunas 
associated with anchored FADs, but no 
published studies have examined fish 
associated with drifting objects. More- 
over, during ultrasonic telemetry exper- 
iments, the assumption is that neither 
the transmitter nor the tracking oper- 
ation alters the behavior of the fish. 
Some yellowfin tuna, however, have 
developed associations with the track- 
ing vessel — a rare behavior previously 
observed on two occasions (Cayre et ah, 
1996; Brill et ah, 1999). In other words, 
in these situations, the vessel is not fol- 
lowing the fish but the fish is following 
the vessel. 
In our study, we examined the move- 
ments of three yellowfin tuna, which 
clearly followed the tracking vessel dur- 
ing ultrasonic telemetry experiments. 
Our objective was to characterize these 
associations and to compare them with 
other types of association behavior. We 
discuss these observations in relation 
to some hypotheses on the nature of 
tuna associations with floating objects 
and propose ideas for future studies. 
Materials and methods 
Fish movements were monitored with 
acoustic telemetry techniques from the 
research vessel RV Alis. Tracking oper- 
ations were conducted between Octo- 
ber 1995 and April 1996 in French 
Polynesia. The depth-sensitive acous- 
tic transmitters carried by the fish 
and the ultrasonic receiving equipment 
were built by VEMCO (Shad Bay, Nova 
Scotia, Canada), and are described in 
detail in Dagorn et al. (2000). 
Fish were caught on vertical longline 
gear and transmitters were attached 
externally with either nylon tie-wraps 
(as described by Holland et al., 1990) or 
a stainless steel dart (as described by 
Brill et al., 1993). 
During tracking operations, simulta- 
neous acoustic data were collected be- 
