765 



Abstract— Ninety-six bigeye tuna (88- 

 134 cm fork Icnfjth) were caught and 

 released with imphinted archival (elec- 

 tronic data storage) tags near fish- 

 aggregating devices (FADs) in the 

 equatorial eastern Pacific Ocean (EPO) 

 during April 2000. Twenty-nine fish 

 were recaptured, and the data from 

 lui'iity-seven tags were successfully 

 downloaded and processed. Time at 

 liberty ranged from 8 to 446 days, and 

 data for 23 fish at liberty for 30 days 

 or more are presented. The accuracy 

 in geolocation estimates, derived from 

 the light level data, is about 2 degrees 

 in latitude and 0.5 degrees in longitude 

 in this region. The movement paths 

 derived from the filtered geolocation 

 estimates indicated that none of the 

 fish traveled west of 110°W during the 

 period between release and recapture. 

 The null hypothesis that the movement 

 path is random was rejected in 17 of 

 the 22 statistical tests of the observed 

 movement paths. The estimated mean 

 velocity was 117 km/d. The fish exhib- 

 ited occasional deep-diving behavior, 

 and some dives exceeded 1000 m where 

 temperatures were less than 3°C. 

 Evaluations of timed depth records, 

 resulted in the discrimination of three 

 distinct behaviors: 54.3% of all days 

 were classified as unassociated (with a 

 floating object) type-1 behavior, 27.7% 

 as unassociated type-2 behavior, and 

 18.7% as behavior associated with a 

 floating object. The mean residence 

 time at floating objects was 3.1 d. Data 

 sets separated into day and night were 

 used to evaluate diel differences in 

 behavior and habitat selection. When 

 the fish were exhibiting unassociated 

 type-1 behavior (diel vertical migra- 

 tions), they were mostly at depths of 

 less than 50 m (within the mixed layer) 

 throughout the night, and during the 

 day between 200 and 300 m and 13° 

 and 14°C. They shifted their average 

 depths in conjunction with dawn and 

 dusk events, presumably tracking the 

 deep-scattering layer as a foraging 

 strategy. There were also observed 

 changes in the average nighttime 

 depth distributions of the fish in rela- 

 tion to moon phase. 



Movements^, behavior^ and habitat selection of 

 bigeye tuna (Thunnus obesus) in the 

 eastern equatorial Pacific, 

 ascertained through archival tags 



Kurt M. Schaefer 

 Daniel W. Fuller 



Inter American Tropical Tuna Commission 



8604 La Jolla Shores Dnve 



La Jolla, California 92037 1508 



E-mail address (for K M Schaefer) kschaefer(5)iattc.org 



Manuscript accepted 24 May 2002. 

 Fish. Bull. 100:76.5-788 (2002). 



Bigeye tuna (Thunnus obesus) occur 

 in subtropical and tropical oceanic 

 waters throughout the world, except 

 in the Mediterranean Sea (Collette 

 and Nauen, 1983; Collette et al, 2001). 

 They are captured by longliners oper- 

 ating in the eastern Pacific Ocean 

 (EPO) from about 40°N to 40°S and 

 by purse seiners from about 5°N to 

 lb°S (Miyabe and Bayliff, 1998; Bayliff. 

 2000). They are economically the most 

 important species of tuna harvested 

 by longline fisheries in the EPO. The 

 estimated average catch of bigeye tuna 

 by the longline fleets of Japan, Taiwan, 

 and the Republic of Korea in the EPO 

 has declined from an average of about 

 79 thousand metric tons (t) during 

 198.5-94 to about 40 thousand t during 

 1995-98 (Bayliff, 2000). The longline 

 fishery targets medium to large bigeye 

 tuna. There is a growing purse-seine 

 fishery that catches primarily small to 

 medium bigeye tuna, most of which are 

 associated with drifting fish-aggregat- 

 ing devices (FADs). The purse-seine 

 catch in the EPO has dramatically 

 increased from an estimated annual 

 average of about 5 thousand t during 

 1964-93, to about 45 thousand t during 

 1994-98 (Bayliff, 2000; Lennert-Cody 

 and Hall, 2000). There is concern that 

 the longline fishery is being adversely 

 affected by the purse-seine fishery and 

 that current catch rates are not sus- 

 tainable (Watters and Maunder, 2001). 

 Elucidating the environmental vari- 

 ables that define bigeye tuna habitat 

 can be used to adjust gear vulner- 

 ability, to standardize catch and effort 

 data, and thus improve bigeye tuna 

 stock assessments (Brill, 1994; Hinton 



and Nakano, 1996; Brill and Lutcav- 

 age, 2001; Hampton et al.M. It is also 

 necessary to understand the affinity 

 of bigeye tuna to drifting FADs, now a 

 common feature of their habitat in the 

 equatorial EPO, and to quantify the ef- 

 fect of FAD densities on vulnerability 

 to capture. Sonic tracking studies (Hol- 

 land et al, 1990; Holland et al„ 1992; 

 Daggorn et al., 2000), along with stud- 

 ies of catches per unit of effort (CPUEs) 

 by longlines (Suzuki and Kume, 1982; 

 Hanamoto, 1987; Boggs, 1992; Nakano 

 et al., 1997) have provided information 

 on differences in depth and tempera- 

 ture distributions for bigeye tuna by 

 size and time of day. 



Studies of fine-scale movements and 

 short-term behavior, based on data 

 from acoustic tags, have been reported 

 for bigeye tuna associated with an- 

 chored FADs (Holland et al., 1990) and 

 in the open ocean (Dagorn et al., 2000), 

 but not for fish associated with drifting 

 FADs. Bigeye tuna tend to stay within 

 close proximity of anchored FADs dur- 

 ing the day, move away at night, and 

 return the next morning (Holland et 

 al. , 1990). When not associated with 

 FADs, they swim within the mixed 

 layer above the thermocline at night 

 and well below the thermocline during 



Hampton, J., K. Bigelow, and M. Labelle. 

 1998b. Effect of longline fishing depth, 

 water temperature and dissolved oxygen 

 on bigeye tuna iThunnus obesus) abun- 

 dance indices. Working Paper 17, 18 p. 

 Eleventh meeting of the standing commit- 

 tee on tuna and billfish, Sec. Pac. Comm., 

 Noumea, New Caledonia. Oceanic Fish- 

 eries Programme, SPC, B.R D5, 98848 

 Noumea Cedex, New Caledonia. 



