Abstract.- The horizontal and 

 vertical movements of yellowfin tuna 

 Thunnus albacares and bigeye tuna 

 T. obes2is captured near fish-aggre- 

 gating devices (FADs) were deter- 

 mined using pressure-sensitive ultra- 

 sonic transmitters. The movements 

 of these FAD-associated fish were 

 compared with the tracks of yellow- 

 fin tuna not associated with FADs. 

 Tracks from 1 1 yellowfin and 4 big- 

 eye tuna were obtained; these in- 

 cluded 23 complete 24-hour periods 

 of observation. Whether associated 

 with FADs or a 40-fathom (75-m) 

 reef dropoff, most yellowfin and big- 

 eye tunas exhibited similar diurnal 

 patterns. The fish tended to remain 

 tightly associated with FADs or the 

 reef dropoff during the day, move 

 away at night, and return the next 

 morning. The maximum range of 

 these nighttime excursions averaged 

 approximately 5 nmi. These tunas 

 apparently treated the FADs as out- 

 liers of the coastal topography. This 

 may not be the same behavior that 

 results in the association of these 

 species with drifting objects such as 

 logs. Tuna can learn FAD positions 

 and navigate precisely between 

 FADs that are at least 10 nmi apart. 

 When not associated with FADs or 

 the 40-fathom dropoff, yellowfin 

 tuna oriented to the bottom of the 

 mixed layer (50-90 m) in daytime, 

 whereas the bigeye occupied depths 

 between 190 and 250 m. The daytime 

 distribution of bigeye tuna seemed to 

 be influenced by the depth of the 

 15°C isotherm. Both species swam 

 closer to the surface at night. Swim- 

 ming strategies possibly associated 

 with energy and thermoconservation 

 were observed. 



Horizontal and Vertical 

 Movements of Yellowfin and 

 Bigeye Tuna Associated with Fish 

 Aggregating Devices* 



Kim N. Holland 



Hawaii Institute of Marine Biology, University of Hawaii 

 PO Box 1346, Coconut Island, Kaneohe, Hawaii 96744 



Richard W. Brill 

 Randolph K.C. Chang 



Honolulu Laboratory, Southwest Fisfneries Science Center 

 National Marine Fisheries Service, NOAA 

 2570 Dole Street. Honolulu, HI 96822-2396 



Tropical and subtropical pelagic fishes 

 aggregate around natural, drifting 

 debris such as logs and mats of algae. 

 Man-made, anchored floating objects, 

 known as fish aggregating devices 

 (FADs), have also proven effective in 

 attracting and holding commercially 

 important pelagic species (Shomura 

 and Matsumoto 1982, Matsumoto et 

 al. 1981, Brock 1985). Species com- 

 monly found around FADs in Hawaii 

 are yellowfin tuna Tliunnus aWamres, 

 bigeye tuna T. obesus, skipjack tuna 

 Katsuwcmus pelamis, dolphin or mahi- 

 mahi Coryphaena hippurus, and wa- 

 hoo or ono Acanthocybium solandri. 

 The first commercial FADs were de- 

 ployed in the calm waters of the Phili- 

 ppines in the early 1970s to attract 

 yellowfin tuna (Kihara 1981), and in 

 1977 experimental FADs designed 

 for use in high-energy, deep-water 

 environments were anchored around 

 the Hawaiian islands (Matsumoto et 

 al. 1981). Due to the success of these 

 bouys in aggregating fish, FADs have 

 come to play an important role in the 

 commercial, subsistence, and recrea- 

 tional fisheries of all the tropical and 

 sub-tropical oceans of the world. 



Manuscript accepted .5 February 1990. 

 Fishery Bulletin, U.S. 88:493-507. 



*Sea Grant Publication UNIHI-SEAGRANT- 

 JC-90-01, and Hawaii Institute of Marine 

 Biology Contribution No. 774. 



Despite the widespread use of FADs, 

 left unanswered are important ques- 

 tions concerning their range of influ- 

 ence, optimal placement, and impact 

 on surrounding fish populations. The 

 behavioral patterns that result in fish 

 being associated with floating objects 

 of any type are poorly understood 

 and not widely agreed upon (Gooding 

 and Magnuson 1967, Hunter and Mit- 

 chell 1967, Fedoryako 1982). How- 

 ever, the underlying mechanisms of 

 aggregation have generally been as- 

 sumed to be the same for both free- 

 floating phenomena such as logs, and 

 anchored man-made objects such as 

 FADs (Shomura and Matsumoto 

 1982). To better understand the be- 

 havior of tunas associated with FADs, 

 the movements of yellowfin and big- 

 eye ttina caught within 500 m of these 

 devices were tracked using pressure- 

 sensitive ultrasonic transmitters. The 

 movements of these fish were com- 

 pared with those of tunas that were 

 not associated with FADs, but with 

 the reef perimeter surrounding the 

 islands. In Hawaii and elsewhere in 

 the Pacific, yellowfin tuna can be 

 found in the vicinity of the 40-fathom 

 (75-m) isobath where the island topog- 

 raphy abruptly descends into very 

 deep water. In fact, the dropoff of the 

 outer reef edge is often so steep that 



493 



