Schaefer and Fuller: Movements, behavior, and habitat selection of Jhunnus obesus 



785 



hypothesis of Marsac et al. (2000) that association 

 with FAD.s causes bigeye tuna to be retained within 

 areas or transported to new areas, thus creating an 

 ecological trap. An alternative hypothesis, suggested 

 by Hunter and Mitchell ( 1966), is that FADs function 

 by simply providing a visual stimulus in an optically 

 void environment. An extension of their hypothesis 

 should include the fact that FADs provide a general 

 sensory stimulus, including sound produced by the 

 FAD and associated fauna, which may be the mecha- 

 nism by which tunas locate FADs. In addition to a 

 general sensory stimulus. FADs may also function as 

 reference points (Freon and Dagorn, 2000). 



Archival tags have provided data on the type of 

 habitat selected (light levels, depths, and tempera- 

 tures) that should be useful for standardizing the 

 catch per unit of effort (CPUE) of bigeye tuna by 

 surface and longline fisheries in the EPO. The data 

 in Table 5 indicate that bigeye tuna greater than 

 110 cm spend Sl^i or more of their time above 50 m, 

 in the mixed layer at night and 53''v or more of their 

 time between 200 and 300 m during the day. Habi- 

 tat-based stock assessment models (Hinton and 

 Nikano. 1996; Hinton and Deriso, 1998; Hampton 

 et al. ') have been developed for the integration of 

 data, such as those provided in Table 5. to adjust 

 effort based on estimated fishing depth of longline 

 gear (Mizuno et al.. 1999) in relation to the vertical 

 distribution of target species by time of day. The 

 fishing depth of longline gear has been shown to be 

 an important source of variation in the CPUE for 

 bigeye tuna (Hanamoto, 1987; Boggs, 1992); higher 

 catch rates of bigeye tuna have been associated 

 with greater fishing depths of the longline gear. 

 This has been interpreted previously as a prefer- 

 ence of bigeye tuna for 10° to 15°C water during 

 daylight hours (Hanamoto. 1987; Holland et al., 

 1990; Boggs, 1992; Brill. 1994). 



We suggest that bigeye tuna are most likely not 

 selecting their daytime and nighttime habitats 

 based on temperature, depth, or light preferences, 

 but on the distributions of their preferred prey. 

 Cephalopods and mesopelagic fishes also show 

 diel vertical migrations (as do other organisms) as- 

 sociated with the DSL. We suggest that the depths 

 and temperatures preferred by bigeye tuna during 

 daylight hours when exhibiting unassociated type- 

 1 behavior are the environmental variables associ- 

 ated with their preferred prey. 



The greater depths and lower temperatures at 

 which bigeye tuna are caught during the daytime 

 in the CWPO (Hampton et al.''; Miyabe") may be a 

 function of the greater daytime depths of the DSL 



' Miyabe, N. 199.5. Follow-up study on the stock status 

 of bigeye tuna in the Pacific ocean. Western Pacific 

 Yellowfin Research Group 5. working paper 12; 21-23 

 August 1995, 15 p. Oceanic Fisheries Programme, 

 SPC, B.P D5, 98848 Noumea Cedex, New Caledonia. 



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