Itoh et al.: Swimming depth, water temperature preference, and feeding of Thunnus onentahs 



541 



and dusk were defined as explained above, a one-hour pe- 

 riod centered on the time of first or last detected daylight. 

 Changes in visceral temperature of "type B" (increase only) 

 were observed most frequently (51.4% of total observed 

 feedings), followed by "type C" (decrease only, 45.2%). Bi- 

 polar events (type A) were as few as 3.5%. 



When averaged over individual months, the number of 

 feeding events per day per individual ranged from 0.9 in 

 January to 2.2 in June and averaged 1.5. Feeding events 

 were observed all year, although they were slightly more 

 frequent in May and June than in other months (Fig. 9). 



Discussion 



Diurnal and seasonal change of swimming depth 



Young Pacific bluefin tuna were previously assumed to 

 swim near the sea surface based on the fact that most of the 

 catch was made by surface fishing gear and fish 

 schools were observed at the sea surface (Yabe et 

 al., 1953). However, details of their vertical swim- 

 ming behavior and relationships between their 

 behaviors and oceanic structures have not been 

 well investigated. Recently, Marcinek et al. (2001) 

 observed during an acoustic tracking experiment 

 over several days that Pacific bluefin tuna in the 

 eastern Pacific Ocean spent the majority of their 

 time in the top portion of the water column. Our 

 archival tag data showed that young bluefin 

 tuna in the western Pacific Ocean also ordinarily 

 stayed within the surface mixed layer and most 

 frequently near sea surface, regardless of the 

 time of day or the season. The vertical distribu- 

 tion of fish changed according to the seasonal 

 change in depth range of the surface mixed layer 

 and appeared to be controlled by the depth of the 

 thermocline. Restriction by the thermocline was 

 also observed for yellowfin tuna (T. albacares) and 

 bigeye tuna (T! obesus) (Carey and Olson, 1982; 

 Holland et al., 1990b; Cayre and Marsac, 1993; 

 Block et al., 1997). Occasionally young Pacific 

 bluefin tuna dived through the thermocline into 

 deep, cooler water, but they returned to the surface 

 mixed layer after a short period. 



Diurnal change in swimming depth, i.e. deeper 

 swimming depth during daytime, was reported 

 by acoustic tracking studies not only for Thunnus 

 species, such as yellowfin tuna (Carey and Olson, 

 1982;Hollandetal.,1990b;Cayre,1991;Yonemori-) 

 and bigeye tuna (Holland et al., 1990b), but also 

 for other large pelagic species, such as skipjack 

 tuna, Katsuwonus pelamis (Yuen, 1970; Dizon et 

 al., 1978); swordfish, Xiphias gladius (Carey and 



30 



o 25 



20 



15 



10 



3 4 5 6 



9 10 11 12 



Month 



Figure 5 



Monthly change of average water temperature (O) and 

 average visceral temperature (•) in young Pacific bluefin 

 tuna with archival tags. Average values of each individual 

 were averaged. 



^Yonemori.T. 1982. Swimming behavior of tunas by 

 the use of sonic tags — a study particularly of swim- 

 ming depth. Far Seas Fish. Res. Lab. Newsletter 

 44:1-5. Pelagic Fish Resource Division, 5-7-1 Shi- 

 mizu-Orido, Shizuoka, Shizuoka, 424-8633, Japan. 

 [In Japanese.] 



o 



25 



20 - 



E 15 - 



10 



C A C C C C 



T^'^^^^^rtT- 



Oh 2h 4h 6h 8h lOh 12h 14h 16h 18h 20h 22h 

 Time of day (h) 



Figure 7 



An example of visceral temperature change in a wild young Pacific 

 bluefin tuna recorded by an archival tag. Visceral temperature (thick 

 line) and water temperature (thin line) are shown. Shadows indicate 

 nighttime. Data are from a fish in the Sea of Japan on 2 May 1996. A, 

 B, and C indicate the types of visceral temperature changes described 

 on page 540. 



