Itoh et a\ Migration patterns of Thunnus onentalis determined with archival tags 



531 



These horizontal swimming speeds (1-4 knots) are com- 

 parable to those of larger young Pacific bluefin tuna and 

 same-size fish of other Thunnus species, namely yellowfin 

 tuna ( T. albacares ), bigeye tuna ( T. obesus ), and albacore (T! 

 alalunga), determined from acoustic tracking experiments 

 (Laurs et al., 1977; Carey and Olson, 1982; Holland et al., 

 1990; Block et al., 1997; Marcinek et al., 2001 ). Sustainable 

 swimming speeds based on oxygen demand and supply for 

 yellowfin tuna and skipjack tuna (Katsuwonus pelamis) of 

 1.5-2 kg in body weight were estimated to be 2-4 times FL 

 per second (Brill, 1996;Korsmeyeret al., 1996), correspond- 

 ing to 2.3-4.7 knots for fish of 60 cm FL. Applying to young 

 bluefin tuna the same rule (2 -4 times FL) used by those 

 workers as a summary of their data, the expected range 

 of sustainable swimming speeds that do not accumulate 

 an oxygen debt would cover the range of estimated aver- 

 age swimming speeds during traveling phases. Of course, 

 the swimming speed of young Pacific bluefin tuna based 

 on a constant moving speed between two successive daily 

 locations obviously carries some errors. First, a fish might 

 not maintain a constant swimming speed all day long. 

 For example, the daytime swimming speed of albacore 

 observed in an acoustic tracking experiment was reported 

 to be 1.3-2.1 times as great as that at night (Laurs et al., 

 1977). Second, the influence of water current which should 

 be taken into consideration (Brill, 1996) was completely 

 ignored. Third, assuming straight-line travel between two 

 daily positions would lead to underestimation of actual 

 daily distances traveled, even though the direction during 

 traveling phases could be approximated as a straight line. 

 Even if these errors had been large and the true swimming 

 speed had been twice as large as that which was estimated, 

 these estimated average swimming speeds were still with- 

 in the range of sustainable speeds. 



Although the horizontal movement clearly differed, many 

 features regarding vertical movement were the same for 

 both residency and traveling phases. One parameter that 

 did differ was that of temperature, where the difference 

 between water and fish viscera was 1.0°C larger during the 

 traveling phase than during the residency phase. Feeding 

 causes an increase of visceral temperature in tuna (Carey 

 et al., 1984; Gunn et al., 2001; Itoh et al., 2003). However, 

 the slightly more frequent feeding in traveling phases was 

 not enough to explain the large thermal excess. In addition, 

 the larger temperature difference was observed not only at 

 daytime when the visceral temperature was usually higher 

 because of more frequent feeding, but also at night when 

 the visceral temperature was usually lower (Itoh et al., 

 2003). The visceral temperature seemed to be raised by 

 high muscle temperature during traveling phases. If this 

 is indeed the case, this would lead to an increase in the 

 delivery rate of oxygen to muscle, which would make the 

 fish less tired and more able to travel (Stevens and Carey, 

 1981; Brill, 1996). The distinct traveling phase might be 

 one of the tactics adopted by young Pacific bluefin tuna to 

 use energy most efficiently for long distance travel. 



During the traveling phase, the frequency of feeding in- 

 creased slightly and the fish dived to water deeper than 150 m 

 depth more frequently. The fish would feed and seek food at 

 least as aggressively as in the residency phase. 



Ambient water temperature is one of the most important 

 environmental factors for young Pacific bluefin tuna (Sund 

 et al., 1981; Koido and Mizuno, 1989; Ogawa and Ishida, 

 1989; Itoh et al., 2003). The onset of most traveling phases 

 were preceded by specific water temperature changes that 

 reached the upper or lower limit of the preferable water 

 temperature of 14— 20°C for young Pacific bluefin tuna (Itoh 

 et al., 2003). Changes in ambient water temperature ap- 

 pears to be a possible trigger for a fish to move. Because no 

 remarkable change in frequency of feeding was observed 

 within several days before or after traveling began, the 

 possibility that shortage of prey is a trigger for migration 

 does not seem to be plausible. 



If the impulse to travel in young Pacific bluefin tuna is 

 regulated only by the search for the preferred water tem- 

 perature range and the aim of traveling is to reach the pre- 

 ferred water temperature range, the ambient temperature 

 would be expected to be out of the preferred range at the 

 onset of travel and within that range at the end. However 

 this did not occur in the fish studied. In addition, half of 

 the observed traveling phases were continued after the fish 

 encountered along the way the same temperature that was 

 present at the end of traveling. According to these results, 

 it appears that the preferred water temperature is neither 

 the sole regulator of traveling in young Pacific bluefin tuna 

 nor is it the sole aim of traveling. 



Data from tagged fish released around Japan in the 

 1980s revealed that some fish released from Nagasaki 

 prefecture migrated to the Sea of Japan and others to 

 the Pacific Ocean (Bayliff et al., 1991). Archival tag data 

 showed that fish released in the same season and same 

 area migrated in various patterns involving different onset 

 times and different destinations when traveling from the 

 East China Sea. In addition, some fish continued to remain 

 in the East China Sea. The migration scenario seems not 

 to be fixed or limited for age 0-1 fish distributed around 

 the East China Sea. A detailed examination of fish be- 

 haviors relating to the water temperature has suggested 

 that although young Pacific bluefin tuna prefer a specific 

 temperature range, they can still tolerate temperatures 

 outside of this range (Itoh et al., 2003). This temperature 

 tolerance would contribute to the diversity of migration 

 scenarios for the species. 



The trans-Pacific migration 



The trans-Pacific migration of Pacific bluefin tuna was 

 originally validated by tagging tuna both from the west- 

 ern Pacific Ocean to the eastern Pacific Ocean and from the 

 eastern Pacific Ocean to the western Pacific Ocean (Orange 

 and Fink, 1963; Clemens and Flittner, 1969). The duration 

 required for trans-Pacific migration was estimated as 215 

 days from the shortest interval between the release offish 

 from one side of the Ocean to the recovery of fish on the 

 other side (Bayliff et al., 1991). The present study obtained 

 a full record of daily locations during a trans-Pacific migra- 

 tion of one fish. The fish took two months to traverse the 

 whole Pacific Ocean, which was much shorter than expected 

 from previous records. The starting time for trans-Pacific 

 migration was estimated by Yamanaka (1982) as May- 



