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



543 



Preferred water temperature 



Water temperature is thought to be one of the most impor- 

 tant environmental factors controlhng the distribution of 

 young Pacific bluefin tuna (Sund et al., 1981; Koido and 

 Mizuno, 1989; Ogawa and Ishida, 1989). Kitagawa et al. 

 (2000) attached importance to the gradient of water tem- 

 perature; however Uda (1957) emphasized the absolute 

 value of temperature, although his study was presumably 

 for large-size fish. Data from the archival tags indicated 

 that young Pacific bluefin tuna seemed to prefer to remain 

 in water of 14-20°C. When there was no accessible water 

 within this temperature range, the fish tended to stay in 

 water of a temperature as close as possible to this range. 

 In addition, archival tags showed that the vertical distri- 

 bution of young Pacific bluefin tuna was restricted by the 

 thermocline, even when the temperature below the ther- 

 mocline was in the tunas preferred temperature range 

 (14-20°C). These observations support the importance 

 of water temperature as shown in previous studies and 

 suggest that both the absolute value and the gradient of 

 water temperature are important as environmental factors 

 controlling the distribution of young Pacific bluefin tuna. 



Feeding 



Visceral temperature of pen-held Pacific bluefin tuna with 

 archival tags changed in a specific way during feeding. 

 Stomach temperature changes have also been observed 

 in pen-held giant bluefin tuna in the Atlantic and in pen- 

 held southern bluefin tuna (Carey et al., 1984; Gunn et al., 

 2001). The cycle of visceral temperature change for young 

 Pacific bluefin tuna was completed in 21 hours (shorter 

 than that observed in previous studies of 1.5 to 2 days) 

 probably due to the smaller size of the fish. Similar vis- 

 ceral temperature changes were also noted in the records 

 of archival tags recovered from wild fish, ranging from a 

 distinct pattern the same as that observed in pen-held fish 

 (type A) to less distinct ones such as type B or type C, which 

 were observed more frequently. All of these changes could 

 be distinguished quite easily from gradual decreases of vis- 

 ceral temperature when fish dived into cold water There- 

 fore, we assumed that these three types of temperature 

 changes were caused by feeding. Temperature changes of 

 type A could be expected if fish consumed a large amount of 

 food at one time as they do when fed in a pen. However, wild 

 fish may seldom have an opportunity for such large meals, 

 and the apparently more frequent small meals would be 

 expected to cause the less dramatic visceral temperature 

 changes of types B or C. 



In the present study, a visceral temperature change was 

 taken to indicate feeding only when that change could not 

 be explained by a change in water temperature. Also, when 

 the water temperature changed very frequently, it was dif- 

 ficult to decide whether water temperature could account 

 for a feeding event and it was not counted as such. Finally 

 it is possible that feeding might not cause a recognizable 

 change in visceral temperature. As a result of these three 

 factors the feeding frequency estimated in our study might 

 have been underestimated. 



Frequencies of feeding events did not change much over 

 the year, although there was a slightly higher frequency 

 in early summer Growth in length of young Pacific bluefin 

 tuna is known to become slow in winter (Yukinawa and 

 Yabuta, 1967; Bayliff, 1993). Because fish weight at a 

 length was constant throughout the year for wild young 

 Pacific bluefin tuna (Itoh, 2001), food consumption in win- 

 ter appears not to be used for increasing weight at a length. 

 We did not reach a conclusion on this question and further 

 investigation of seasonal change in food items and of the 

 physiology of tuna is needed. 



Acknowledgments 



We thank the staff of Marino Forum 21 and the Kagoshima 

 Fisheries Experimental Station for their cooperation in the 

 pe-held fish experiment. We also thank troll fishermen, 

 staff in the Kamiagata Fisheries Cooperative Association, 

 the Tsushima Fisheries Extension Service, and the Naga- 

 saki Fisheries Experimental Station for their cooperation 

 with the experiment on wild fish. We greatly acknowledge 

 fishermen, consumers, and staff at the Inter-American 

 Tropical Tuna Commission for their kindness in returning 

 recovered archival tags bearing the information necessary 

 for our study. We especially thank J. Gunn at CSIRO for 

 giving us valuable information about implanting the archi- 

 val tag in fish. We are also grateful to the staff of Northwest 

 Marine Technology Inc. and Tanaka Sanjiro Co., Ltd., for 

 providing us with tags. We would like to thank P. Ekstrom 

 of Northwest Marine Technology Inc. for his critical review 

 and help with the English text. We thank our staff in Japan 

 NUS Co., Ltd., the Suidosya Co., Ltd., and the National 

 Research Institute of Far Seas Fisheries, and also T. 

 Kitagawa in the Ocean Research Institute of the Univer- 

 sity of Tokyo, for their efforts in regard to implantation 

 of the tags in fish. We gratefully acknowledge S. Kume of 

 Japan NUS Co., Ltd., N. Baba of Fishery Research Agency, 

 Z. Suzuki, and Y. Uozumi of National Research Institute of 

 Far Seas Fisheries for their critical review. 



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