Fisheries Research 149 (2014) 86–91 Contents lists available at ScienceDirect Fisheries Research journal homepage: www.elsevier.com/locate/fishres Short communication Vertical movements of Pacific bluefin tuna (Thunnus orientalis) and dolphinfish (Coryphaena hippurus) relative to the thermocline in the northern East China Sea Seishiro Furukawa a,∗ , Yuichi Tsuda b , Gregory N. Nishihara a , Ko Fujioka c , Seiji Ohshimo d , Seitaro Tomoe e,f , Naoyuki Nakatsuka a , Hideaki Kimura g,h , Takashi Aoshima g , Hisao Kanehara g , Takashi Kitagawa i , Wei-Chuan Chiang j , Hideaki Nakata a , Ryo Kawabe a a Graduate School of Fisheries Science and Environmental Studies, Nagasaki University, Taira-machi, Nagasaki 851-2213, Japan b Fisheries Laboratory, Kinki University, Nishimuro, Shirahama 3153, Wakayama 649-2211, Japan c National Research Institute of Far Seas Fisheries, Fisheries Research Agency, Shimizu, Shizuoka 424-8633, Japan d Seikai National Fisheries Research Institute, Fisheries Research Agency, Taira-machi, Nagasaki 851-2213, Japan e Japan Overseas Cooperation Volunteers, Japan International Cooperation Agency (JICA), Tokyo, Japan f Service Départemental de Pêche et de la Surveillance de Mbour, Mbour, Senegal g Faculty of Fisheries, Nagasaki University, Bunkyo-machi, Nagasaki 852-8521, Japan h Itochu Logistics Corporation, Head Office, Akasaka 3-Chome, Minato-ku, Tokyo 107-0052, Japan i Graduate School of Frontier Sciences, Atmosphere and Ocean Research Institute, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8564, Japan j Eastern Marine Biology Research Center of Fisheries Research Institute, No. 22, Wuchuan Rd., Chenggong, Taitung 96143, Taiwan article info Article history: Received 26 February 2013 Received in revised form 27 August 2013 Accepted 3 September 2013 Keywords: Thermocline Immature Pacific bluefin tuna Dolphinfish Vertical habitat separation Water temperature preference abstract Pacific bluefin tuna, Thunnus orientalis, and dolphinfish, Coryphaena hippurus, are temporally sympatric top predators in the north East China Sea. To clarify their vertical habits in relation to the thermocline, we analyzed swimming depth and ambient temperature data for Pacific bluefin tuna that were obtained from animal-borne data-loggers in April, May, and November 2007. We also reanalyzed published elec- tronic tagging data for free-ranging dolphinfish in the Tsushima Straight. When the vertical temperature structure was homogeneous, Pacific bluefin tuna made vertical excursions to the bottom layer in early May and November. When the thermocline developed in late spring, bluefin tuna made frequent dives below the thermocline, and their main distribution depth shifted to above the thermocline from the sur- face. During the same period, dolphinfish in this area remained near the surface and did not dive across the thermocline. However, swimming data for dolphinfish that were obtained in September and October showed that dolphinfish extended their vertical depth ranges as the thermocline depth increased. © 2013 Elsevier B.V. All rights reserved. 1. Introduction The distribution and abundance of many marine fish are often driven by physical oceanographic conditions, especially water tem- perature (Wildhaber and Crowder, 1990, 1991, 1995; Krause et al., 1998). Changes in water temperature with depth are believed to strongly influence the distribution of pelagic species because ther- mal responses vary among species (Galli et al., 2009; Hochachka and Somero, 2002; Krebs, 2009). The thermocline may play a key role in decision making about vertical habitat preferences in a vari- ety of pelagic predatory fish (Block et al., 2005; Brill et al., 1999). Thus, understanding how such species use their habitat with regard ∗ Corresponding author. Tel.: +81 95 850 7311; fax: +81 95 840 1881. E-mail address: seishirou07@gmail.com (S. Furukawa). to temperature changes with depth requires detailed knowledge about their responses. This can be used to strengthen the founda- tion for the spatial management of marine ecosystems. In the northern East China Sea (ECS), Pacific bluefin tuna, Thun- nus orientalis, and dolphinfish, Coryphaena hippurus, are temporally sympatric top predators. Immature Pacific bluefin tuna are dis- tributed in temperate waters and favor water temperatures of 12–21 ◦ C(Uda, 1957). However, dolphinfish are found mainly in subtropical and tropical waters, but migrate to the northern ECS via the Tsushima Warm Current in late spring, when the surface seawater temperature (SST) is >20 ◦ C(Kojima, 1966; Palko et al., 1982). Hence, in late spring, the habitat of these 2 top predators can overlap along a horizontal scale near the 20 ◦ C SST isotherm in this region. Indeed, dolphinfish are frequently captured as by-catch in bluefin tuna surface longline fisheries in the northern ECS. In the temperate waters of the continental shelf, seasonal changes in thermocline depth and strength can be clearly observed 0165-7836/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.fishres.2013.09.004