Vertical behavior of bigeye tuna (Thunnus obesus) in the northwestern Pacific Ocean based on archival tag data |
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Authors: | Takayuki Matsumoto Takashi Kitagawa Shingo Kimura |
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Affiliation: | 1. National Research Institute of Far Seas Fisheries, , Shimizu, Shizuoka, 424‐8633 Japan;2. Atmosphere and Ocean Research Institute, University of Tokyo, , Kashiwa, Chiba, 277‐8564 Japan;3. Graduate School of Frontier Sciences, University of Tokyo, , Kashiwa, Chiba, 277‐8561 Japan |
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Abstract: | The behavior of bigeye tuna (Thunnus obesus) in the northwestern Pacific Ocean was investigated using archival tag data for 28 fish [49–72 cm fork length (FL) at release, 3–503 days] released in Japanese waters around the Nansei Islands (24–29°N, 122–132°E) and east of central Honshu (Offshore central Honshu, 32–36°N, 142–148°E). Vertical behavior was classified into three types based on past studies: ‘characteristic’ (non‐associative), ‘associative’ (associated with floating objects) and ‘other’ (behavior not fitting into these two categories). The proportion of fish showing associative behavior decreased and that of characteristic behavior increased as fish grew, and this shift was pronounced at 60–70 cm FL. The fish usually stayed above the 20°C isotherm during the daytime and nighttime when showing associative behavior and below the 20°C isotherm during daytime for characteristic behavior. A higher proportion of characteristic behavior was seen between December and April around the Nansei Islands, and between September and December for offshore central Honshu. Seasonal changes in vertical position were also observed in conjunction with changes in water temperature. In this study, ‘other’ behavior was further classified into five types, of which ‘afternoon dive’ behavior, characterized by deep dives between around noon and evening, was the most frequent. The present study indicated that in the northwestern Pacific Ocean, the vertical behavior of bigeye tuna changes with size, as well as between seasons and regions. |
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Keywords: | archival tags bigeye tuna habitat selection northwestern Pacific vertical behavior |
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