Vertical movement characteristics of tuna (Thunnus albacares) in Pacific Ocean determined using pop-up satellite archival tags

Abstract: Pacific yellowfin tuna (Thunnus albacares) is widely distribute in subtropical and tropical waters, extending to 40°N in the North Pacific Ocean. Understanding the biology of tunas requires the knowledge of where it lives and its various movements between different habitats. But, the movement and habitat of yellowfin tuna in the Northwest Pacific Ocean have been poor understood in the world. A total of 36 yellowfin tunas Thunnus albacares were tagged using Pop-up Satellite Archival Tags (PSATs) and released in 2010-2012 to examine their vertical movement patterns. The results showed that the return rate of 8 tunas deployed from purse seine fishery was 100%, but the recorded period of tags was only 0.5-5 days. The return rate of those tunas deployed from handline fishery was 75%, while the recorded period of tags was 0.5-91 days and the tag of 2 tunas were normally popped up. In this study, the recorded period was lower than 10 days for 18 tags, was 10-20 days for 3 tags and was longer than 20 days for 8 tags. The longest recorded time of fish ID 33339 was 91 days and its straight-line distance was 822 km from the deployed site. The second longest recorded time of fish ID 33869 was 89 days because of the 3 months set time, while its straight-line distance was only 10 km. The tunas deployed from handline fishery provided loner-time information. About 85.9% of time for yellowfin tunas lived in the 0-150 m depths, 13% of time in ≥150-250 m and only 1.1% of time lived in the ≥250 m depths. As for as water temperature, 81.7% of time yellowfin tunas lived in ≥24℃, 16.2% of time in 16-24℃ and only 2.1% of time lived in ≤16℃. In 0-50 m depth, the occurrence frequency of yellowfin tunas in night was 2 times higher than that in day, while the occurrence frequency of them in day was larger than that in night in 50-500 m depths. From the most tags data, we found that the yellowfin tuna can go up to surface water layer (<10 m) and the maximum habitat depth of 53.3% individuals both above 300 m. For example, the fish (tag 33869) have reach to 1 100 m water depth and the corresponding temperature only was 4.9℃. In the activity layer of yellowfin tunas, the minimum temperature of 80% of tuna were above 10℃, while the maximum temperature of tuna were more than 26℃. As a whole, the daily depth of yellowfin tuna was obviously deeper than the nightly depth at 23.5 m. About 87.5% individuals have the phenomenon of obvious diurnal vertical migration. In the whole, about 68% of individuals begun to move the shallow waters at dusk (18:00), and then they dived to relative deepwater at dawn (06:00). For example, the data of tag 33869 and 33882 showed that these two tunas have similar movement patterns in 24 hours. They both begun to dive deeper water layers from 5 to 6 clocks in the early morning, then maintain at the 60-160 m layer, and begun to rise to 20-50 m layer to habitat at dusk (17:00-18:00). In general, this preliminary study on the tag and release of yellowfin tuna has proved to be successful and can be used as a protocol for tagging study of tunas in the future.