Mahi-mahi are active in surface waters, and Taiwan, China and Japan are joining forces to advance research in the North Pacific

The Hsingkang mahi-mahi season has "timed" its arrival well, and the Eastern Marine Biology Research Center of the Fisheries Research Institute and the research team from National Taiwan Ocean University have also brought good news from mahi-mahi research. Combining the results of mahi-mahi tagging and release in Taiwan, China and Japan from 2016 to 2018, they found that mahi-mahi inhabit waters at a depth of about 70 to 90 m, where the surface water temperature differs by 5 to 6 degC. It can be inferred that the seasonal distribution of mahi-mahi populations will shift under the influence of the marine environment. This research result has been published in the Japanese international academic journal Fisheries Science.

At the end of 2014, the Hsingkang area began promoting the "Mahi-mahi Fishery Improvement Project" (mahi-mahi FIP), and the following year the Fisheries Research Institute cooperated with National Taiwan Ocean University on the "Taiwan, China Mahi-mahi Population Identification and Resource Research." Jiang Weiquan, associate researcher at the Eastern Marine Biology Research Center of the Fisheries Research Institute, explained that current monitoring shows mahi-mahi visit Taiwan, China twice each year: once from April to June, when mahi-mahi swim from south to north, and once from October to December, when mahi-mahi swim from north to south. Because the mahi-mahi fishing grounds of Taiwan, China and Japan both belong to the western Pacific, the research team not only successfully tagged and released mahi-mahi in Chenggong, Taitung, but also cooperated with Nagasaki University in Japan in 2016 and 2018 to tag and release mahi-mahi in the waters of Kagoshima and northern Nagasaki, recording data such as temperature, depth and light intensity. They hope to understand the relationship between Taiwan, China's two fishing seasons and to explore the population structure and movement characteristics of mahi-mahi in the northwestern Pacific.

The research team cooperated with Nagasaki University in Japan, and in 2018 went to the Ikitsuki custom fishing grounds in northern Nagasaki, Japan, to conduct mahi-mahi tagging and release research. During tagging, characteristics such as fish body length and sex were measured. The monitoring results showed that the satellite tags all detached in the northern East China Sea, and a total of 5 to 30 days of data on mahi-mahi moving northward were recorded.

The study found that mahi-mahi off eastern Taiwan, China inhabit shallower water layers above 56.6 m in summer at water temperatures of 25 to 29.7 degC, and in winter inhabit shallower water layers above 83.4 m at 23.2 to 29.1 degC. In Japan, mahi-mahi are seen only in winter each year. Mahi-mahi in the waters of Kagoshima Bay inhabit shallower water layers above 52.4 m at 20.5 to 23.7 degC, while those in the waters of northern Nagasaki inhabit shallower water layers above 94 m at 17.8 to 23.1 degC. It can be seen that the habitat depth of mahi-mahi is related to the temperature of the sea area, and they dive only within waters where the depth differs from the surface water temperature by 5 to 6 degC. Therefore, changes in the marine environment can easily affect the seasonal population distribution of mahi-mahi, and may in turn affect catches.

Jiang Weiquan added that, regrettably, because mahi-mahi are highly active, the satellite tag recorders attached to the fish bodies were easily shaken off by the research team's subjects. At present, the longest recording time is only 30 days, so the research team has not yet grasped the migratory route of mahi-mahi, and therefore cannot infer the relationship between the mahi-mahi populations of Taiwan, China and Japan. This year they will continue the tagging and release program for mahi-mahi in Taiwan, China and Japan, and will also add tag recorders with photographic functions, hoping to learn about mahi-mahi movement and feeding in the sea and to improve understanding of mahi-mahi populations.

Chen Junru, Director-General of the Fisheries Research Institute, said that according to United Nations Food and Agriculture Organization (FAO) statistics, global mahi-mahi production in 2015 was about 120,000 metric tons, with Taiwan, China ranking 4th with an annual output of 9,000 tons. In response to export market requirements for capture fisheries, Taiwan, China promoted the mahi-mahi FIP in 2014, developing early-warning strategies and catch control measures, making it the first fish species in the country to undergo a fishery management plan. To implement an FIP, rigorous scientific research is needed as a basis to know whether overfishing is occurring. Therefore, the Fisheries Research Institute will also continue cooperating with fishery scientists from Japan, the United States and other countries, hoping to understand variables such as mahi-mahi life history, population movement characteristics, age structure and fishing gear, so as to carry out mahi-mahi resource assessment and ensure the sustainable operation of the domestic mahi-mahi capture fishery.

Overlaying the movement routes of mahi-mahi with a sea surface temperature map (red triangles are release sites after tagging; pink stars are the positions where tags detached; blue lines are estimated movement positions) shows that the habitats of the two mahi-mahi populations in seawater are similar, in waters about 70 to 90 m deep where the temperature differs from the surface water by 5 to 6 degC.

Jiang Weiquan, associate researcher at the Eastern Marine Biology Research Center of the Fisheries Research Institute, demonstrates how a satellite tag is inserted into a fish body. Because mahi-mahi are highly active, they shake off the tag, so the recording time is only 30 days at most, and a mahi-mahi migratory route cannot be established. This is the research threshold that must be overcome next.