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The brown trout, a giant invasive fish that swallows ayu whole, is increasing in number in various regions. As the ayu fishing season approaches, local fishermen and others carried out a mass eradication operation in a river in Gifu Prefecture.
■3,000 eggs, spawning multiple times Brown trout are currently wreaking havoc in rivers across Japan. The captured fish was 70 centimeters long, about the same size as an 8-month-old baby. Brown trout, which swallow native fish whole, lay 2,000 to 3,000 eggs at a time. Unlike salmon, they can spawn multiple times in their lifetime, so their numbers will only continue to increase unless they are eradicated. The Miyagawa River flows through northern Gifu Prefecture. The released juvenile ayu grow in the clear waters surrounded by the rich mountains of Hida. They are called “natabera ayu” because they are wide and thick like a hatchet or spatula. However, they are an invasive species that is devouring the bounty of Hida. We followed the brown trout eradication operation closely.
■Also known as “Chamasu” in Japanese The location is the Kotorigawa River, a tributary of the Miyagawa River. The Miyagawa Lower River Fisheries Cooperative and university researchers investigating the establishment and impact of invasive fish species are participating in the brown trout eradication. Entering the river, the eradication of brown trout begins. The eradication method uses a device called an “electric shocker,” which sends electricity into the water to temporarily paralyze the fish, allowing them to be captured with nets. Assistant Professor Peterson of the Faculty of Science, Toyama University: “These are reeds, but the water is very shallow inside the reeds. There are quite a lot of large brown trout in places like this.” And then, almost immediately… “We’ve caught a brown trout. It has a slightly brown body, and its English name is “Brown,” so it’s also called “Chamasu” in Japanese. It has a brown body with red spots, and red dots in the middle, which makes it easy to identify.” In just a short time since the eradication began, they were catching them one after another… “More and more brown trout are coming in. There are lots of smaller brown trout around here.”
■63 fish caught in one hour of culling Brown trout are widely distributed from Europe to West Asia. They prefer cold water and their lifespan in rivers is said to be 5 to 8 years. They have now entered a slightly deeper area. There seemed to be more small brown trout in the areas with a lot of grass, so they have moved into the deeper area. 50 minutes after the start of the culling… Assistant Professor Peterson: “We’ve caught a big brown trout. We’ve caught the big brown trout we were aiming for. This one is about 30 centimeters long. When they get this size, we are very concerned about their impact as predators.” When we measured it, it was 35.7 centimeters from head to the tip of the tail fin. “They generally start becoming piscivorous at around 30 centimeters, so this 35-centimeter brown trout has definitely eaten plenty of fish. Brown trout grow quickly and tend to get quite large. Last year, we caught some that were 60-70 centimeters here. They tend to grow larger than native trout species, especially in small rivers.” On this day, 63 brown trout were caught in the Kotori River in just one hour of culling.
■Does it taste like salmon? Eating and utilizing it What does brown trout taste like? We tried grilling one fresh with salt. The texture and taste are soft, like river fish such as ayu, and it’s light, refreshing, and delicious. To make the mild-tasting brown trout even more delicious, we went to a fish expert. We had brown trout that had been aged at minus 20 degrees Celsius for over a week prepared as sashimi. The color is like white fish, but first, Shigehisa Hayatsu, president of the Tanashi Fishing Port Direct Sales Store, who knows everything about fish, tried it… “It’s fatty and sweet. It’s like salmon. I didn’t know it was this fatty.” “I’m surprised.” Next, announcer Maya Ki tried it. “It’s soft and fatty. It would be delicious even if it were served as sashimi in a restaurant.” The Miyagawa Lower Reaches Fisheries Cooperative culled 1,117 brown trout last year. Shinji Morishita, head of the Miyagawa Lower Reaches Fisheries Cooperative: “I wonder if simply culling them is the right thing to do. I would like to research what kind of utilization would be best in the future.” (From the broadcast on June 19, 2026) [TV Asahi NEWS] https://news.tv-asahi.co.jp
