Acoustic Tracking of Aquarium Reared Akame (Lates Japonicus) Released in Shimanto Estuary
Akame, Lates japonicus, is a rare top-predator fish endemic of Japan and found in estuaries and coastal waters on the east Pacific coast of the country. This species has become a symbol fish in the sport fishing world because of its big size and rareness of capture, and it has been catalogued as endangered by the Ministry of Japan, but very few studies on it had been conducted. One of the areas where it can be found frequently is the Shimanto estuary, in Kochi prefecture. To help in the conservation of this endangered predator, a local aquarium of Shimanto city, the Akame Gakuyukan, catches juvenile fishes in Shimanto estuary, rears them in the aquarium until they reach over 40 cm total lengths, and then releases them back together with children from local schools. Since the natural mortality of the juveniles is believed to be higher compared to bigger sizes, they expect that this activity will increase the number of adult Akame in Shimanto estuary, helping to ensure the continuity of the species in the area. However, it is unknown whether the released fishes can indeed survive after they enter the wild, and if they remain inside Shimanto estuary after they are released. To answer these questions, in October of 2014, an acoustic pinger (Vemco, V9P) was inserted into four individuals before release, and the fishes were tracked inside the estuary with nine acoustic receivers (Vemco, VR2W). The results showed that all fishes were alive and moving around the estuary after release, and remained inside it for over six months. Additionally, it was observed that two of the released individuals shifted between residencies following daylight, and another one moved daily following the tides during spring. The results of this experiment suggest that the release of Akame in Shimanto estuary has a high chance of increasing the total number of Akame, and if results can be extrapolated to the wild ones, that the habitat use of Akame of 40 cm in total length is limited to the brackish water areas located around three kilometres upstream of the sea mouth.
Index Terms- biotelemetry, fish release, residence shifts, Lates japonicus, Akame, Shimanto.