Some alien fishes, such as the bluegill (Lepomis macrochirus macrochirus) pose a significant threat to the native biodiversity in Japan. Therefore they have become strictly prohibited across the nation. All activities related to the possible introduction of the species into the wild are currently punishable by a fine of up to $30,000 for a person ($900,000 for corporations), or even a prison sentence of up to 3 years.
In June 2015, Akinori Teramura tweeted two photographs of the invasive bluegill, both adults and juveniles, along with two young goldfish, which do not belong to the local fauna, either. In his post he identified the species and shared his surprise at the irresponsibility of the people who had released the fish. When a researcher saw the tweet, he decided together with a colleague and the discoverer to publish this information as a scientific report.
The student found all fish in an outdoor public pool in Yokohama City, Japan, while it was being cleaned before opening for the summer. Usually, these facilities are closed to the public during the colder seasons and it is then when native aquatic insect species, such as dragonflies and diving beetles, use them as spawning and nursery habitats. Curiously enough, though, the pool had been isolated from natural waters since its construction.
Therefore, the researchers conclude that the alien fishes have most likely been released from an aquarium from a local shop or an aquarist who no longer wanted them. However, keeping bluegill fish in a home aquarium is illegal as well.
Such aquarium releases are not uncommon and invasive snakeheads in the US and the invasion of lionfish in the Caribbean are well documented examples on what can go wrong if people act irresponsibly. This study is a nice example of applied citizen science and how social media can help to quickly disseminate information.