Supportive breeding of the Tokyo bitterling in Tochigi Prefecture, Japan

Summary

The Tokyo bitterling (Tanakia tanago), a Vulnerable cyprinid endemic to central Japan's Kanto region and dependent on freshwater mussels for egg-laying, survives in just one wild population confined to a single 900-meter irrigation ditch in the Tone River system, fragmented into inbred sub-groups by small weirs. Genetic surveys since 2006 estimated the population at only about 104 individuals with lower diversity than a captive line founded in 1994, prompting Tochigi Prefectural Fisheries Experimental Station to launch a supportive breeding program using fresh wild founders instead of the older, possibly captivity-adapted line, deliberately pairing fish from genetically distant sampling sites. In autumn 2007, 42 wild fish were caught and re-matured under extended photoperiod and warm water, then bred as 15 separate pairs producing 1,415 offspring; 296 juveniles plus their parents, selected to equalize family representation, were released throughout the habitat in summer 2008 with genetic diversity matching the 2006 wild baseline. By autumn 2009, monitoring found 132 newly emerged young and an estimated total population exceeding 200, more than double the 2006 baseline, while genetic relatedness among individuals isolated above weirs had dropped to statistically unrelated levels, confirming enhanced natural reproduction. Major difficulties included the habitat's severely limited carrying capacity, the labor cost of genetically managed captive pairing, and ongoing poaching by aquarists despite community surveillance. The project rates itself Successful, citing confirmed reinforcement through natural reproduction, though it notes long-term persistence still requires habitat improvement not yet undertaken.