Utilization of artificial spawning beds by endangered bitterling fish in an agricultural channel in southern Okayama, western Japan

We focused on artificial spawning beds for a conservation method of endangered bitterling fish inhabiting agricultural channels. To clarify any preference for host mussel species by bitterling fish, we conducted a field experiment to set artificial spawning beds in an agricultural channel in southern Okayama, western Japan, between March and August 2014. We set nine artificial spawning beds (50 cm×36 cm×8 cm) each with a single species of three mussel species (Unio douglasiae nipponensis, Pronodularia japanensis and Lanceolaria grayana cuspidata) made of rectangular plastic trays at the experimental site. After three weeks, we retrieved the mussels from the spawning beds and then individually reared them in aquaria in the laboratory to observe the number and the species of bitterling juveniles that emerged from each host mussel. The total number of juveniles that emerged from the host mussels was 679( 420 from U. douglasiae nipponensis and 259 from P. japanensis; no individuals from L. grayana cuspidata). We identified four bitterling species including three endangered native Tanakia limbata, Tanakia lanceolata and Acheilognathus rhombeus, as well as the invasive Rhodeus ocellatus ocellatus, but the number of A. rhombeus was only 9 because this fish is an autumn-spawning species and thus spawned before the experiment (i.e., in 2013). Rhodeus ocellatus ocellatus spawned U. douglasiae nipponensis and P. japanensis, but T. limbata and T. lanceolata utilized only P. japanensis as host mussels, indicating that host mussel preference differed among the three bitterling species. In this study, an endangered species Rhodeus atremius suigensis, inhabiting the experimental channel, designated as a Nationally Endangered Species of Wild Fauna and Flora by the Ministry of Environment of Japan did not utilize the spawning beds. We need to clarify the preference for mussel species and appropriate artificial spawning beds for R. atremius suigensis.