Downstream migration and mortality of larval Ayu (Plecoglossus altivelis altivelis) on the Sanriku coast, northern Japan

The ecology and mortality of larval ayu (Plecoglossus altivelis altivelis) during downstream migration was investigated during three spawning seasons (2008-2010) in the Unosumai River, Iwate Prefecture, Japan. Downstream larval migration occurred from early September to mid-December, peaking from late September to mid-October. Larval drift numbers recorded at the lowest reach spawning ground peaked from 18:00-21:00, twenty-four-hour-surveys in 2008 and 2010 resulting in total daily numbers of drifting larvae of 2,855,660 and 785,880, respectively, with 88.1 and 88.7%, respectively, occurring between 17:00-24:00. A twenty-four-hour-survey at the river mouth in 2010 recorded larvae between 19:00-06:00 (following day), peaking at 21:00 (67.5% of larvae occurred between 21:00-23:00), approximately 2 h later than the peak at the lowest reach spawning ground. The river mouth survey estimated a total larval number of 113,724, corresponding to 14.5% of the number recorded from the lowest reach spawning ground on the same day, indicating a mortality rate of 85.5%.