Spawning activities and physical characteristics of the spawning ground of Lethenteron reissneri at the headstream of the Himekawa River, central Japan

 The spawning period of the Far Eastern brook lamprey, Lethenteron reissneri, in the headstream of the Himekawa River is estimated to be between mid-March and late May with the peak of spawning activity between early April and early May. The sex ratio (female:male) in 1999 ranged from 1 : 2.5 to 1 : 3.0 (mean 1 : 2.8) and in 2000 from 1 : 0.8 to 1 : 4.0 (mean 1 : 2.4). In >90% of the observations of spawning nests, males outnumbered females. The construction area of spawning nests tended to shift upstream during the spawning period. The nests were constructed at water depths between 5 and 70 cm, water velocity between 10 and 30 cm/s, and on substrate with pebbles of 5–20 mm in diameter. Lethenteron reissneri constructed nests on substrate similar with Petromyzon marinus, but at shallower points and in areas with a slower water velocity. Received: April 2, 2001 / Revised: December 12, 2001 / Accepted: December 27, 2001