Spawning Habits of the Dwarf Ayu-fish in Lake Biwa

Abstract

The spawning habits of the dwarf Ayu-fish, Plecoglossus altivelis, in Lake Biwa were studied in the autumn of 1972–1974. Spawning grounds were mainly located in shallow riffles of lower reaches of rivers flowing into the lake. As is the case with the amphidromous type, the principal physical characters indicating an area as the spawning ground of the dwarf type seemed to be the type of substrate and water velocity. General conditions of the spawning grounds were unsettled sandy-pebble bottoms, 10–30 cm in depth and 30–70 cm/s in water velocity. Spawning usually occurred in the nighttime. Spawners crowded together in the spawning grounds. Prior to extruding eggs, one of the males followed and nudged a female. The female then pushed her snout to the bed of the river. Simultaneously the male dashed and swam in parallel with the female, and both fishes quivered closer together. As to the sex ratio, males predominated in the spawners crowded in the spawning ground, while females predominated in the pool near the spawning ground in the nighttime. In contrast, in the daytime a greater proportion of males was recorded in the pool as compared with the spawning ground. Gonad conditions and stomach contents of the Ayu-fish taken from the spawning ground differed from those in the pool. These facts suggest that spawners acting in the nighttime move to the pool in the daytime. Whereas, immatures resting in the pool in the nighttime move to the riffle and feed on algae attached to stones in the daytime. © 1978, The Japanese Society of Fisheries Science. All rights reserved.