Multiple spawning and related variations in female reproductive parameters of amphidromous type ayu

Adult ayu Plecoglossus altivelis were sampled from the Nezugaseki River, Yamagata Prefecture to investigate the reproductive parameters and analyze their variations related to multiple spawning. Ovarian histology (mainly based on the occurrence of postovulatory follicles) indicated that female ayu can be divided into four groups, which are considered to correspond with unspawned, once spawned, twice spawned, and three (or more) times spawned individuals. Comprehensive analysis of the ovarian state with field surveys revealed that most of the female ayu achieve second spawning and that third spawning is very common in this river. Mean relative batch fecundity (RBF) for the second, third, and fourth spawning indicated 69, 42, and 22%, respectively, of the value for the first spawning, showing a decline with the number of spawning batches. Mean gonadosomatic index (GSI) values of non-ovulating fish showed a similar decline during the main spawning period (October). Condition factor (without gonads) during autumn also decreased with the number of batches spawned, suggesting that multiple spawning is achieved partly by transfer of body mass nutrients to the ovary. Size dependency in reproductive parameters such as occurrence of the multiple spawning, RBF values, and GSI values of the unspawned fish, was not evident except for GSI values in early September (generally during the early phases of ovarian development), when larger fish had larger GSI.