The number of pyloric caeca in the naturally spawning chum salmon Oncorhynchus keta from the Izari River was determined to detect variations by run timing. Pyloric caeca were surveyed from September to December for three brood years. The mean number of pyloric caeca changed according to the run timing. The early run population showed low mean values, whereas the late run population showed high mean values. There were no differences in the mean number of pyloric caeca among the brood years. Thus, the number of pyloric caeca may be a useful trait to detect seasonal runs in naturally spawning chum salmon.
