Spawning habitat characteristics and egg mortality in relation to river geomorphology and run-times of chum salmon (Oncorhynchus keta) in a metropolitan river system, northern Japan

We investigated the geomorphology and environmental variables in which early- and late-run chum salmon groups spawn in an urban section of the Toyohira River, northern Japan, in relation to egg mortality, where a braided riverbed had been developing before river improvement occurred. Geomorphic units in the river channel having the highest proportions were riffles > the upwelling zone of gravel bars > pools > secondary channels. Most redds (> 60%) in the early-run group were built in the upwelling zone of gravel bars in the primary stream, indicating that salmon chose this geomorphic unit for spawning. A greater proportion of spawning redds in the late-run group occurred in secondary channels (i.e., smaller subsidiary channels that branch from the main, active channel). The buried-egg experiment showed that egg mortality was lower in the early-run group and higher in spawning redds that were shallower and had a higher maximum water temperature in winter. Late-run salmon need to select habitat with relatively higher water temperatures for spawning to compromise between egg mortality risk and the later timing of offspring hatching. Annual variation in the number of out-migrating fry was most associated with the number of spawning redds of the early-run group. A metropolitan river system may be highly regulated by humans and represents a monotonous river morphology, which nonetheless provides favorable spawning habitat for certain run-time populations because wild salmon may facultatively utilize limited diverse environments for natural reproduction.

Automated summary

This study investigated the geomorphology and environmental variables of chum salmon spawning in an urban section of the Toyohira River in northern Japan. It found that early-run salmon preferred the upwelling zone of gravel bars, while late-run salmon preferred secondary channels. Egg mortality was lower in the early-run group, but higher in shallow spawning redds with higher water temperatures. The number of out-migrating fry was most affected by the number of spawning redds in the early-run group. Despite human regulation and monotonous river morphology, metropolitan river systems can still provide favorable habitat for spawning.