Abundance of salmon carcasses at the upper reach of a fish trap

Abundance, biomass and retention of salmon carcasses (Oncorhynchus keta and O. masou masou) were studied in a spring-fed tributary, the Naibetsu River, Chitose River system, Hokkaido, northern Japan, in 1998–2003, where almost all adult salmon were caught for a salmon enhancement program by a fish trap at the lower reach every fall. The abundance and biomass fluctuated seasonally, with two peaks in fall and winter, and annually. The total number of carcasses in fall was significantly correlated with the highest water level at lower reach in fall, indicating that the adult salmon swam over the trap at flood. Carcass abundance in winter may depend on the population size spawned in the wild at the upper reach, since the trap was removed in this season. Since the large wood debris and gravel were absent or scarce, many carcasses were lost from the study site within 20 days and a few carcasses remained over 31 days. Carcass removal by some terrestrial animals from the study site was detected.