Evaluation of population viability of wild chum salmon Oncorhynchus keta in the Toyohira River, Sapporo metropolitan watershed, Japan

¹Sapporo Foundation for Green Park, Sapporo Salmon Museum, Sapporo, Hokkaido 005-0017, ²Hokkaido National Fisheries Research Institute, Fisheries Research Agency, Sapporo, Hokkaido 062-0922, ³National Research Institute of Aquaculture, Fisheries Research Agency, Nikko, Tochigi 321-1661, Japan

Natural spawning of chum salmon Oncorhynchus keta has been observed during the past 30 years in the Toyohira River in the Sapporo metropolitan area, an area from which they were once exterminated. However, the population viability of wild chum salmon is still unknown as hatchery salmon from non-local broodstock have been released into the Toyohira River throughout this time. For brood years 2003-2006, all hatchery released fry from the Toyohira and Chitose hatcheries were marked within the riverine system. For these brood years, wild salmon accounted for 59.2-75.9% of the total number of spawning adults in the Toyohira River, and straying of salmon from the Chitose River hatchery was very low (0.6%). If the marine survival rates are assumed to be similar between wild and hatchery salmon, the egg to fry survival rate of naturally spawned fish is estimated to be 12.6%. This suggests that the egg-to-fry survival rate is low in the Toyohira River, possibly because of poor in-stream environmental conditions. Interestingly, the age at maturity and spawn timing differed between wild and hatchery salmon, indicating that adaptations to the environmental conditions of the Toyohira River may occur after approximately seven generations.