The Coho Salmon Oncorhynchus kisutch from Iturup Island

Abstract

A total of 122 coho salmon specimens were examined. The fish were caught in the coastal zone and in the rivers of Iturup Island (southern Kuril Islands) during their spawning migration in August–October 2021. Almost all females and males close to maturity in that year had spent 1 year at sea and were at ages of 1.1+ (66.7%) and 2.1+ (30.0%). Fish that had spent 2 years at sea were found singly: one specimen at an age of 1.2+ and three specimens at an age of 2.2+ (2.5%). Taking into account the age at maturity and the body weight (an average of 2.63 kg for females and 2.44 kg for males), an assumption can be made that the examined fish bred in short streams. The high value of the gonadosomatic index (up to 20–22% for some females) did not indicate that all the fish caught in the coastal waters would have run to other spawning grounds. At least a portion of them should have spawned in the water courses of Iturup Island, as was evidenced by the captures of a few coho salmon immediately in the rivers. Along with the fish running to the spawning grounds, two actively feeding coho females at an age of 0.0+ (0.55 kg) and 1.0+ (1.11 kg) were also caught with stake nets.