Natural spawning of chum salmon Oncorhynchus keta within a hatchery stock enhancement program : a case in the Otsuchi River at the Sanriku coast

Natural spawning of chum salmon, Oncorhynchus keta, in the Otsuchi River within a hatchery-based stock enhancement program was assessed for the first time in the 2014–2015 spawning season. Adult chum salmon, their spawning redds and carcasses were visually counted biweekly by walking along the river 1.0 to 4.0 km from the river mouth, during 15 September 2014 to 27 January 2015. A total of 160 spawning redds and 808 carcasses were found and at least 904 adults were counted upstream of the hatchery weir, suggesting that chum salmon spawn naturally in the Otsuchi River within a hatchery stock enhancement program. The majority of spawning redds (91.3%) were found in the area of 2.0–2.5 km from the river mouth and the main spawning area appears to have shifted more than 500 m upstream compared to the previous description in 1962. Retention of carcasses in the Otsuchi River (less than 14 days) was significantly shorter than Hokkaido (30–40 days during autumn and 70–80 days during winter). Warm water temperatures and relatively rapid flow by the steep gradient of the river in Sanriku Ria likely made carcasses quickly decomposed or washed away. This study suggested a possible contribution of naturally spawning chum salmon for returning adults in the Otsuchi River and a particular ecological role of salmon carcasses in the riverine ecosystem in Sanriku Ria