Sea-entry conditions of juvenile chum salmon Oncorhynchus keta that improve post-sea-entry survival: a case study of the 2012 brood-year stock released from the Kushiro River, eastern Hokkaido, Japan

We examined the sea-entry conditions (i.e., timing and body size) of juvenile chum salmon that improve their survival during their coastal residency. On 25 June 2013, we sampled 365 juvenile chum salmon [57.5–98.6 mm fork length (FL)] off Konbumori, eastern Hokkaido, ⁓ 20 km east of the Kushiro River mouth, which originated from a hatchery in the Kushiro River. Sea-entry conditions of these Konbumori juveniles back-calculated using otolith daily increment analysis were compared with data from 373 juveniles released from the same hatchery that were captured at the mouth of Kushiro River (i.e., just before sea entry) from April to July 2013. Most of the Konbumori fish were estimated to have entered the sea from 25 May to 5 June, when coastal surface temperatures constantly exceeded 5 °C, which is considered favorable for juveniles. The estimated FLs at sea entry of the Konbumori fish were larger than FLs of fish sampled at the river mouth during a comparable period, which suggests that size-selective mortality existed. Back-calculated post-sea-entry growth rates of fish with larger FL at sea entry, particularly those with FL > 65 mm, tended to be high enough for survival among the Konbumori fish. Assuming growth-dependent mortality, this case study suggests that the release of larger-sized juveniles under favorable coastal temperature conditions improves their survival.