Initial growth characteristics of poor-return stocks of chum salmon Oncorhynchus keta originating from the Okhotsk and Nemuro regions in Hokkaido on the basis of scale analysis

For Japanese juvenile chum salmon, mortality during coastal residency and northward migration to the Sea of Okhotsk is considered to affect their stock size. Although poor adult returns by the 2012 and 2013 brood years were widely observed along Japanese coasts, the mechanisms underlying mortality have not been defined. We collected scales from age-4 homing chum salmon captured in September–November in 2005–2017 (i.e., the 2001–2013 brood years) from three rivers in the Okhotsk and Nemuro regions, northern/eastern Hokkaido. Scale analyses revealed that growth rates during coastal residency for the 2012 and 2013 brood years of the three rivers were all significantly lower than those in other brood years. Coastal sea surface temperature (SST) in the target regions in May 2013 (i.e., the main ocean-entry timing for the 2012 brood year) was the coldest among those in 2002–2014 and was assumed to be caused by the terrestrial cold climate. Meanwhile, in May 2014, coastal SST was the second coldest and was likely influenced by offshore cold currents. These results suggest that cold coastal conditions triggered by different sources resulted in lower growth rates during coastal residency and therefore poor adult returns.