Decadal Rapid Change in Energetic Carrying Capacity for Juvenile Chum Salmon in the Sanriku Coast

ABSTRACT Adult returns to coastal rivers in Sanriku, located near the southernmost area of chum salmon distribution, have declined since 2010. Climate‐induced fluctuations in juvenile prey availability may affect growth and survival, which are linked to adult returns. This study integrated 10‐year time series data of physical variables and the growth trajectory of Sanriku juveniles into a Dynamic Energy Budget model to determine whether low prey availability for Sanriku juveniles might result in a low‐return rate for its brood year (BY). The laboratory validation of the model revealed a low assimilation rate with juvenile skeleton ossification and a large energy flux for somatic growth at high temperatures. These ontogenetic and thermal effects on the bioenergetics of chum salmon indicate that anomalously warm conditions can substantially increase the energy requirements and decrease the energy storage capacity. Notably, the application of our model to field observations demonstrated that juveniles required increased energy in the low adult return regime (after the 2015 BYs) than in the high‐return regime (before the 2007 BYs). In contrast, prey availability during the low‐return regime was one‐fifth that observed during the high‐return regime, indicating an energy deficit for juvenile growth and swimming. The potential decrease in the coastal carrying capacity could affect the ability to reach the Okhotsk Sea, and consequently, lower adult returns. Our results highlight the importance of integrating long‐term monitoring data with bioenergetics to understand the effect of environmental variability on salmon populations, within the scope of “Salmosphere” research.