Comparisons of growth of chum salmon fry released into rivers of which temperature and prey abundance conditions were different

摘要

Riverine environments are important factors that must be considered when deciding on which rivers to carry out hatchery programs of chum salmon Oncorhynchus keta fry. In this study, the surveys were conducted in the Chitose River and its tributary, the Mamachi Stream, in order to compare the growth of released fry. These rivers were different in thermal condition and prey abundance, i.e. the Chitose River was characterized as a high prey abundance, low temperature environment, and vice versa for the Mamachi Stream. In the two rivers, we monitored temporal changes of body size from 2008 to 2012, and in addition to body size, stomach fullness in 2012. The results showed that growth was greater in the Mamachi Stream than in the Chitose River. In addition, stomach fullness was greater in the Mamachi Stream than in the Chitose River, and the difference became greater with time. These results suggest that high temperatures caused high growth through high foraging efficiency. The relationships between prey abundance and growth were still unclear because of failure to evaluate the density of interacting fishes with released fry.