Abstract
We examined the combined effects of fasting in freshwater and water temperature during the transition from freshwater to seawater on growth/metabolic parameters in juvenile chum salmon Oncorhynchus keta. Juveniles of 1 g in freshwater were first subjected to a 5-day fasting period at 10 or 5 °C, acclimated to either 10 or 5 °C seawater and fed ad libitum for 30 days. Control fish were transferred from 10 °C freshwater to 10 °C seawater and fed ad libitum throughout the experimental period. Serum insulin-like growth factor (IGF)-I was measured to evaluate growth status/potential and liver glycogen as an index of energy storage. Fasting in freshwater for 5 days negatively affected body size. Growth of juveniles kept at colder temperatures was retarded in seawater for at least 20 days, which may partly be explained by a lower feeding rate in cold seawater. Serum IGF-I levels were lower in fasted fish in freshwater at both temperatures and colder seawater had a negative effect on restoring serum IGF-I levels after refeeding for 20 days. Liver glycogen content was low in fish fasted in freshwater for 5 days. After refeeding in seawater for 10 days, liver glycogen content increased significantly in juveniles kept at colder temperatures. Colder water temperatures in both salinities positively affected glycogen content for 30 days after transfer to seawater, suggesting that juveniles allocated energy stores to the liver rather than growth under suboptimal feeding and temperature conditions. The findings of the present study suggested that relatively cold freshwater could negatively affect juvenile chum salmon growth soon after sea entry.
Highlights
Chum salmon Oncorhynchus keta is an important commercial fish along the coasts of Hokkaido and northern Japan
Serum insulin-like growth factor (IGF)-I levels were not recovered in these fish, and liver glycogen content was very high (Nakamura et al 2019). These results suggested that when juvenile chum salmon experienced fasting during downstream migration and entered cold seawater, they sacrificed growth and stored energy in the liver to prepare for future nutritional deficit
The present study is an expansion of Nakamura et al (2019), who reported the synergetic negative effect of fasting in freshwater and transfer to cold seawater on the growth of juvenile chum salmon in a short-term
Summary
Chum salmon Oncorhynchus keta is an important commercial fish along the coasts of Hokkaido and northern Japan. They are obligatory anadromous fish: hatching in freshwater, remaining in rivers for a few to several months depending on the region and river system, and migrating to the ocean in the spring of their first year (Salo 1991; Urawa et al 2018). Released chum salmon juveniles can adapt to seawater through hypo-osmoregulatory changes (Kashiwagi and Sato 1969; Wong et al 2019).
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