Changes in plasma prolactin and growth hormone concentrations during freshwater adaptation of juvenile chum salmon (Oncorhynchus keta) reared in seawater for a prolonged period |
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| Authors: | Tsuyoshi Ogasawara Tetsuya Hirano Toshio Akiyama Shigeru Arai Masatomo Tagawa |
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| Institution: | (1) Ocean Research Institute, University of Tokyo, Nakano, Tokyo, 164;(2) National Research Institute of Aquaculture, Nansei, Mie 516-01, Japan;(3) Ocean Research Institute, University of Tokyo, Nakano, Tokyo 164, Japan |
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| Abstract: | Freshwater adaptability of chum salmon was examined in juvenile fish reared in seawater for 4 months. The fish, weighing about
40g, were transferred directly to fresh water in October, when their cohorts are migrating in the North Pacific Ocean. Plasma
sodium concentration decreased from 167 mM in seawater to about 130 mM during the first 24h, and increased gradually during
2–7 days after the transfer. No immunoreactive prolactin (PRL) was detected in the plasma of the seawater-adapted fish nor
during the first 24h in fresh water. Significant levels of PRL were detected after 2–3 days. The maximal level (2.6 ng/ml)
was observed after 5 days and became undetectable again after 7 days; no significant correlation was seen between the changes
in plasma sodium and PRL levels during the transfer. Plasma growth hormone levels were relatively constant, except for a significant
decrease 12h after the transfer. Although plasma thyroxine levels were highly variable during the experiment, a significant
decrease and an increase were observed 12h and 5 days after the transfer, respectively. The present study indicates that juvenile
chum salmon retain hyperosmoregulatory ability even after prolonged rearing in seawater. Examination of turnover rates, rather
than changes in plasma levels, seems to be essential to clarify the osmoregulatory roles of the hormones. |
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| Keywords: | chum salmon Oncorhynchus keta osmoregulation prolactin growth hormone thyroxine |
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