Changes in plasma somatostatin associated with seawater adaptation and stunting of coho salmon, Oncorhynchus kisutch

Somatostatins (SSs) are a family of peptide hormones made up of different molecular forms that have been found to modulate various aspects of growth, development, and metabolism of vertebrates. In this study, coho salmon (Oncorhynchus kisutch) were used to evaluate the role of SSs in seawater (SW) adaptation. In experiment 1, pre-smolt and smolt coho salmon (age = 1-1.5 years) were transferred to SW and sampled at zero, one, two, and seven days later. For both pre-smolts and smolts, plasma SS-25 concentrations rose following SW exposure and reached peak levels two days after transfer; thereafter, levels gradually declined. Interestingly, SS-25 levels were higher in pre-smolts than those in smolts, a difference that was similar to that noted for rectal [Na⁺] and indicating a greater state of SW readiness for the smolt group. In experiment 2, pre-smolt coho salmon were transferred to SW and sampled eight weeks later. Pre-smolt SW exposure resulted in significant growth retardation (stunting). Plasma levels of both SS-25 and SS-14 were significantly higher in stunts than those in their normal (SW smolt) counterparts. These results indicate that changes in SSs accompany SW adaptation and that SSs may be involved with stunting associated with premature SW transfer.