Changes in plasma glucose, amino acid nitrogen and growth hormone during smoltification and seawater adaptation in coho salmon, Oncorhynchus kisutch

Abstract

Weekly samples of plasma were obtained from coho salmon held under natural temperature and photoperiod conditions during the parr to smolt transformation. The samples were measured for glucose, amino acid nitrogen (AAN) and growth hormone (GH). In laboratory studies, fish from the same population were injected with salmon GH at different stages of the smoltification process. Plasma glucose and AAN were measured in these fish. Coho smolts were also exposed to sea water and after various time intervals, levels of plasma glucose, AAN and GH were determined. During smoltification the weights of the fish, as well as plasma levels of glucose and AAN, varied with peaks at, or near, full moons. Plasma GH also varied but did not appear to be associated with lunar cycles. Changes in plasma GH levels correlated with changes in fish weight as well as with plasma glucose and AAN. Injections of salmon GH caused elevations in plasma glucose but had no effect on plasma AAN. The values of these blood parameters varied depending on the stage the fish were at during the smoltification process. Exposure to sea water caused increases in circulating levels of GH but had little effect on plasma glucose or AAN. These results are discussed with respect to the role of GH during smoltification and subsequent seawater adaptation.