R.M. Sweeting

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.

The effect of thyroid hormone and thyroid hormone blocker on visual pigment shifting in juvenile coho salmon (Oncorhynchus kisutch)

Abstract Juvenile coho salmon were exposed to either dim-light (9.1×10 −7 lx)/5°C water or bright-light (2.6×10 −6 lx)/15°C water conditions to affect retinal dominance in porphyropsin or rhodopsin, respectively. The administration of thyroid hormone over a 5-week period resulted in the rhodopsin-dominated retinae of the fish in the bright-light/15°C tanks to shift to porphyropsin dominance whereas the fish in the dim-light/5°C tanks had their porphyropsin-dominated retinae exhibit a small, but, significant increase in the relative proportion of rhodopsin. When methimazole, an anti-thyroid drug, was administered for 4 weeks to fish in dim-light/5°C that were transferred to bright-light/15°C tanks, the shift to rhodopsin dominance was significantly less than in nontreated controls. Similarly, 6 weeks of methimazole treatment of fish in bright-light/15°C that were transferred to dim-light/5°C tanks exhibited significantly less porphyropsin than in nontreated controls. Methimazole treatment to nontransferred fish showed no effect on visual pigment levels. The response of the retinae to thyroid hormones and the impairment of response when treated with thyroid hormone blocker suggests that during smoltification, changes in visual pigment composition may be, in part, attributed to alteration in thyroid function.

Growth and hormonal changes associated with precocious sexual maturation in male chinook salmon (Oncorhynchus tshawytscha (Walbaum))

Abstract One-year-old chinook salmon ( Oncorhynchus tshawytscha (Walbaum)), reared in seawater, develop a bimodal weight-frequency distribution prior to the precocious sexual maturation of some of the males (jacks). Although jacks are larger than immature fish, they are still too small to be of commercial value. To investigate the differential size between jacks and immature fish, chinook salmon were tagged and reared in seawater for 5 months prior to sexual maturation (March–August). During this time, immature males, females, and jacks were sampled monthly for wet weight and plasma concentrations of triiodo- l -thyronine (T3), testosterone, and growth hormone (GH). Fish were also sampled in the preceding December as controls. The weight-frequency distribution of the sampled fish became significantly bimodal in May, with the upper modal group consisting mostly of jacks. Jacks had the highest relative growth (RG) rates from April–June. Jacks had significantly elevated plasma T3 concentration in March only, and had significantly elevated testosterone levels throughout the experiment. There were no significant differences in plasma GH concentrations between jacks and immature fish; however, in jacks, plasma GH concentrations were positively correlated with RG during April–May, and plasma testosterone levels were positively correlated with RG for June–August. Although a number of hormonal differences between jacks and immature fish coincided with the differential growth rates, no measured hormone concentration could explain the observed differences in growth and size.

Changes in plasma growth hormone and various metabolic factors during smoltification of coho salmon, Oncorhynchus kisutch

Abstract Plasma and tissue samples were collected weekly from juvenile coho salmon during the parr-smolt transformation as well as for an additional 12 weeks in freshwater during desmoltification. The parameters measured were: plasma growth hormone, muscle glycogen, liver glycogen, liver glucose, muscle protein, muscle amino acid nitrogen, and individual plasma saturated and unsaturated free fatty acids. Plasma levels of growth hormone increased over the sampling period. Muscle and liver glycogen decreased during smoltification but increased again over desmoltification. Liver free glucose was unchanged over the sampling period except for low levels seen over a 3-week period immediately following normal release from the hatchery. The ratio of liver free glucose to glycogen increased during smoltification and then decreased. Muscle protein levels were fairly stable over both smoltification and desmoltification. Amino acid nitrogen content of the muscle fluctuated weekly but generally increased over the latter weeks of the study. Both the saturated and unsaturated plasma free fatty acids remained stable initially, but increased dramatically near the end of the study. Correlations between the various parameters and plasma growth hormone are examined to ascertain possible roles of growth hormone in the physiology of the smoltification and desmoltification processes.