Osmoregulatory ability of chum salmon,Oncorhynchus keta, reared in fresh water for prolonged periods

Abstract

The osmoregulatory ability of chum salmon (Oncorhynchus keta), reared in fresh water for a prolonged period, was examined by transferring them directly to seawater and then back to fresh water. When fry and juveniles weighing 0.3-125g, reared in fresh water for 1.5-13 months, were transferred directly to seawater, they adjusted their plasma Na(+) concentration to the seawater-adapted level within 12-24h. When they were transferred back to fresh water after having been adapted to seawater for 2 weeks, the plasma Na(+) level gradually decreased during the first 12-24h, and then increased to reattain the initial freshwater level after 5-7 days. No mortality was observed during the experiment except among the smallest fry weighing about 0.3g after transfer to seawater (2.1%). The maintenance of good osmoregulatory ability of the chum salmon for a prolonged period in fresh water seems to be unique among Pacific salmon, with the possible exception of the pink salmon.Changes in plasma levels of hormones during the transfer experiments were recorded in juveniles reared in fresh water for 13 months. Prolactin levels increased maximally 3 days after transfer from seawater to fresh water, as would be expected from its well-established role in freshwater adaptation in several euryhaline teleosts. In addition, an increase in plasma growth hormone was observed during the first 12h after seawater transfer, along with a tendency towards a decrease during freshwater transfer, suggesting an important role for this hormone in seawater adaptation. There were no consistent changes in plasma levels of thyroxine and cortisol during freshwater to seawater or seawater to freshwater transfer.

Development of radioimmunoassay for eel growth hormone.

“…In the past decade, GHs have been isolated from the pituitary of several teleosts: tilapia Oreochromis mossambicus2.3) sturgeon Acipenser guldenstadti4) chum salmon Oncorhynchus keta5-3) carp Cyprinus carpio9) and eel Anguilla japonica.10) A radioreceptor assay was developed for a tilapia GH, and secretion of GH from the incubated pituitary was analysed.11,12) Recently, homologous radioimmunoassays (RIAs) for carp and chum salmon GHs were developed and used for measurement of the plasma and pituitary levels of the goldfish and the salmonid fishes.9, [13][14][15][16][17] Control of GH and prolactin (PRL) secretion from the organ-cultured pituitary has been studied in the European eel A. anguilla. [18][19][20] In these studies, the hormone content in the pituitary and in the medium was measured by densitometry after separation of the hormones by gel electrophoresis.…”

Functional Morphology of Mitochondrion-Rich Cells in Euryhaline and Stenohaline Teleosts

“…In these species, gill Na + /K + -ATPase activity increases during smoltification, in correlation with increased salinity tolerance and also with the increase in number and size of branchial MR cells (Langdon and Thorpe 1985;Richman et al 1987;McCormick 1995). In contrast with these salmonids, chum (O. keta) and pink (O. gorbuscha) salmon go downstream to the sea in early life stages, acquiring hypoosmoregulatory ability during the alevin stages (Weisbart 1968;Iwata et al 1982;Hasegawa et al 1987).…”

Expression of insulin-like growth factor I gene in osmoregulatory organs during seawater adaptation of the salmonid fish: possible mode of osmoregulatory action of growth hormone.

“…Bolton et al (12) and Collie et al (13) have demonstrated that the effect of GH on seawater adaptation of rainbow trout (Oncorhynchus mykiss) is independent of the growth-promoting actions of GH. Plasma levels and turnover rates of GH increase after transfer of several salmonids to seawater (14)(15)(16)(17)(18)(19). Recently, we have shown that bovine IGF-I also enhances the hypoosmoregulatory ability of rainbow trout by inhibiting the increase in plasma osmolarity and sodium levels after transfer to seawater, independently of its growth-promoting effect (20).…”

“…Furthermore, no specific assay is available to determine IGF-I protein levels in fishes since IGF-I protein has not been obtained from teleosts; only the cDNA of coho salmon IGF-I is known (21). Therefore, we examined the effect of GH on IGF-I mRNA levels in the liver, gill, and body kidney and their changes during seawater adaptation of rainbow trout, when GH levels are known to increase (14)(15)(16)(17)(18)(19).…”