Variations in the Plasma-Level of Chloride in Hatchery-reared Yearling Atlantic Salmon during Parr–Smolt Transformation and following Transfer into Sea-Water

Abstract

THE successful adaptation of juvenile Atlantic salmon to sea-water requires a prior physiological ‘metamorphosis’—parr–smolt transformation¹. The occurrence of this change appears to depend in part upon age, and in part upon the complex of factors controlling the growth-rate of the parr². In addition to external evidences of parr–smolt transformation such as loss of parr marks, decrease in condition factor and deposition of guanine there must also be some variation in biochemical systems associated with the regulation of body electrolyte-levels relative to those of the environment. Evidence for such changes may be seen in the changing plasma chloride-levels of two migratory salmonids, Oncorhynchus masou, and Salmo gairdneri, during transition from parr to smolt stages³. Several investigators have also reported marked development of Keys–Willmer cells, units thought to have an ion secretory role, in a number of species of salmon prior to downstream migration⁴. Fontaine, however, could find no significant variation in blood chloride-levels of smolting Atlantic salmon, although a decrease did occur in tissue concentration⁵.