Criteria for adaptation of salmonids to high salinity seawater in France

Abstract

Tolerance to high salinities was investigated in four species of salmonids using direct or progressive adaptation to 35‰ salinity seawater. Smoltifying species, or populations, included: coho ( Oncorhynchus kisutch) of different ages (0 and 1), sea-run brown trout yearlings ( Salmo trutta); nonsmoltifying species or populations included domestic rainbow trout ( Salmo gairdneri), brown trout ( Salmo trutta) and brook trout ( Salvelinus fontinalis). Smolts of migrating species, characterized by a seasonal increase in gill Na +K +-ATPase activity in fresh water could be successfully directly transferred to seawater during narrow “windows” of time corresponding to the gill enzyme activation. Juveniles of these species were capable of rapidly controlling fluctuations in plasma osmolality associated with the transfer to seawater. Variations were greater and longer in duration when the fish entered seawater outside of the period of increased gill Na +K +-ATPase. Despite the lack of any seasonal gill enzymatic activation in the strains of nonmigratory species tested, transfers to seawater could be achieved during certain periods with domestic rainbow and brown trout of large size, whereas brook trout yearlings were unable to control the osmotic stress which resulted in 100% mortality. Rainbow and brown trout showed well-defined transitory osmotic modifications in blood plasma, which required a much longer period to effectively regulate ions than did the anadromous species.