Acclimation to seawater of dwarf nonanadromous Atlantic salmon, Salmo salar

Abstract

The ability of nonanadromous Atlantic salmon to regulate hydromineral balance in seawater was assessed by determining plasma Na+ and Cl− concentrations, gill Na+–K+ ATPase activity levels, and gill chloride-cell length in fish exposed to gradually increasing salinity. Plasma ion levels were much higher in nonanadromous salmon exposed to seawater than in anadromous smolts in the same condition, and in both forms held in fresh water. This indicates a lower level of hypoosmoregulatory ability in the nonanadromous form. The activity of gill Na+–K+ ATPase was low in nonanadromous salmon at the beginning of the experiment but increased in response to elevated external salinity. Mortality approached 100% despite increased enzyme activity levels. Salmon retained in fresh water did not exhibit increased enzyme activity. Anadromous smolts exposed to the same conditions exhibited somewhat elevated yet nonlethal plasma ion concentrations. Gill chloride cells increased in size in salmon exposed to seawater and decreased in size in fish held in fresh water. Hypoosmoregulatory inability and a general suppression of the smolting process in some stocks of nonanadromous salmon may be associated with sexual maturation at a young age. However, this is not universal, since nonanadromous salmon in some other stocks mature later yet show clear evidence of reduced smolting.