Osmoregulatory ability of anadromous Arctic char, Salvelinus alpinus (L.), migrating towards the sea

Abstract

Abstract. Hypoosmoregulatory capacity was examined in anadromous Arctic char, Salvelinus alpinus (L.), caught whilst migrating towards the sea in the river Å-elva, northern Norway (69°04′N and 17°00′E). Metacercariae of the marine digenean Cryptocotyle lingua (Creplin) were found on 47% of the fish investigated, demonstrating that they had been in brackish or sea water on at least one previous occasion. The fish were randomly divided into three groups, and exposed to sea water (35 ppt salinity at 5–6°C) for 7 days- one group immediately after capture, and the other two groups after 16 days of acclimation in fresh water and brackish water (15–17ppt), respectively. Blood plasma osmolality, Na+ and Mg2+ levels showed only a small and transient rise during seawater exposure, and there were only minor changes in muscle water content, irrespective of prior treatment. A slight, but insignificant, reduction in hypoosmoregulatory capacity appeared to occur in the groups retained in fresh water or brackish water for 16 days prior to seawater exposure. The results did not indicate that the fish show any improvement of hypoosmoregulatory capacity following a period of acclimation in brackish water. The hypoosmoregulatory capacity of seaward-migrating Arctic char is well developed before they enter the sea, and may be comparable to that seen in salmonids which undergo a parr-smolt transformation. There was a negative correlation between blood plasma electrolyte levels and both body length and weight, indicating that hypoosmoregulatory capacity was affected by the size of the fish. The effects of body size upon the development of hypoosmoregulatory capacity are discussed with respect to estuarine residence and the constraints placed upon the survival of small fish in full-strength sea water.