Effects of temperature and feeding on smolting and seawater survival of Atlantic salmon ( Salmo salar)

Abstract

Atlantic salmon were reared in fresh water at ambient temperature or under conditions of constant 11°C temperature from November to April. One of the ambient temperature groups was not fed during November and December in order to produce small fish for study during the smolting period. Yearling fish were transferred to seawater net-pens as yearlings at the end of April. Survival of the fish in seawater was compared among the groups. For determination of smolt development, gill Na +K +ATPase activity and plasma concentration of thyroid hormones were measured during the spring in fish in fresh water. Gill ATPase activity was elevated in the ambient temperature groups in February compared with the group in heated water. Plasma thyroxine showed an initial peak value in the ambient temperature-fasted group in March. During April and May all groups showed similar changes in ATPase and thyroid hormones, suggesting completion of smolting by the beginning of May. Survival of fish in seawater was greatest in the ambient-fed group and poorest in the heated-fed groups. All groups in seawater showed two periods of increased mortality. The first mortality occurred immediately after seawater entry; a second period of increased mortality occurred between the middle of June and middle of July. It is suggested by these results that warm water temperature in the winter may be detrimental to the subsequent survival of the fish in seawater.