Smolting in anadromous and landlocked strains of Atlantic salmon (Salmo salar)

Abstract

Abstract Juvenile Atlantic salmon from a landlocked population (‘Blege’, lake Byglandsfjord, south–central Norway) and an anadromous population (river Vosso, southwestern Norway) were reared in freshwater on simulated natural photoperiod (60°25′N) between December 2000 and June 2001. Fish from the Blege (mean length: 13.4 cm) and Vosso (mean length: 13.9 cm) strains were transferred to seawater tanks in late May for studies of long-term post-smolt performance. Condition factor of fish from the Blege strain increased throughout the study, whereas in the anadromous Vosso strain condition factor increased until April followed by a decrease between April and June. Smolts from the Vosso strain showed all the morphological characteristics of smolting, whereas smolt-sized fish from the landlocked population obtained a lower degree of silvering, and distinct parr-marks and red dots in the skin prevailed. Gill Na + ,K + -ATPase activity remained low in both strains in January and March, with a significant increase in the Vosso strain in April and May to peak levels of approximately 17 μmol ADP/mg protein/h. In contrast, only a slight increase was seen in the Blege strain in May, with peak level of approximately 8 μmol ADP/mg protein/h. In mid-June Na + ,K + -ATPase activity was down to parr levels. Improved short-term hypoosmoregulatory ability (seawater challenge tests, 34‰, 96 h) was observed in both strains; however, the Vosso strain showed better seawater tolerance than the Blege strain in March and April. There was no mortality among the smolt-sized fish of the Vosso strain transferred to seawater in late May, whereas high mortalities (40%) were observed in the Blege strain during the 2 weeks of seawater rearing. Our findings show that landlocked Atlantic salmon, represented here by the Blege population, having been isolated from the sea for approximately 9000 years, may have abandoned key elements of the parr–smolt transformation associated with marine life. Our results further emphasize the importance of the gill Na + ,K + -ATPase for long-term seawater performance.