Smolting of sea trout ( Salmotrutta L.) in northern Norway

Abstract

Seawater tolerance and development of the hormones thyroxine and cortisol were studied in hatchery-reared 2-year old sea trout ( Salmo trutta L.) smolts during the smolting period in 1995. The fish were reared under a simulated natural photoperiod and ambient water temperature. The fish were divided into two size groups. Fish from the small size group (mean 180 mm, range 143–210 mm) had an increasing seawater tolerance from the first sampling time, passing a plasma chloride value of 160 mmol l −1 by the end of June. This group also showed a peak in plasma thyroxine and plasma cortisol concentrations by the middle of May and by the end of June, respectively. Sea trout from the large size group (mean 199 mm, range 172–228 mm) given the same light and temperature regimes as the small size group established their seawater tolerance about 2 weeks before the small size group. Also, the osmoregulatory capacity of fish from the large size group was better when compared to the small size group. It is concluded that fish size is important for the development of the hypoosmoregulatory capacity in sea trout. It is also shown that sea trout develop the same hormonal pattern (thyroxine and cortisol) as other salmonid species during the smolting period.