A Comparative Study of Smoltification in Sea Trout(<i>Salmo trutta</i> L.) and Atlantic Salmon (<i>Salmo salar</i> L.): Seawater Tolerance and Thyroid Hormone Titres

Abstract

Plasma thyroxine (T4) concentrations and seawater tolerance were measured in one-year-old (SI) and two-year-old (S2) Atlantic salmon (Salmo salar L.) smolts and S2 sea trout (Salmo trutta L.) smolts reared under natural temperature and photoperiod regimes in western Ireland (09?34/W 53?55/N). The S2 salmon and sea trout showed significant surges in T4 titres during April-May, which coincided approximately with the period of maximum seawater tolerance. However, no T4 surges were observed in SI salmon during the sampling period (February?June). Nevertheless, SI salmon exhibited a clear increase in seawater tolerance, indicating that they can complete smoltification without an observed elevation in T4 titres. The seawater tolerance of S2 sea trout progressively increased during the smolt period. The results demonstrate clear differences in seawater tolerance and the development of smoltification between sea trout and salmon. D.T.G. Quigley (corresponding author; e-mail: declanquigley? eircom.net), Carrigadrohid Smolts Ltd, Macroom, Co. Cork; M.J. Harvey, Martin Ryan Institute, Cama Research Laboratories, Cama, Co. Galway; T.J. Hayden and C. Dowling, Department of Zoology, University College Dublin, Belfield, Dublin 4; M.P. O'Keane,