Steriod hormone levels and patterns of growth in the early part of the reproductive cycle of adult Atlantic salmon (Salmo salar L)
Steroid hormone levels and patterns of growth have been examined between December and April in captive, marked Atlantic salmon destined to mature some months later (November). Changes in maiden fish of both sexes becoming mature after 1 year (grilse) or 2 years of sea-life have been described. Two groups of grilse were distinguished on the basis of their appearance in late June and their steroid hormone profiles and growth in spring. Serum 11 -oxotestosterone levels were raised in the preceding December in that group of male grilse which could be identified as such by their appearance in late June. Levels peaked in February and early March. Maturing male grilse which could be identified only later in the summer did not show these changes. Maturing female grilse showed no changes in serum oestradiol over the sampling period irrespective of whether or not they could be identified as malurers in June. Body weights in male and female fish which could be identified as malurers in June were indistinguishable from each other over the sampling period but, after late March, were significantly greater than those in the other maturera and the fish which remained immature. Lengths differed similarly but only at a later date. However, analysis of specific growth rates suggests that the differences in both body weight and length began to be established in the period between February and March. Final maturity rate as grilse in the monitored group of fish greatly exceeded that in the parent group from which they had been derived. In males becoming mature after 2 years in the sea 11 -oxotestosterone was raised throughout the sampling period, reaching peak values in January. In maturing females of the same age oestradiol levels were elevated throughout the sampling period and peaked in February.