During the spawning period, the plasma levels of the androgens 11-ketotestosterone, testosterone, 11β-hydroxytestosterone, 11β-hydroxyandrostenedione, 11-ketoandrostenedione, and of the progestin 17α-hydroxy-20β-dihydroprogesterone were higher in mature compared with immature male Baltic salmon parr. 11-Ketotestosterone was always found to be the predominant androgen in the plasma of mature male parr and mature anadromous males. Mature male parr and large mature anadromous males had similar androgen profiles during the main spawning period, although 11-ketotestosterone tended to be higher in the mature anadromous males and testosterone was always higher in the mature male parr. At the time of the main spawning period, plasma levels of 17α-hydroxy-20β-dihydroprogesterone rose sharply in mature male parr, these levels being similar to those found in the plasma of large mature anadromous males. All measured androgens were low or nondetectable in castrated parr. Treatment of castrated parr with Silastic capsules containing 11-ketoandrostenedione resulted in high plasma 11-ketoandrostenedione levels and even higher levels of 11-ketotestosterone, indicating an extratesticular site of androgen conversion.

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At the breeding season (November) the mature male parr had a thicker epidermis than had the immature parr. In winter (February) there was no difference, while in May, at the smoltification period, the previously mature male parr had a thicker epidermis than the immature parr. In both categories of fish the epidermis was thinner in May than in November. The epidermal thickness in castrated male p a n was the same as in immature pan. Treatment with 1 1-ketoandrostenedione (1 1 kA4) increased epidermal thickness in castrated males. Treatment with testosterone and 1 1 kA4 increased epidermal thickness in previously mature males in spring. There was no development of anadromous secondary sexual characters (hooked lower jaw, adult nuptial colouration) in any male.

One-year-old Atlantic salmon improved their seawater adaptability in late spring, at a time when 2-year-old fish smoltify, though not to the same degree as the latter. One-year-old males that matured precociously were less able to adapt to seawater than immature fish. Both 1- and 2-year-old immature salmon increased their plasma levels of thyroxine when entering the smolt stage; however, both the increase and the levels were much lower in the 1-year-old fish. The seasonal patterns of plasma triiodothyronine and thyroxine were studied in immature parr and precocious males held in fresh and brackish water. The most consistent difference was that the levels of triiodothyronine and thyroxine were higher in mature males than in immature fish during winter.

LINDAHL, K., H. LUNDQVIST, and M. RYDEVIK. 1983. Plasma thyroxine levels and thyroid gland histology in Baltic salmon (Salmo salar L.) during smoltification. Can. J. Zool. 61: [1954][1955][1956][1957][1958]. Plasma thyroxine (T4) levels in Salmo salar L. showed a clearly defined surge which coincided with a rapid rise in the ambient water temperature and with the period of natural seaward migration. It is sugggested that the elevation of T4 levels has a major role in the completion rather than the onset of the smoltification in Baltic salmon parr. The body coloration (i.e., smolt or parr appearance) could not be correlated to the levels of plasma T4 and silvering does not at all times reflect the degree of smoltification. A relationship between size (length and weight) and plasma levels of T4 was not detected for smolting Baltic salmon parr. Increased circulating levels of T4 is not reflected by an overall morphological change of the thyroid cells during smoltification.LINDAHL, K., H. LUNDQVIST et M. RYDEVIK. 1983. Plasma thyroxine levels and thyroid gland histology in Baltic salmon (Salmo salar L.) during smoltification. Can. J. Zool. 61: [1954][1955][1956][1957][1958]. Les concentrations plasmatiques de thyroxine (T4) subissent une augmentation appreciable au moment de l'augmentation de la temperature de l'eau et de la periode naturelle de migration vers la mer chez le saumon de la Baltique. I1 semble que cette augmentation joue un r61e important au moment oh le tacon achkve de se transformer en saumoneau plutat qu'au debut de la transformation. La coloration du corps, c.-a.-d. l'aspect du tacon ou du saumoneau, n'est pas en correlation avec la concentration de T4 du plasma et l'apparition des reflets argent n'indique pas toujours le degre du developpement en saumoneau. I1 ne semble pas y avoir non plus de relation entre la taille du poisson (longueur et masse) et les concentrations de T4 du plasma chez le tacon en transformation. L'augmentation des concentrations de T4 en circulation ne s'acccompagne pas de changements morphologiques importants des cellules thyroi'diennes durant la transformation en saumoneau.[Traduit par le journal]