Translation Series No.1211

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area of distribution is formed by a line between Regen and Schonen accor-
ding to Swedish,Finnish,.Polish and German taggings.
4.1. The divisions of the periodic migration
• The periodic migration is determined by the life cycle of the
salmon. It takes place in three divisions, the emigration from the river,
that is connected with the transformation into the smolt stage, the search
for food in the Baltic Sea and the return to the home stream during the
spawning migration.
4.1.1. The migration of the smolt
K. A. Pyefinch (1955) calls the transformation into the smolt
stage a series of morphological and physiological changes. The most striking
external phenomenon is the change. from the "trout type" (parr) into the
"silvery fish" through deposition of guanin. The physiological process of .
the transformation into the smolt stage is not yet completeliunderstood,
although werk on the ion budget and the components of haemoglobin (H. Koch
1963), the activity of the glands (Hoar 1953, Fontaine : 1954, after7 Pyefimàh
1955) and the action of hormones (D. H. Piggins 1962) have provided partial
knowledge.
With reservations one can at present consider the process of
the transformation into the smolt stage to take place about as follows.
(various authors, according to W. S. HOar 1957).
When the parr have attained a certain size, internal changes
take place that trigger the process. There is agreement that during this
period the thyroid plays a role by causing the deposition of guanin and
'influencing processes of morphogenetic growth. D. J. Piggins (1962) suc-
ceeded in increasing the number of, young salmon capable of emigrating
(smolts) by feeding bovine thYroids. However, W. S. Hoar (1957) attributes
r