Translation Series No.1211

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the initial effect to'the hypophysis, which controls not only the growth
but also the function of the thyroid. In this connection the hormone som-
atotropin appears to be of great importance,Secause it regulates growth
and in low concentrations raises the tolerance for salt. There is therefore
the possibility that the changes in the hypophysis-thyroid system result
in a mineral imbalance that can be compensated only in an environment with
a higher salt content. This means then that the smolt has not to tolerate
the higher salinity as a disturbance of its physiological state, but that
it needs it in order to remove an existing disturbance.
When the transformation into the smolt stage has.taken place,
the beginning of the emigration must be stimulated through various environ,
Mental factors. H. C. White (1939) does not consider rain or high water
levels to be decisive, but temperature increase and the amount of light
intensity. G. Alm (1934) also considers the temperature to be important.
'After they have reached the region of the river mouth, perhaps
in May/June, the smolt from the rivers of the Gulf of Bothnia remain in
a transitional area for a short while (days or weeks). A more rapid growth
begins already at this time. The fish probably eat.insects that live on
the surface of the .water (A. Lindroth 1961b). Their movements are still
non-directiOnal and do not extend fax. During the course of . summer and
fall they colonize the Gulf of Bothnia during the transition into the post-
smolt stage.
4.1.2. The feeding migrations
In Table 22 have been assembled and recalculated some results
of tagging experiments from the Indalsglv and the Lulegiv (B. Carlin 1959a,
1963) and from the Oulujoki (E. Halme 1961). The number of reports of re-
boveries has been given for the different periods and the three regions
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