Translation Series No.1211

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tagging experiments'that salmon from the Oulujoki return along the Finnish
coast of the Gulf of Bothnia.
4.2. Local movements
In general we must assume that the hourly and daily movements
of the salmon in the Baltic Sea are determined by the search for food. There
can be no doubt that these local movements are of the utmost interest for
the practical fishery, because they are the cause of the short-period
fluctions in the yield. The ascertaining of such local movements and of
their immediate causes requires, however, an extensive material that can
be obtained only with difficulty. If we want to learn more about the causes
of these local movements, we have to have recourse to the results of the
critical examination of the captures per unit effort. We could start from
the fact that the catches can be influenced on the one hand by the specific
effects of the fishing gear and on the other hand by environmental factors.
The latter have the effect to induce the salmon to stay at certain places
or to travel. We found such factors to be:
(1) temperature,
(2) wind conditions,
(3) competition for food,
(4) occurrence of cod.
We can neglect the temperature conditions because they lead to
seasonal vertical movements of the salmon. The catches with nets are af-
fected only by the wind conditions, the line catches, on the other hand,
are affected by all other factors. If we now enquire which of them can
cause local movements of the salmon, we are left with only the food and
perhaps also the wind conditions and currents.
It has already been pointed out that a pelagic gregarious fish,
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