Translation Series No.1211

only in December or even in January. It is thus possible that further fish
did still rise steadily from the deeper layers of the water. With such a
fluid transition it is perhaps not correCt to speak of a critical ;tern-
core..k.es
perature. To judge by the size of the-cep-tuxes per unit effort, only about
one-half of all fish have returned to the surface in October, when a tem-
perature of about 10 ° C prevails there.
tC
Further changes in the captur -e-s per unit effort are not correl-•
ated with the course of temperature. Beginning in March (for line catches
already beginning in February), when the temperature is lowest, the yields
are already dropping again. The salmon season comes close to its end at
. the beginning of June, when the surface temperature passes 10 ° C.
As regards an explanation for the reduction in values for the
ccutclee
-cap:tee:re-8 per unit effort during February/March, we are reduced to assump-
tions. There is the possibility that temperatures of from 2 ° to 8 °C cons-
titute an optimum range for salmon. The area of its habitat lies in the
[P. 243]
summer probably under the layer of the temperature discontinuity (at a
depth of 25 to 50 m), where the temperature is 2 ° to 3 ° C. It must be as-
sumed that the fish do not dwell in still greater depths, because in the
bottom layers of tlie large basin of the Baltic Sea there is an occasional
lack of oxygen and sulfuretted hydrogen may be present. (Salmon need a
minimum of 5 mg 02/1.) If thé fish wish to escape from water that is too
cold in February/March, they will have to pass through the layer of dis-
continuity, which at that time lies at a depth of 60 to 70 m. Below this
the temperature is about 5 ° C. (A. GloWinska 1961, J. Piechura 19 6 1)
HoweVer, this question Of behaviour is at the môment not
ecdeiter,
decisive for us; because the question is in how far the eaptueee per
unit effort as expression for the stock density are infltlenced by other
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