Translation Series No.1211

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ceptu.ye per unit effort of 1957/58 increaes bs ten per cent, then Z2 rises
from 0.05 to 0.15. Increases in the capttbre.pr unit effort by 20, 30, 40
per cent result in corresponding values of Z. ) of .0.23, 0.31, 0.39. Even
this amount must be considered as too small. Un the other hand, we cannot
Cc
assume that the -e-sekttte,e per unit effort in 1957/58 should have amounted
to- mere than 14.7 instead of 10.5. It is, however, also possible that not
only the value for 1957/58, but also that for the following season is
affected . by errors. gowever, if both data are changc-sd then we find consid-
erable effects on the results of the -iirculations for the neXt fishing
season. After it has been shown (2.3.3.2.) that the -captures per unit ef-
fart are subject to other influences than that of the itrength of the stock,
and that these influenees cannot be eliminated readily, one has to reckon
after all with certain effects on the calculations of the mortality.
There is still another problem to be considered. The data for
the present estimates are based on the yield of the German salmon fishery
that amounts to only 15 per cent of the total. Changes that were caused
by the efforts of other fleets cannot be expressed in the present data. We
have therefor e . to reckon with incorrect individual values under certain
circumstances.
• The figures for Z that have been obtained with the aid of the
captures by net per unit effort (Table 55) change little. The average
values for Z 2 are slightly smaller; theSe for Z 3 are slightly greater
than. those in Table 54.
[P. 349]
For the determination of the total mortality .can also be used
the method of the virtual population. The virtual population of a certain
V 'Ye '>‘
Yt‘raer4-7--Set is the total ntimber of all captured fish of this ydrarle sct.
The mortality in the year x + 1 is the natural logarithm of the ratio
between the virtual populations of the age of x-+ 1 years and of x years