Íslenskar landbúnaðarrannsóknir - 01.09.1978, Page 160

158 ÍSLENZKAR LANDBÚNAÐARRANNSÓKNIR
Fig. 1. Run pattern of Icelandic salmon streams approximated by the cumulative catches expressed in
percent.
these exponentially decaying pulses will
cascade into a composite gamma function,
which for practical reasons in most cases
can be approximated by a normal curve.
The cumulative catch curves for
Icelandic salmon streams have a sigmoid
pattern. Variability in timing between
years can be seen in Fig. 1 where the catch
curves from Laxá in Kjós have been
drawn for five consecutive years. The
maximum variation in timing when the 50
percentage point was reached is less than
2 weeks. As a corollary an average catch
curve based on all available years can be
used to predict when a specified percen-
tage of the catch will be reached, with
great accuracy in the range of 10-90% of
total catch. The necessary supposition
that changes in fishing effort are small
from year to year is true for Icelandic sal-
mon fisheries.
Only in a couple of streams in Iceland
are there available total rivercounts and
then not necessarily at the river mouth.
When such counts are smoothed by
plotting in a cummulative fashion along