Íslenskar landbúnaðarrannsóknir - 01.09.1978, Síða 161

SALMON MANAGEMENT AND OCEAN RANCHING 159
TABLE 1. Mean constants (a and b) derived from expression (1).
River Years a Standard error of the mean b Standard error of the mean
Ellidaár run 1961-73 6.85 0.48 -0.84 0.06
catch 1949-73 4.95 0.17 -0.64 0.02
Nordurá run 1972, 73, 75 6.96 1.05 -0.86 0.14
catch 1949-7*5 4.82 0.13 -0.67 0.01
Grímsá 1972-75 4.03 0.28 -0.51 0.03
Laxá í Adaldal 1949-75 5.24 0.15 -0.73 0.12
the time axis, a normal sigmoid pattern
appears. Mathematically this can be de-
scribed as a function of time by
y(0 = } -)- g - (a + bt)
where y is the cumulative run at time t.
The two constants a and b represent, re-
spectively, the slope and intercept of the
linearized form
ln ( 1 ) = ~ (a + bt)
Each year’s counts furnish an estimate
of a and b and from their mean values can
be constructed an average entry curve
with associated confidence limits (Table
!)•
The asymptotic limits, which reflect the
magnitude of the total runs, differ from
year to year but can be estimated when
three points on the ascending limit have
accumulated.
Even though the curves have the same
slope, the timing of the runs differs from
year to year. Therefore, if the cumulative
curve in one year is compared directly to
the average curve on a Julian calendar
basis, both timing and differences in run
strength are confounded. However, the
best fit can be found by double minimiza-
-tion process as developed for salmon runs
in Bristol Bay, Alaska (MuNDYand Mathi-
sen 1977).
If the catches form a fixed percentage of
the total run at any time during the sea-
son, the cumulative catch curve should
then be proportionally identical to the to-
tal run curve. The data availale for Ell-
idaár largely confirm this, and the exact
amount of variability can be estimated in
this river from the seasonal rate of fishing
mortality.
The first exploitation rate in Table 2 is
based on total rod catch and total run. In a
few years missing data have been esti-
mated by methods discussed by MuNDYet
al. (1978). These range from 20 to 58%,
but, aside from these two extremes, in
general the rates are rather stable. From
1935 to 1955 about 40% of the total run
was removed by anglers. During the next
decade the rates fell to 25% or less,
whereupon they reverted back to their
former levels. During the last fewyears the
rates are again low and about 20-25%.
In Ellidaár the weir trap counts are
made about V2 km from the estuary. The
exploitation rates, based on rod catches
made above the trap and the total trap