Íslenskar landbúnaðarrannsóknir - 01.09.1978, Blaðsíða 129

GROWTH RATES OF TROUT AND CHAR 127
TABLE 1. Back-calculated length of brown trout in Laxá, measured in cm.
Upper section n li 12 13 l4 lS 16 1? 18 19 110
1974 26 5.1 11.2 19.0 27.4 34.3 42.6 47.5 50.8
1975 47 4.8 11.2 18.4 26.4 34.7 41.8 48.4 53.7 56.0
Mean 5.0 11.2 18.7 26.9 34.5 42.4 48.0 52.3 56.0
Lower section
1974 6 5.3 12.8 22.2 31.4 39.7 45.4 50.6
1975 31 4.31 11.1 19.9 29.4 37.8 44.8 50.7 52.1 58.2 69.8
Mean 4.8 12.0 21.1 30.4 38.8 45.1 50.7 52.1 58.2 69.8
measured annual phytoplankton produc-
tion of 110 g C/m2/yr in the south basin
Qónasson el al. 1977). The amount of or-
ganic material flowing out of this lake has
been estimated to be 80 tons/day from
May to October in 1972-1973 (Ólafsson,
personal communication). The benthic
production has not been measured; but
the topography of the river creates very
favorable conditions for Similium species
on which the trout feed heavily through
the summer.
Population estimate
In 1976 a tagging experiment was con-
ducted in the upper section of Laxá. Prior
to the fishing season a total of 75 fish was
caught on rod and reel and marked with
Floy’s spaghetti tag. During the fishing
season 16 tagged fish were recaptured in a
catch of 1,772 fish. All of the tagged fish
were caught at the same place where they
had been released.
A population estimate was made using
a simple Peterson estimate:
Sj =
M (C+l)
R + 1
75(1772+1)
16+ 1
7822
Here:
N = 7822, estimated population size at
time of tagging,
M = 75, number of tagged fish,
C = 1722, number of fish examined,
R = 16, number of recovered tags.
If it is assumed that the natural mor-
tality was negligible during the experi-
ment, the ratio R/M = 16/75 = 0.21 is an
estimate of total fishing mortality during
the season.
The 1974-1976 catch
Traditionally, the river was used by the
farmers themselves. They fished with
gillnets, seines, or rods. In other cases in-
dividual farmers rented out their fishing
rights. No reliable information exists for
the catch until 1974, when the entire river
was rented to one angling club. The club
sets its own regulations: fly fishing only
with a lower size limit of 35 cm and a catch
limit of 12 trout per day. Each fisherman
must give full information about his catch,
such as the length of each fish and the
location where the fish was caught.
Table 2 summarizes the average catch
for the entire season at the various fishing
sites in Laxá in 1974—1976. For both the
upper and the lower sections there is a
decline in total landings over this time
period. A more sensitive measure is the
catch per rod per day. The lower sections