Íslenskar landbúnaðarrannsóknir - 01.09.1978, Blaðsíða 122

120 ÍSLENZKAR LANDBÚNAÐARRANNSÓKNIR
in cold; with steady exponential
growlh, it is true then that 50 =
0.25eG "lu Thc weights shown under
rule 3 were obtained by solving for G
and then using the equation Wi =
WoeG', where Wo is the initial weight
and Wi is the weight after time t.
Sketchy data suggest that the schedule
shown is slighlly conservative in the
early months and slighlly ambitious in
the later months. Also, the weights
shown are mean weighls of an un-
graded population. In practice, grad-
ing occurs regularly so that ponds are
divided into populations growing fas-
ter than the mean weight and popula-
tions growing slower.
3. Some growth does occur in unheated
water, but the data are extremely
sketch)-. While this would appear to
yield conservative results, the effects of
this assumption are possibly important
and deserve more careful scrutiny.
4. Data on the number of fish that can be
held per unit ofwater were incomplete.
However, it was known that 20.000, 50
g fish could be held in heated water in
two large ponds of the second-stage
rearing house. The maximum number
of fish to be held at other weight was
scaled from this known point accord-
ing to the schedules used in
Washington State salmon hatcheries.
For the first-stage rearing house, var-
ious densities were scaled from the
known point of 150.000, 3.2 g fish in 40
heated ponds. The same maximum
number was assumed for both heated
and unheated water, although it is
known that more fish can be held in
cold water.
Using the previously stated rules, the
chart in Fig. 4 was filled in and a number
of options were considered. Specifically,
the following options were examined:
1. Assume first-stage rearing house re-
mains heated year-round, then raise as
many 1-year smolts as possible to (a)
12 cm, (b) 14. cm, (c) 16 cm (approxi-
mately 19.1, 30.9, and 50 g, respec-
tively), plus the maximum number of
additional 2-year smolts.
2. Allow the first stage rearing house to
use unheated water from September
through February, and raise as many
1- year smolls to (a) 12 cm,(b) 14
cm,(c)16 cm, plus as many additional
2- year smolts. as possible.
3. Relax the rule ofa June 1 release, and
allow releases on May 15, June 15, or
July 15.
Table 2 provides a summary of the
maximum number of smolts that can be
raised under these options. Som impor-
tant ideas become apparent from the
numbers provided in the chart and table.
These ideas, which deserve further study,
are discussed below.
In the second-stage rearing house,
ponds with heated water ought to be kept
at maximum density throughout the
growing period. As the fish grow,
threatening to exceed capacity, the excess
can be transferred to the cold water ponds.
Because of experimental needs, this idea
has not been practiced in the past, but on a
production basis it offers a key way to
utilize available hot water.
In the past, heated water has been used
in the first-stage rearing house throughout
the winter. If the first-stage house were left
unheated from September 1 to March 1,
two extra ponds of hot water would be-