Íslenskar landbúnaðarrannsóknir - 01.09.1978, Side 111

AN EVALUATION OF SMOLT RELEASES 1Q9
retrieve detected tags. The fishermen con-
scientiously brought in their catches and
consented to have a core or plug contain-
ing the tag taken from their salmon. Occa-
sionally, however, a fisherman would not
show up and was subsequently traced by
the Institute staff. In most cases, these
fishermen did not catch any salmon.
The plugs containing the tag were taken
to the Institute for excisron and identifi-
cation, usually done by the same person
who was on duty at the river on that par-
ticular day.
The tag codes along with length and
weight of the fish, their place of recapture,
and a few other parameters of interest,
were punched on computer cards for sub-
sequent analysis on the computer.
The escapement
In order to be able to estimate the total
return rates for the tagged smolts, it was
necessary to know the escapement accu-
rately. Originally, we intened to use the
counts of the mechanical counter on the
river and by closing the trap for 24 hours
once a week, to obtain the tagged-to-un-
tagged ratio. However, the counter was
very unreliable and was removed fre-
quently for repairs. Also, it was found that
the smaller salmon were passing through
the trap through a hole beetween the
metal grid and the side of the trap.
Therefore, on August 4, 1976, the trap
at Ellidaár was continously closed except
for weekends. At 2:00 every weekday af-
ternoon, the salmon were dip-netted from
the trap. All salmon with microtags were
slauhgtered for tag identification, while
the untagged salmon were released up-
stream with a hole punched in their
adipose fin. A total of 294 salmon with
holes in their adipose fins was released
upstream. This number proved to be
satisfactory for a Peterson estimate of the
population from recoveries in the brood
stock seining data.
Estimation of total survival
The escapement above the trap was esti-
mated using a simple Peterson estimate.
r (Total No. seined) X (Total No. marked)
Escapemcn. = ■■■■ f^al'number m^ks recovered-
= 272 X 294 = 1800
43
According to the data, about 12%, or
226 salmon, above the trap were mic-
rotagged. These salmon were then divided
into the representative tag groups based
on a ratio obtained from the fishery in that
location.
Similar procedures were used for the
escapement below the trap.
The total escapement in 1976 was esti-
mated to be 2,245, and 1,690 salmon were
caught, giving a total run of close to 4,000
salmon.
The actual recoveries and estimated
survivals for the various smolts released
are shown in Fig. 2. The survival of wild
smolts based on actual recoveries is in-
creased 150% if estimated returns are in-
cluded, but is only increased 50% for
hatchery smolts. These differences be-
tween hatchery and wild smolts are due to
the fact that close to 60% of the wild
smolts were caught above the trap, but
only 15% of the hatchery smolts were
caught in that location. Thus, the hatch-
ery smolts migrated considerably shorter
distances up the river and did not contri-
bute significantly to the greater part of the