Íslenskar landbúnaðarrannsóknir - 01.09.1978, Page 149

HYDROACOUSTIC ASSESSMENT 147
map, boat speed was calculated. The ei'-
fective beam angle was estimated to be
9.54° full angle. Table 1 gives the result oí'
the measurement at three depths and Ta-
ble 2 lists the effective pulse sampling vol-
umes.
CALCULATION OF FISH DE-
NSITIES AND POPULATION ESTI-
MATES
The calculation of fish dcnsities, most
conviently expressed as fish/1000 m3, is
straightforward when the total number of
target delections and the total sampling
volume are known, b\' use of the following
equations (Nunnallee ancl Mathisex
1972b). (2)
(1000) (lotal lish detections)
Fish/1000 m3 = -—------------------------
(eíTccti\e pulse \'olumc) (total pulses)
Also, the number of físh/100 m2 surfacc
area of thc lake can be calculated from the
rcsults ofequation 2 b\ use of the follow-
ing: (3)
Fish/100 m2 = 1)11 ~ —1 (Ni)
\^\
where, Du ancl Du = the upper and lower
depth limit of stratum i, respecti\el\'
n = the number of depth strata
Ni = Fish/1000 m3 in stralum i
The next logical step, after a suíFicient
number of measures of fish density/unit
surface area ha\re been made, is to esli-
mate the population of fish in the lake.
Various methods can be used to make a
population estimate, the simplest being
merely to expand the mean desity/unit
surface area over the area of the lake. A
better method is to stratify the lake on the
basis of target densities and to expand the
strata density/unit surface over the rc-
spective lake area. Finally, densitics of
fish/unil volume can be stralified o\’cr
3-dimensional unils, such that fish de-
nsities within each stratum are cxpanded
over their individual volumes and sum-
med to produce a population eslimate.
The flrst method was uscd in the popula-
tion eslimates for Thingvallvaln ancl
Skorradalsvatn because insufllcienl sam-
pling efforl was expendcd to clearly deflne
the areal and depth dislributions of the
fish. Further surveys can be designed to
gathcr the necessary data for the second or
third method now that the exploratory
surveys have given an indication of the
expectcd fish densilies ancl distributions.
TEST FISHING IN THINGVALLA-
VA'I’N LAKE
Hydroacoustic Fish stock assessment
lechniques are nol complete in themselves
since species, size, and age composition of
the ílsh cannot be determined directly. It
therefore is necessary to catch ílsh by var-
ious representative means, in order to
monitor these population parameters.
A midwater frame trawl was used in
Thingvallavatn (Nunnallee 1974b) and
three hauls were made near the central
part of transect 2 (Fig.2), where consid-
erable densities offish were observed. The
net was submerged to the depth of highest
Ftsh abundance and was towed for 30, 20,
and 30 min for the three respective hauls.
Tite total catch, however, amounted to
only four fish, all of which were approxi-
mately 18 cm long. All of the Fish were
adults and were approaching spawning
ripeness. No small fish of any kind was