Íslenskar landbúnaðarrannsóknir - 01.09.1978, Side 143

ISL. LANDBÚN.
J. AGR. RES. ICEL. 1978 10,2: 141-155
Hydroacouslic assessment of the lake Thingvallavatn
and lake Skorradalsvatn fish populations
Edmund p. Nunnallee
College of Fisheries, Universily of Washinglon,
Sealtle, Washinglon
and
Jón KristjAnsson
Inslilule of Freshwaler Fisheries,
Reykjavík
ABSTRACT
A hydroacoustic survey was made of pelagic lish populations in lakes Thingvallavatn and Skorradalsvatn.
Estimates were made of total number of targets at both places, but lack of fishing capabilities prevented a
direct signature of the targets. They are known to consisl cxclusively or predominantly of char (Salvelinus
alpinus).
IXTRODUCTION
Hydroacoustic surveys oí Thingvallavatn
and Skorradalsvatn were completed dur-
ing the nights of August 27 and September
1, respectively (Fig. 1). Experimental
transects were made during both the day-
time and nigthtime prior to the surveys to
determine the diel movements of the fish.
It was found that during the day the fish
formed small, tight schools from 20 to 40
m deep, making it impossible to determine
the numbers in any school. During the
nighttime, however, the schools dis-
pcrsed, and nearly all ofthe deteclable fish
were scattered throughout the water col-
umn, mostly at depths from 10-25 m.
Prior to this first survey of Thingvalla-
vatn, nothing was known of the areal or
depth distributions of the fish or of what
densities they would be found. Therefore,
an exploratory survey was designed by
allocating equal sampling effort to all
areas of the lake. This was done by trans-
ecting across the long axis of the lake with
approximately equal spacing between
each transect. In all, five crossings were
made, as shown in Fig. 2. The highest fish
densities were found near the northern
and southern ends of the lake, with lower
densities in the central part.
The survey of Skorradalsvatn was also
exporatory and the same type of sampling
design was used as in Thingvallavatn. In
all, six transects were run across the long
axis of the lake, progressing from the west-