Íslenskar landbúnaðarrannsóknir - 01.09.1978, Page 148

146 ÍSLENZKAR LANDBÚNAÐARRANNSÓKNIR
TABLE 1. Estimation of the rad ius of the effective sampling beam in the Skorradalsvatn survey.
Depth (m) 1 2 Detections per fish 3 4 5 6 7 Mean Det/fish Radius
16.5 9 15 6 1 2 2.15 1.60
20.1 5 12 9 4 1 2.48 1.85
27.4 2 4 4 7 2 1 3.35 2.50
Elíective beam radius f'or all depths: r = 0.202 + 0.084 (D).
ESTIMATION OF EFFECTIVE
SAMPLING VOLUMES
The method used to estimate eíl'ective
hydroacoustic sampling volumes has been
described previously (Nunnallee 1972b).
Basically, the method relates the average
number of'times that individual fish are
detected by consecutive transmissions oí'
the echo sounder, the pulsing rate oí’ the
transmitter, and boat speed, in order to
oblain a measure of the diameter oí the
continuum of target detectability at var-
ious depths. After the dimensions are de-
termined, a pulse volume can be calcu-
lated for any deplh stratum. The product
of one pulse volume and the total number
of pulses in a transect or section are equi-
valent to the total sampling volume.
During the Thingvallvatn survey there
was no reliabie wav to measure the boat
speed, so the sampling volume for the sur-
vey was based on a 10° cone. From past
experience in a large number ofsurveys of
other lakes, measured conic angles have
varied from 8° to 10° using similar
methods. Therefore, the assumption of a
10° cone is conservative in the worst
situation. Also, the signal to noise ratio at
the deepest depths of interesl suggests,
frorn the directivity pattern of the trans-
ducer, a beam angle of' from 9° to 9.5°,
again making 10° a conservalive estimate.
l'he sttrve) of Thingvallavatn was
anah zed in 3.7 m (5 ms) depth intervals,
from 3.7 m to the lake bottom, or until
significant ftsh densities were no longer
encountered.
During the Skorradals\atn sur\e\,
transect 2 was run at a very slow speed
between two easily distinguishable lancl-
marks to facilitate calculation of the eíl'ec-
tive pulse sampling\-olume. B\ measuring
the time of the run and the dislance on a
TABLE 2.
EfTective pulse sampling volumes based
on beam radius estimates for Skorradalsvatn.
Depth
ms m pulse volume (m3)
5 to 10 3.7 to 7.3 5.05
- 15 - 11.0 10.81
- 20 - 14.6 18.74
- 25 - 18.3 28.87
- 30 - 21.9 41.13
- 35 - 29.6 55.57
- 40 - 29.3 72.16
- 45 - 32.9 90.99
- 50 - 36.6 111.93
- 55 -40.2 135.03
- 60 -43.9 160.30
- 65 - 47.5 187.83