In situ acoustic target strength of juvenile capelin
Richard L. O'Driscoll
0
George A. Rose
0
0
R. L. O'Driscoll: National Institute of Water and Atmospheric Research
,
PO Box 14901, Kilbirnie, Wellington
,
New Zealand.
G. A. Rose: Fisheries Conservation
,
Fisheries and Marine Institute, Memorial University of Newfoundland
,
PO Box 4920, St. John's
A dispersed, monospecific aggregation of juvenile (0+) capelin was detected acoustically in shallow (20-70 m) water in Bonavista Bay, northeastern Newfoundland in January 2000. This provided a rare opportunity to measure acoustic target strength (TS) of very small (mean length=51 mm) capelin in situ. Mean observed TS at 38 kHz was 61.0 dB. Observed TS was similar to TS predicted by the Norwegian-Icelandic capelin TS-length relationship (TS=19.1 log L (cm) 74), but 2 dB lower than predicted by the existing TS-length relationship for capelin in Newfoundland waters at 38 and 49 kHz (TS=20 log L (cm) 73.1). Combining present data with previous 38 kHz data indicates the relationship TS=23.3 log L (cm) 77.1 (r2=0.95, n=6) for capelin of lengths 5-14 cm in Newfoundland waters.
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Capelin (Mallotus villosus Mu ller) are small pelagic fish
that are an important forage and commercial species in
northern waters. Abundance of capelin is routinely
assessed using acoustic echo integration methods (e.g.
Vilhja lmsson, 1994; Toresen et al., 1998). Despite the
importance of acoustic target strength (TS) in scaling
echo integrator intensity to fish density, relatively few
measurements have been made of capelin TS. Target
strength models currently used in Norwegian and
Icelandic capelin acoustic surveys (Dommasnes and
Rttingen, 1985) and Canadian surveys (Rose, 1998) are
based on in situ and/or ex situ experiments conducted
with a limited length range of fish. Rose (1998) measured
in situ TS of capelin from 9.515.2 cm in Newfoundland
waters. The TS model of Dommasnes and Rttingen
(1985) incorporates ex situ measurements made by
Dalen et al. (1976) on capelin from 1318 cm as well as
in situ measurements (by echo trace counting) on capelin
of unspecified lengths. It is not known how well existing
models describe capelin TS outside these length ranges.
In this paper we present in situ measurements of acoustic
target strength of very small ( 5 cm) juvenile capelin.
We compare observed TS to predicted values from
existing TS-length/weight relationships.
Materials and methods
We encountered a large, monospecific aggregation of
juvenile capelin in Sweet Bay in the southern part of
Bonavista Bay, northeast Newfoundland (48 26.4 N
53 42.4 W) on 13 January 2000. The aggregation
consisted of a thick layer of capelin dispersed throughout
the water column (2070 m water depth).
Target strength measurements were made using a
Simrad EK500 echosounder with a hull-mounted
splitbeam 38 kHz transducer. Pulse length was 1.0 ms. The
echosounder was calibrated using a standard tungsten
carbide sphere prior to (June 1999) and following (April
2000) this experiment according to recommended
procedures (Foote et al., 1987). There were no major changes
in calibration between these times (<0.2 dB).
Measurements of capelin TS were made during darkness (1900
2030 local time) at a vessel speed of 35 knots over
8 km of acoustic transects in Sweet Bay. Data
collection was limited to targets with normalised echo lengths
from 0.81.8, gain compensation <4.0 dB, standard
phase deviation <2.0 steps, and with corrected TS values
> 70 dB (SIMRAD, 1997).
Following measurements of TS, species composition
and size of capelin were obtained by fishing with a
Campelen 1800 shrimp trawl fitted with a fine mesh
codend liner towed in mid-water ( 25 m depth) through
the aggregation. The catch was 100% juvenile capelin. A
random sub-sample of 200 fish was measured (total
length from tip of the mandible to the end of the ventral
lobe of the tail) and weighed.
The number of fish per effective reverberation volume
(Nv) was calculated from estimates of volumetric fish
density (capelin m 3) and ensonified volume (m3)
according to Sawada et al. (1993). The beam width of
the transducer was 7 between half power points with an
equivalent beam angle, =0.0085 sr. Approximate mean
volumetric fish densities were calculated for bins 5 m
deep by 30 pings (5080 m) long using a backscattering
cross-sectional area equivalent to 61 dB. Only TS
values from bins with Nv<0.4 were used in this paper
(see below). Mean TS was calculated as the logarithm of
arithmetically averaged backscattering cross-section
values.
Results and discussion
Both fish length and TS histograms were unimodal
(Figure 1). Mean capelin length was 51 mm. Mean
weight of individual capelin was 0.30 g. Juvenile capelin
were not aged, but observed lengths were consistent with
fish spawned the previous summer (0+). Peak larval
emergence on Bellevue Beach in adjacent Trinity Bay
was on 22 July 1999 (Nakashima and Slaney, 2000),
suggesting these capelin were probably 175 days old.
We are not aware of any previous TS experiments wi (...truncated)