Acoustic target strength of capelin measured by single-target ...

ICES Journal of Marine Science, 59: 1081–1085. 2002
doi:10.1006/jmsc.2002.1239, available online at http://www.idealibrary.com on
Acoustic target strength of capelin measured by single-target
tracking in a controlled cage experiment
Roar Jørgensen and Kjell Olsen
Jørgensen, R., and Olsen, K. 2002. Acoustic target strength of capelin measured by
single-target tracking in a controlled cage experiment. – ICES Journal of Marine
Science, 59: 1081–1085.
Knowledge of the acoustic target strength (TS) of capelin, Mallotus villosus (Müller),
is important to scale integrator outputs for use in acoustic surveys, but experimental
measurements of capelin TS are scarce. This paper reports a new ex situ experiment on
capelin TS. Small groups of the fish were kept in a large sea cage that allowed them to
swim freely between 4.5 and 10 m deep. TS measurements were made vertically with a
scientific split-beam echosounder. Differences in behaviour could induce at least a 5 db
change in TS owing to changes in tilt angle. The observed mean TS for capelin from
the local Balsfjord stock was >1 db higher than expected from the TS–length
relationship for Barents Sea capelin used during abundance estimation.
2002 International Council for the Exploration of the Sea. Published by Elsevier Science Ltd.
All rights reserved.
Keywords: cage experiment, capelin, target strength, target tracking.
Received 24 July 2001; accepted 30 March 2002.
R. Jørgensen and K. Olsen: Norwegian College of Fishery Science, University
of Tromsø, N-9037 Tromsø, Norway; tel: +47 77 646084; fax: +47 77 646020;
e-mail: roarj@nfh.uit.no.
Introduction
The stock of capelin, Mallotus villosus (Müller), in the
Barents Sea is potentially the largest in the world,
its biomass in some years reaching 6–8 million tons.
It is the largest stock of pelagic fish in the Barents Sea
and supports many populations of predatory fish,
marine mammals, and seabirds. It has provided an
annual fishery harvest of up to 3 million tons
(Gjøsæter, 1998). Acoustic surveys in addition to
information from catch statistics have provided the
only information on stock status to date (Gjøsæter,
1998; Toresen et al., 1998). In situ and (or) experimental
measurements of capelin acoustic target
strength (TS) are scarce, but knowledge of capelin TS
is important for scaling integrator output for use in
acoustic surveys. Dalen et al. (1976) measured dead or
stunned capelin and derived an equation for total
length dependency of the maximum dorsal aspect TS
for a mixture of several physostome fish (including
capelin). Olsen and Angell (1983) conducted TS
measurements on small groups of live capelin enclosed
in a cage and found that the TS of mature fish was
tilt-angle dependent, when observed by a 38 kHz echosounder.
The mean target strength currently used for acoustic
abundance estimates of the Barents Sea capelin stock is
a pooled result of experimental and field measurements,
including counting/integration on single-fish echo traces
(Dalen and Nakken, 1983; Dommasnes and Røttingen,
1984). The equation used is
TS=19.1log 10(l)74 (1)
where l is total length.
This paper reports on capelin TS at 38 kHz measured
by single-target tracking in a controlled experiment.
Material and methods
The experiment was carried out in a net pen at the
fish farming plant of the Ka˚rvika Aquaculture Station,
situated at Ringvassøya, an island northwest of Tromsø
in northern Norway. Capelin were captured from the
local stock in Balsfjord, close to Tromsø, during April
2000. They were caught during beach-spawning with a
small shore-seine and carefully transported to Ka˚rvika
in a tank with oxygenated water. The fish were kept in
larger tanks at the station, and were fed until the day
they were transferred to the net pen.
1054–3139/02/101081+05 $35.00/0 2002 International Council for the Exploration of the Sea. Published by Elsevier Science Ltd. All rights reserved.

1082 R. Jørgensen and K. Olsen
Figure 1. Schematic overview of the experimental set-up: (1)
pier, (2) net pen, (3) net roof, (4) big weight, (5) small weight,
(6) air-filled aluminium frame, (7) split-beam transducer, (8)
cable, (9) dry lab with echosounder and computer equipment.
The arrows show how the net pen and the aluminium frame
could be lifted and suspended by pulling the smaller weights.
Small groups of capelin were allowed to swim freely in the pen;
only one fish is shown here.
The experimental net pen was 5510 m with a
mesh size of 11 mm (Figure 1). The bottom of the pen
was attached to a 5.25.2 m air-filled aluminium frame
stabilized by four 20 kg weights. The net pen and the
aluminium frame could be lifted and suspended by
pulling smaller weights attached to the aluminium frame
(Midling et al., 1998). To keep the capelin well outside
the transducer nearfield, a ‘‘roof’’ was installed in the net
pen at a depth of 4.5 m. Under the roof, capelin were
allowed to move freely between 4.5 and 10 m deep in a
120 m 3 cage shaped as a 554.5 m box with an
additional roof extension 1 m high. The echo from the
net roof (TS

Table 1. Parameter settings and calibration parameters for the EY500 system.
Parameter
maximal vertical movement between consecutive detections
was 10 cm. The tracking output files were reformatted
and imported to SYSTAT (SPSS Inc.) and
MATLAB (Math Works Inc.) for final statistical
analysis.
TS measurements were conducted on several different
groups of capelin. However, results from just one group
passed the criteria for single-fish echo traces: other
groups did not disperse sufficiently to avoid multiple
echoes. This actual group was transferred to the net pen
on 22 June 2000; it consisted of 12 female and 3 male
post-spawning Balsfjord capelin. The fish were allowed
to acclimatize for 10 h in the net pen prior to TS data
collection. In all, 5 fish, all females, died during the two
2 d experiments. Fish schooled during daylight, but
partly dispersed at night and in the early morning.
Echo-trace data were recorded between 5.8 and
9 m deep; they consisted of 6569 TS data distributed
in 273 echo traces, with 5–91 TS detections in each
trace.
A relationship between tilt angle and target strength
for an individual capelin was estimated by relating TS to
the movement of a fish in a selected 33-ping-long echo
trace. For one transmission (‘‘ping’’), n, the angles and
define the target direction relative to the acoustic
axis in alongship and athwartship planes respectively.
For small angles, the angle from the acoustic axis is
Date (2000)
05 June 05 June 05 June 26 June 26 June
Sound speed (m s 1 ) 1472 1472 1472 1480 1480
Ping rate (s) 1 1 1 0.0 1
Range (m) 4 5 6 6.4 6.4
RMS (dB) 0.10 0.08 0.08 0.12 0.10
TS gain (dB) 21.3 21.2 21.1 20.5 21.1
3dBbeamwidth()
Athwartship 11.8 12.0 12.1 12.1 12.0
Alongship 12.0 12.3 12.4 12.4 12.4
Offset angle ()
Athwartship 0.19 0.04 0.19 0.00 0.22
Alongship 0.14 0.00 0.18 0.01 0.18
Table 2. Target strength detection parameters for the EY500.
Parameter Value
Minimum value
Echo length
70 dB
Minimum 0.8 ms
Maximum 1.8 ms
Maximum gain compensation 6.0 dB
Maximum phase deviation 3.0 steps
Acoustic target strength of capelin
given by:
n=√( 2
n+ 2
n) (Reynisson, 1999) (2)
The three-dimensional movement of the capelin at a
given range (r n) was thus transformed to Cartesian
coordinates alongship:
x n=r n sin( n) (3)
and athwartship:
y n=r n sin( n) (4)
In depth this gives:
1083
z n= r n cos( n) (5)
Random variations of target positioning were
removed by smoothing each coordinate with kernelweighted
polynomial regression (Wilkinson, 1999), with
time as independent variable. The angle, n, between the
line of movement from ping n1 to ping n and the
transducer’s face was given by:
n=atan{(z nz n1)/√[(x nx n1) 2 +(y ny n1) 2 ]} (6)
The tilt angle in the first ping was estimated to be
equal to the tilt angle in the second ping ( 1= 2).
The smoothed angle from the acoustic axis ( n) was
computed from the smoothed Cartesian coordinates:
n=acos{√[z n/(x 2
n+y 2
n+z 2
n)]} (7)
Because the fish in the selected trace moved approximately
parallel to the alongship axis (x), the tilt angle
could be computed by simply adding n to n (x n>0) or
subtracting n from n (x n

1084 R. Jørgensen and K. Olsen
Count
1000
800
600
400
200
0
–57
Results
–56 –55 –54 –53 –52 –51 –50 –49
Target strength (dB)
Figure 2. Target-strength distribution of all observations in the
ex situ experiment (n=6569). The vertical line shows the mean
target strength, 52.5 db.
Depth (m)
5.55
5.6
5.65
5.7
5.75
5.8
5.85
–0.4
–0.2 0 0.2 0.4 0.6
Alongship (m)
Figure 3. Vertical movements of a capelin in one echo trace
(n=33). Crosses show actual measurements and arrows show
the direction of movement. The smoothed line describes
estimated movement alongship and in depth.
The total length of the 15 fish ranged from 10.5 to
12.3 cm, with a mean of 11.3 cm (s.d.=0.5). The mean
length of the 10 fish alive at the end of the experiment
was 11.4 cm (s.d.=0.5).
The TS distribution of all valid measurements (Figure
2) is rather narrow, ranging from 49 to 57 dB.
By computing the mean of the acoustic backscattering
cross-section, the corresponding mean TS was
52.5 dB (vertical line in Figure 2).
The swimming behaviour of a fish in an echo trace is
shown in Figure 3. This fish moved approximately
parallel to the alongship axis through the centre of the
beam. The effect of tilt angle on TS for this trace
Target strength (dB)
–50
–51
–52
–53
–54
–55
Head up
Head down
–56
30 20 10 0 –10 –20 –30 –40 –50
Tilt angle (°)
Figure 4. Successive target-strength observations (circles) in
relation to estimated tilt angle during one echo trace (n=33).
observation is estimated and plotted in Figure 4. For
this single capelin, the dotted lines that connect the
estimated points (circle) in Figure 4 suggest a maximum
TS with near-horizontal swimming behaviour. The echo
trace has about 5 dB range in TS.
Discussion
If the current TS-length relationship used for Barents
Sea capelin surveys (equation 1) is applied, a mean TS of
53.8 dB for the 10 capelin that survived the experiment
would be generated. This is 1.3 dB lower than
estimated during the current measurements. This discrepancy
could be due to differences in behaviour of
captive capelin compared with fish in the open ocean.
Capelin in their natural environment may have a different
tilt-angle distribution than in the net pen, and this
could influence the mean TS (Olsen and Angell, 1983).
Target strength in this experiment has been measured
close to the surface, whereas TS at greater depth may be
lower owing to there being no compensation for
the swimbladder during vertical migration (Ona, 1990).
Furthermore, seasonal variations caused by the fat cycle
and gonad development can effect the TS of fish such as
herring Clupea harengus (Ona, 1990) and capelin (Rose,
1998). These factors may cause the target strength of the
lean post-spawning Balsfjord capelin to be different
from that of Barents Sea capelin during the acoustic
assessment survey in autumn, when capelin have a
higher fat content. Reduced swimbladder size as a result
of higher fat content may in fact lead to a reduced mean
target strength. A general conclusion is, however, that,
at surface levels, the target strength of capelin is lower
than that of other species such as cod Gadus morhua and
herring.

The bimodality of the TS distributions shown in
Figure 2 may be related to up-and-down swimming
behaviour of the capelin or differences in the acoustic
backscattering properties for different fish in the group.
The 30 dB
range found in a similar experiment with mature herring
(Zhao, 1996). At 38 kHz, the drop in target strength
attributable to changes in tilt angle is expected to be
considerably smaller for a more omnidirectional backscattering
target such as a 11 cm capelin, than for a
30 cm herring (Nakken and Olsen, 1977). This is because
the difference between fish length and the wavelength of
the transmitted sound at 38 kHz is smaller for capelin
than for herring.
Acknowledgements
We thank E. Ona and I. Svellingen (IMR) for technical
assistance and for valuable comments on the manuscript.
Special thanks to local fisher H. Frydenlund for
catching the Balsfjord capelin, and to J. S. Christiansen
and B. S. Sæther for keeping them in tanks and providing
us with live material. We are grateful to L. P.
Jørgensen and his crew at the fish farming plant at the
Tromsø Aquaculture Research Station for all their various
assistance. The Norwegian Research Council funded
the study.
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