Download In situ target-strength measurement of young hairtail (Trichiurus

ICES Journal of Marine Science, 63: 46e51 (2006)
doi:10.1016/j.icesjms.2005.07.010
In situ target-strength measurement of young hairtail
(Trichiurus haumela) in the Yellow Sea
Xianyong Zhao
Zhao, X. 2006. In situ target-strength measurement of young hairtail (Trichiurus haumela)
in the Yellow Sea. e ICES Journal of Marine Science, 63: 46e51.
The target strength of hairtail (Trichiurus haumela) in the Yellow Sea was measured in situ
with a 38 kHz, split-beam echosounder on 2 January 2001. The fish measured were of the
2000 year class, its anal length ranged from 62 to 115 mm, with a mean of 89.8 mm. The
mean target strength of these young hairtail was estimated to be 49.2 dB, with a 95% confidence interval of (49.4, 49.0) dB. This provided a rare and useful reference for the
acoustic-abundance estimation of hairtail.
Ó 2005 International Council for the Exploration of the Sea. Published by Elsevier Ltd. All rights reserved.
Keywords: hairtail, target strength, Yellow Sea.
Received 29 May 2003; accepted 30 July 2005.
X. Zhao: College of Marine Life Science, Ocean University of China, Qingdao, China, and
Key Laboratory for Sustainable Utilization of Marine Fisheries Resources, Ministry of Agriculture, Yellow Sea Fisheries Research Institute, Chinese Academy of Fishery Sciences,
106 Nanjing Road, Qingdao 266071, China. Correspondence to X. Zhao: Yellow Sea Fisheries Research Institute, 106 Nanjing Road, Qingdao 266071, China; tel: +86 532 858 353
63; fax: +86 532 858 115 14; e-mail: [email protected].
Introduction
Hairtail [Trichiurus haumela (Forsskål)] is one of the commercially and ecologically most important fish species in
the East China Sea and the Yellow Sea (Luo, 1991). It is
a swimbladder-bearing, semi-demersal fish that performs
fairly distinct diurnal vertical migrations. It concentrates
in a narrow layer close to, but often a few metres above,
the bottom during daytime (own observation), and disperses
in the pelagic during night-time. This makes the fish a suitable target for echo-integration surveys. However, hairtail
are often mixed with many other fish of non-negligible
quantity, which renders the in situ measurement of the target strength of this fish a very difficult task.
Hairtail has an extremely elongated and deeply compressed body shape; its unusually thin and long tail is easily
broken, which leads to the measurement of its anal length
instead. Lacking direct target-strength (TS) measurements,
Ona (1987) suggested the following TS to anal-length (la)
relation for hairtail:
the correlation between total length and anal length of
hairtail.
During an acoustic/trawl survey on the recruitment status
of important fish populations in the Yellow Sea in winter
2000/2001, hairtail echo registrations suitable for in situ
target-strength measurement were encountered. The results
are discussed in this paper.
Material and methods
Measurement site
The survey was conducted aboard RV ‘‘Bei Dou’’, a 56.2 m
stern trawler designed for acoustic and trawl surveys. Target-strength measurement of hairtail was carried out in the
western part of the Yellow Sea (Figure 1), close to a traditional spawning ground in Haizhou Bay (Luo, 1991).
Biological sample collection
TSZ20 log la 66:1;
ð1Þ
that has been used whenever needed in surveys (Zhu and
Iversen, 1990). The equation was therefore established on
the basis of comparison between swimbladder morphometries of hairtail and several other North Atlantic fish, and
1054-3139/$32.00
During the survey, at about 18:30 on 2 January 2001, dispersed single-fish echo traces (Figure 2) were found in
the water column from 25 m below the sea surface to
a few metres above the bottom at about 52-m depth. In order to identify the biological origin of the echo traces,
a sampling haul was carried out. In this particular instance,
a bottom trawl, instead of midwater trawl, was used to test
Ó 2005 International Council for the Exploration of the Sea. Published by Elsevier Ltd. All rights reserved.
In situ target-strength measurement of young hairtail
47
38
Qingdao
Yellow Sea
Latitude (N)
36
Haizhou
Bay
34
32
120
122
124
126
Longitude (E)
Figure 1. Area of data collection. The TS measurement site is
shown by the solid dot.
the effectiveness of the bottom trawl as a pelagic sampling
device at shallow depth.
The trawl was monitored via a SCANMAR trawlsounder
(model TS150). At a speed of 3.2 knots, the trawl was initially towed pelagically at 25-m depth by paying out only
100 m of warp; every 20 m additional warp was paid out
after the net had been stabilized for about 10 min at each
sampling depth. The trawl finally stabilized on the bottom
for 10 min at a warp length of 190 m. The tow lasted for
56 min. The start and end positions of the tow were
34(32.4#N 122(15.0#E and 34(32.2#N 122(11.4#E,
respectively.
The trawl was made up of four panels, with a stretched
mesh size of 200 mm in the forward section and 14 mm
in the codend. The vertical opening of the trawl ranged
from 7.7 to 8.2 m when towed pelagically, and from 5.9
to 6.1 m when settled on the bottom; its wingspread was
about 25 m according to previous and later measurements.
The trawl sample was sorted according to the routine
procedure. For hairtail, a subsample of 110 fish was measured to the nearest millimetre (anal length) to give the
length distribution.
Figure 2. Typical echogram showing single-fish echo traces. The
values to the right of the echogram are the depths of the integration
layers with the vessel’s draft included.
by SIMRAD. The calibration results and other important instrument settings are given in Table 1.
Data analysis
Echo-trace data acquisition
When the haul was on deck, the survey was adjourned for
target-strength measurement. The ship sailed back to the
start position of the haul and then turned back to the end
position again at a speed of about 10 knots. The echo-trace
data were collected during these two runs with a SIMRAD
EK500, split-beam echosounder. The echosounder was
calibrated prior to the survey on 26 December 2000 using
the LOBE program (V5.xx, 30 October 1995) provided
The raw echo-trace data were logged via the serial port of the
echosounder and stored on a PC by PROCOMM (V2.4, Datastorm Technologies Inc., 1986). They were processed using
a dedicated Pascal program (Zhao, 1996). First, the echogram
was carefully checked and only those runs with a constant
vessel speed of about 10 knots and clear single-fish echo
traces were selected for further treatment. The corresponding
sub-data set was obtained by specifying the selected time
intervals and range limits through a subroutine of the
program. Data were then reformatted into the standard
48
X. Zhao
Table 1. SIMRAD EK500 echosounder (V5.31) technical specifications and parameter settings for hairtail target-strength
measurements.
Parameter
Settings
Unit
Ping interval
Pulse duration
Bandwidth
Maximum transmitting power
Range
Sv colour minimum
TS transducer gain
Alongship 3 dB beam width
Athwardship 3 dB beam width
Alongship offset angle
Athwardship offset angle
Minimum TS value
Minimum echo length
Maximum echo length
Maximum gain compensation
Maximum phase deviation
1
1.0
3.8
2 000
100
70
27.10
7.07
6.86
0.05
0.07
60
0.8
1.3
6
4
s
ms
kHz
W
m
dB
dB
degree
degree
degree
degree
dB
dB
case-by-variable format and imported into SYSTAT Inc
(1992) for final analysis. In all cases, the averaging process
was done in the linear domain (scattering cross-section)
and then converted into the logarithmic domain (TS).
Results
Biological samples
Hairtail dominated the catch both in number and in weight
(Table 2). Figure 3 shows the length distribution of the hairtail sample. The anal length ranged from 62 to 115 mm,
with a mean of 89.8 mm and standard deviation of
9.7 mm. They were of the 2000 year class, according to
the growth characteristics of this fish (Luo, 1991).
Anchovy (Engraulis japonicus) was the second most
common species in the catch, its total length ranging from
75 to 98 mm. Other fish in the haul were of negligible
amount (Table 2). Some typical bottom-dwelling fish and
small shrimp and crab were also present. They were most
likely caught at the end of the tow when the trawl was on
the bottom, and are not included in Table 2.
Target strength of hairtail
Figures 4 and 5 show, respectively, the target-strength histogram of the echo data selected from 25- to 45-m depth layer
and the layer deeper than 45 m (up to 53 m). It can be seen
from Figure 4 that the target-strength distribution has a long
left tail, with a distinct mode at 48 dB; a smaller second
mode at 59 dB is also visible. The target-strength distribution of the echoes close to the bottom (Figure 5) is relatively
messy, with more large echoes than those observed in the
upper layer of the water column (Figure 4).
Since most of the best single-fish echo traces were concentrated in the 25e45 m layer (Figure 2), and also to avoid
further contamination attributable to other organisms close
to the bottom, only data in the depth range 25e45 m were
used for the hairtail target-strength estimate. The mean target strength of hairtail was thus estimated to be 49.2 dB,
with a 95% confidence interval of (49.4, 49.0) dB. The
mean depth of the echo traces was 38.2 m.
Discussion
Target strength of hairtail
The target strength of a fish is a key parameter for quantitative
acoustic fish-abundance estimation. For some fish, however,
this parameter is often not available or even not easily obtainable. Hairtail is such a fish. It is either mixed with other fish in
non-negligible quantity or aggregated in too high density and
not resolvable as a single target. In fact, I have not found any
references in the literature on the target-strength measurement of this particular fish.
The mean target strength of 9.0 cm (anal length) hairtail
reported in this paper was 49.2 dB, with a 95% confidence interval of (49.4, 49.0) dB. When expressed in
the common TSZ20 log la Cb20 formula, it becomes:
.
ð2Þ
TSZ20 log la 68:3G0:2;
where la is the anal length in centimetres. This is
about 2.2 dB lower than that suggested by Ona (1987,
Equation 1). However, the validity of Equation (2) in the
Table 2. Catches relevant to the single-fish, echo-trace data collection.
Species
Common name
Hairtail
Anchovy
Silver pomfret
Small yellow croaker
Vertical striped cardinal fish
Number of fish
Weight of fish
Scientific name
Individuals
%
g
%
Trichiurus lepturus
Engraulis japonicus
Pampus argenteus
Pseudosciaena polyactis
Apogonichthys lineatus
18 720
1 665
52
2
9
91.55
8.14
0.25
0.01
0.04
180 000
6 660
1 530
146
9
95.57
3.54
0.81
0.08
!0.01
In situ target-strength measurement of young hairtail
150
70
80
90
100
110
Number of fish
100
0.1
50
0
-62
0.0
120
0.2
-58
-54
-50
-46
-42
Proportion per Bar
0.1
10
Proportion per Bar
0.2
20
Echo number
30
0
60
49
0.0
-38
Anal length (mm)
TS (dB)
Figure 3. Length distribution of hairtail caught in the sampling
haul.
Figure 5. Target-strength distribution of the single-fish, echo traces
in the layer deeper than 45 m.
entire length range of hairtail needs to be confirmed by further investigation. As the shape of this fish is quite different
from clupiform or gadiform fish, for which the slope of the
target-strength relation is determined, a different slope in
the target strength-to-size relationship may be found.
Sources of error
For the target-strength measurement of hairtail reported in
this paper the estimate was still open to several sources
of error, even though the data were carefully selected. First,
the minimum target strength (60 dB) used for single-fish,
echo-trace data collection was a little too high, as can be
seen from the truncated left tail of the target-strength
0.2
200
0.1
100
0
-62
-58
-54
-50
-46
-42
Proportion per Bar
Echo number
300
0.0
-38
TS (dB)
Figure 4. Target-strength distribution of selected single-fish echo
traces in the 25e45 m layer.
distribution (Figure 4). This might lead to a slightly higher
TS estimate.
Second is the long recognized multiple-target problem
(Ona and Røttingen, 1986; Soule et al., 1995) although
the performance of the single-target detection algorithm
of the new echosounder software has been greatly improved
(Soule et al., 1997). The maximum sA for the 30e40 m
layer was 21 m2 nautical mile2, corresponding to a fish
density of less than 0.1 fish per sampled volume, or a multiple-target probability of less than 0.05 according to Ona
(1999). The fish density in the 20e30 m layer was much
less. Therefore, failure in the complete rejection of multiple
targets might cause a positive but very small bias.
The third and probably the most complicated source of error is the contamination of hairtail echo data due to the presence of several other fish. Fortunately, anchovy was probably
the only contaminant that was of practical significance in the
depth range 25e45 m. According to Chen and Zhao (1990),
the TS to fish-length relationship of anchovy is about
TSZ20 log l 73:2. The mean target strength of these
75e98 mm anchovy was therefore anticipated to be lower
than that of hairtail, so the estimated mean target strength
of hairtail might be negatively biased.
The extent of these biases, however, is very difficult to
quantify. In the case of the truncated left tail, Foote et al.
(1986) used a compensation procedure based on theoretical
or simulated target-strength distribution; while in the case
of contamination or mixed species, several statistical approaches have been used to extract those target-strength data
that are believed to be connected with the fish in question
(Foote et al., 1986; Barange et al., 1996; MacLennan and
Menz, 1996). A common prerequisite to use of these approaches is that the target-strength distribution of the fish
in question is known or assumed a priori. However, there
is simply no a priori knowledge of the target-strength distribution of the hairtail on which to base an assumption.
50
X. Zhao
Anal opening
Swimbladder
Figure 6. A dissected hairtail showing the relative dimension of the swimbladder and the fish. A photograph of the swimbladder alone is
also shown.
This is mainly due to the uncertainty about the behaviour of
this fish in connection with the peculiarity of its body shape,
behavioural pattern being an important influential factor in
the determination of target strength (Foote, 1980). Moreover, because of the directive nature of sound scattering
on the one hand, and the non-uniform, individual behavioural pattern of the fish on the other, the target-strength distribution of fish can be bimodal (Williamson and Traynor,
1984) or even multi-modal and quite variable (Barange
and Hampton, 1994). Zhao (1996) showed that repeated
measurements on a single free-swimming fish could result
in multi-modal target-strength distribution, and warned that
great care should be paid when trying to link different
modes in the target-strength distribution directly to those
of the fish size distribution.
Hairtail has a thin but long swimbladder (Figure 6). The
ratio between the length of the main chamber of the swimbladder and the anal length of the fish is 0.48 according to
Ona (1987). Therefore hairtail must be a directive scatterer
at 38 kHz, so a target-strength distribution similar to that
shown in Figure 4 can be considered as highly possible.
In fact, multi-modal or even a skewed distribution pattern like
that shown in Figure 4 has also been found with anchovy
(X. Zhao, unpublished observation). Therefore, no attempt
has been made to ‘‘correct’’ the target-strength estimate
of hairtail at present if only because hairtail was the overwhelmingly dominant fish according to the trawl sample
(Table 2). Possible bias associated with the selectivity
of the sampling gear and its avoidance by fish is well
known (Foote et al., 1986; Barange and Hampton, 1994;
MacLennan and Menz, 1996), and will not be discussed
further here.
Future work
The target-strength estimate given in this paper provides
a valuable starting point for the acoustic-abundance
estimation of hairtail not only in the Yellow Sea and East
China Sea, but also in other parts of the world. Further efforts should be made to measure the target strength of this
fish of different lengths and preferably at even purer catch
composition, and, of course, using a lower minimum target
strength for single-fish, echo-trace data collection.
Acknowledgements
The captain and crew of the RV ‘‘Bei Dou’’ are thanked
for their cooperation during the unusual sampling operation, and Yong Wang for his help during target-strength
data collection. Dr E. Ona of the Institute of Marine Research, Bergen, Norway and two anonymous reviewers are
thanked for their valuable comments. This work was carried out under the auspices of the National Fishery Resources Monitoring Program from the Ministry of
Agriculture (HY126-02) and the National Key Basic Research Program from the Ministry of Science and Technology, P. R. China (G19990437).
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