ICES Journal of Marine Science, 60: 584591. 2003
doi:10.1016/S10543139(03)00058-4
Target strength of mesopelagic lanternfishes
(family Myctophidae) based on swimbladder morphology
Hiroki Yasuma, Kouichi Sawada, Tatsuki Ohshima,
Kazushi Miyashita, and Ichiro Aoki
Yasuma, H., Sawada, K., Ohshima, T., Miyashita, K., and Aoki, I. 2003. Target strength
of mesopelagic lanternfishes (family Myctophidae) based on swimbladder morphology. ICES Journal of Marine Science, 60: 584591.
This article reports theoretical values of target strength (TS) for mesopelagic lanternfishes
based on morphological measurements of their swimbladders. Three species of lanternfishes, Diaphus theta (26.977.4 mm standard length (SL)), Symbolophorus californiensis
(85.0108.4 mm SL), and Notoscopelus japonicus (126.0133.2 mm SL), were examined.
After external morphological measurement of the fish body, a specialized ‘‘soft X-ray’’
imaging system was used to map the swimbladders and obtain their morphological
parameters. The swimbladder was inflated in D. theta, uninflated in S. californiensis, and
was absent in N. japonicus. For D. theta, the swimbladder length does not increase in
proportion to the body length, suggesting that the contribution of the swimbladder to
acoustic reflection is reduced with growth in this fish. Based on the morphological
measurements, the theoretical TS of the fish at 38 kHz was calculated using the approximate
deformed-cylinder model (DCM) and the general prolate-spheroid model (PSM). For all
three species, the calculations showed about 3 dB difference between the TS indicated by
the DCM and PSM. Given that the description of body shape is poor in PSM, the DCM
results were adopted for fish without a swimbladder or an empty one. The intercept b20 in
the standard formula TS ¼ 20 log SL þ b20 was 85.7 dB (DCM) for S. californiensis and
86.7 dB (DCM) for N. japonicus. On the other hand, the PSM model was adopted for D.
theta since its swimbladder has too small an aspect ratio to apply the DCM. For D. theta,
the relationship between SL and TS is best expressed by TS ¼ 11:8 log SL 63:5, which
implies that its scattering cross-section is not proportional to the square of the body length.
Ó 2003 International Council for the Exploration of the Sea. Published by Elsevier Science Ltd. All rights
reserved.
Keywords: deformed-cylinder model, lanternfishes, spheroid model, swimbladder morphology, target strength.
H. Yasuma and I. Aoki: Graduate School of Agricultural and Life Science, University of
Tokyo, 1-1-1 Yayoi, Bunkyo, Tokyo 113-8657, Japan. K. Sawada: National Research
Institute of Fisheries Engineering, Ebidai, Hasaki, Kashima, Ibaraki, Japan. T. Ohshima:
Japan Marine Fishery Resources Research Center, 3-27, Kioi-Cho, Chiyodaku, Tokyo,
Japan. K. Miyashita: Field Science Center for the Northern Biosphere, Hokkaido
University, Hakodate, Hokkaido, Japan. Correspondence to H. Yasuma; tel: þ81 3 5841
5281; fax: þ81 3 5841 8165; e-mail: [email protected].
Introduction
Fish target strength (TS) is one of the most important
factors for the reliable interpretation of acoustic data from
field surveys. Estimates of TS have been determined for
many important fish species, either through experimentation or theory (MacLennan and Simmonds, 1992).
Among the many parts of a fish, the swimbladder
contributes 9095% or more to its acoustic scatter (Foote,
1980a), and so its presence and morphological features
are most important considerations with regard to TS.
10543139/03/000584þ08 $30.00
Morphological studies of swimbladders in relation to
acoustic backscatter have been conducted mainly on commercially important fish, such as gadoids (Foote, 1985) and
tuna (Bertrand and Josse, 2000). In those studies, swimbladder morphology was obtained by dissection or slicing
a frozen specimen with a microtome (Foote, 1985). A recently developed ‘‘soft X-ray’’ technique makes it possible
to obtain this information while keeping the fish body intact
(Sawada et al., 1999).
While there is an accumulated knowledge of swimbladder morphology and TS for many fish species, they do not
Ó 2003 International Council for the Exploration of the Sea. Published by Elsevier Science Ltd. All rights reserved.
Target strength of mesopelagic lanternfishes
include the lanternfishes that have a vast biomass and
are thought to play a key role in aquatic ecosystems. Some
histological observations of lanternfish suggest that swimbladder occurrence and morphology may vary significantly
among the species or even among life stages in the same
species (Butler and Pearcy, 1972; Brooks, 1976).
The purpose of this study was to determine the
swimbladder condition of three dominant species of lanternfishes found off Japan (Dhiapus theta, Symbolophorus
californiensis, and Notoscopelus japonicus). We used a
‘‘soft X-ray’’ imaging system and determined the theoretical values of TS based on swimbladder morphology.
Significant progress in the theoretical modelling of fish
TS was made by Foote (1985), who used the Kirchhoff
approximation. Furusawa (1988) was the first to describe
fish swimbladders and bodies as prolate spheroids. Recently, Clay and Horne (1994) and Ye and Furusawa (1995)
used high-frequency approximations to develop simple
deformed-cylinder TS models (DCM), which were subsequently improved using the Kirchhoff integral (Ye, 1997).
In this study, we use both the vacant prolate-spheroid
model (PSM) (Furusawa, 1988) and the DCM (Ye et al.,
1997) to calculate the TS of lanternfishes.
Material and methods
Fish samples
Three species of lanternfishes were obtained from four
cruises conducted around Japan from January to July 2000
(Table 1). Adult D. theta (51.577.4 mm standard length
(SL)) and N. japonicus were captured during daytime by
a commercial midwater trawl (26-m mouth height and 10mm codend mesh) in the mesopelagic layer off southeastern Hokkaido. Juvenile D. theta (26.836.3 mm SL) were
captured at night with an Isaacs-Kidd midwater trawl
(IKMT with 333-lm codend mesh) at a depth of about 20 m
off Sanriku, northeastern Honshu. Adult S. californiensis
were captured at night by another midwater trawl (25-m
mouth height and 3-mm codend mesh) at a depth of about
585
50 m off Hokkaido. All samples were frozen immediately
after capturing and stored at about 35 C on the ship.
Morphological measurements of swimbladder
In the laboratory, the samples were thawed slowly in iced
water over a period of 24 h so that their swimbladder shape
would not change. After external morphological measurement of the fish body, a specialized ‘‘soft X-ray’’ imaging
system (Softex PRO-TEST 100) was used to map the
swimbladders. Six-fold magnification was used to display
the X-ray image on the video monitor and to print the
image. All fishes were X-rayed from both dorsal and side
aspects, following Sawada et al. (1999), and then dissection
was performed to confirm the shape of the swimbladder.
From the X-ray observation and dissection, we classified
the fishes into three categories based on their swimbladder
morphology: (1) thin-walled and gas-filled swimbladder;
(2) atrophied swimbladder without gas; and (3) lacking
swimbladder structure.
The outlines of the dorsal and side-aspect swimbladder
images were traced on translucent paper and measured
with a micrometer to the nearest 0.1 mm. Outlines of
body shape were obtained in the same way when no gasfilled swimbladder was found. Total swimbladder volumes
were estimated by the formula for a prolate spheroid,
VO ¼ 4=3pðaL=2ÞðbL=2Þ2 , where aL and bL are the major
and minor axes, respectively. Whole-body volume was
estimated by submersion in a graduated cylinder and the
note being taken of the difference in the fluid level.
Models of sound scattering by fish
We selected the vacant, PSM (Furusawa, 1988) and the
DCM (Ye et al., 1997) to estimate sound scattering by fish.
The TS of a scatterer is:
TS ¼ 10 logðjfbsj2 Þ
ð1Þ
where fbs is the backscattering form function (Sawada et al.,
1999).
Table 1. Myctophids sampled by midwater trawl and IKMT in the year 2000. The abbreviations a, j, and n refer to adult, juvenile, and
number of fish, respectively.
Sampling location
Date
02
15
26
26
08
08
21
26
19
January
January
January
January
April
April
June
June
July
Time
Latitude
Longitude
Sampling gear
14:2015:09
12:4713:20
10:3611:03
11:4812:38
19:0019:32
22:2122:50
13:2013:56
16:3217:03
23:230:23
42 409N
42 259N
41 559N
41 569N
38 009N
38 209N
41 509N
42 359N
42 349N
144 529E
143 519E
143 419E
143 409E
144 009E
144 009E
142 479E
143 579E
158 469E
Midwater
Midwater
Midwater
Midwater
IKMT
IKMT
Midwater
Midwater
Midwater
trawl
trawl
trawl
trawl
trawl
trawl
trawl
Net depth
(m)
Species
n
Mean length
(mm s.d.)
315
322
330
293
23
20
280
174
50
D. theta (a)
D. theta (a)
D. theta (a)
D. theta (a)
D. theta ( j)
D. theta ( j)
N. japonicus (a)
N. japonicus (a)
S. californiensis (a)
32
51
24
36
19
14
100
100
100
64.0 5.5
62.0 4.6
63.0 5.2
65.1 5.1
29.6 1.1
33.5 1.8
133.1 4.8
130.3 3.8
98.1 11.9
586
H. Yasuma et al.
The PSM approximates the swimbladder as a spheroid,
and the resulting form function is given by:
f1 ðh; /=h9; /9Þ ¼ ð2i=kÞ
1 X
1
X
em =Nmn ðhÞ½Smn ðh; cos hÞ
m¼0 n¼m
Amn Smn ðh; cos hÞcos mð/ /9Þ
ð2Þ
where h ffi kq, k is the wave number, and 2q is the distance
between focal points of the prolate spheroid; h, / and h9, /9
are the spherical-angle coordinates of the scattered and
incident waves, respectively (Figure 1); em is the Neumann
function; Smn is the prolate-spheroidal wave function of
the first p
kind
of order m and degree n; Nmn is the norm;
ffiffiffiffiffiffiffiffiffiffi
and i ¼ ð1Þ. The coefficient Amn is determined from
appropriate boundary conditions.
The DCM describes a swimbladder as a series of
adjacent, disk-like, cylindrical elements. The scattering
function f is given by:
ð
1
X
n
f ðki ; ks Þ ¼ ði=pÞ
Bn ðzÞFn ðzÞðiÞ cos½n/ðzÞ
models show good agreement for prolate spheroids that
fulfil this condition (Ye, 1997).
There are many studies, both experimental (Foote,
1980a, 1985) and theoretical (Ye and Farmer, 1996), reporting that the swimbladder is the main source of echoes and
that echoes from the fish body are negligible. Calculations were therefore done for swimbladders only when an
inflated swimbladder was confirmed. When no gas-filled
swimbladder was found in the fish, the fish body was modelled as a liquid-filled prolate spheroid (liquid-PSM) or deformed-cylinder (liquid-DCM) (Furusawa et al., 1994;
Sawada et al., 1999).
The following parameters are used in both models: the
sound speeds in seawater, in the swimbladder, and in the
fish body are 1522, 340, and 1560 m s1, respectively; and
the density ratios between air and seawater, and between
fish flesh and seawater, are 0.001259 and 1.04 (Furusawa,
1988). The TS is estimated at 38 kHz, which is the most
common frequency in quantitative echosounders used for
stock assessment surveys of fish.
n¼0
exp½iki rðzÞ iks rðzÞdz
ð3Þ
where the integration is done along the deformed-cylinder
axis; ki, ks are incident- and scattered-wave vectors, respectively; Bn(z), Fn(z) are expansion coefficients of order n; /(z) is the azimuth angle between the incident and
scattering directions; and r(z) is range from the datum
point. For further details of this model (see Ye et al., 1997).
We divided the swimbladder outline into 20 equal parts,
with 19 lines drawn perpendicular to the major axis,
following Keys (1981) and Sawada et al. (1999).
Though the DCM describes the swimbladder morphology more precisely and is relatively simple to calculate
compared with the PSM, it has theoretical limitations. A
peculiar limitation of this model is that the aspect ratio has
to be large (approximately >5) and the tilt angle not too
large (<40 ) (Sawada et al., 1999). Results from both
Maximum, average, and normalized TS
We defined the maximum TS as the peak value in the plot
of TS against fish tilt angle. A tilt-angle distribution is
required to calculate the average TS according to Foote
(1980b). In this study, we applied a distribution with a mean
of 5 (5 head down) and a standard deviation of 15 . It
is often convenient to describe the mean TS value by the
normalized TS. This is the constant b20 in the standard
formula TS ¼ 20 log SL þ b20 , where TS is is in dB and
SL is in cm.
Results
Morphological features of swimbladders
All the fish species had different swimbladder morphology.
D. theta had thin-walled, gas-filled swimbladders. All the
S. californiensis had atrophied swimbladders without gas,
and N. japonicus lacked a swimbladder structure. In terms
of acoustic scattering, the key is the presence of gas. In the
subsequent analysis, we therefore treated D. theta as
‘‘bladder fish’’ and S. californiensis and N. japonicus as
‘‘bladderless fish’’.
Swimbladders of D. theta
Figure 1. Swimbladder geometry for the soft-spheroid model.
Thick arrows indicate the directions of the incident and scattered
waves. Positive swimbladder tilt angles are head-up.
We found air bubbles in all the X-ray images of D. theta
specimens. In direct observation by dissection, however,
many of specimens had swimbladders in which the thin
walls appeared to have collapsed. Those bladders may have
ruptured and the gas might have escaped to the interperitoneal cavity; this may have occurred accidentally while
retrieving, freezing, or thawing the catch. We therefore excluded fish with ruptured swimbladders and selected 23 fish
with swimbladders in good condition. This group consisted
Target strength of mesopelagic lanternfishes
587
Table 2. SL and swimbladder dimensions of D. theta. Tilt is the
swimbladder tilt angle with respect to the snout/tail line (Figure 2).
Specimen
number
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
SL (mm)
aL (mm)
bL (mm)
Tilt (degree)
71.7
74.7
60.3
75.5
68.6
70.0
73.2
77.4
33.0
33.3
34.3
36.3
33.9
29.4
34.3
34.5
32.6
33.8
27.2
26.9
31.6
37.7
30.4
6.0
6.9
7.7
8.7
6.3
7.6
7.8
7.4
5.1
5.2
5.4
5.9
5.9
2.8
3.4
3.0
3.3
3.3
4.2
5.5
2.4
6.8
3.4
2.8
3.6
3.4
4.7
3.0
2.9
3.3
3.9
1.9
2.1
2.7
2.3
2.7
1.5
2.1
2.0
1.9
1.8
2.4
2.3
1.8
2.1
1.9
1
14
17
6
17
11
16
14
24
15
20
9
18
11
14
18
7
9
0
0
6
10
9
of eight adults (>60 mm) and 15 juveniles (<35 mm)
(Table 2).
The swimbladder volume increases with the body length
(Figure 2 upper). However, the ratio of swimbladder to
whole-body volume decreases with increasing length
(Figure 2 lower). This finding suggests that growths of
the swimbladder and body length are not proportional.
From logarithmic regression of the swimbladder and body
lengths (Figure 3), we determined a non-allometric growth
relationship for D. theta, as follows:
aL / SL0:68
Figure 2. A plot of swimbladder volume (upper panel) and
swimbladder/whole-body volume ratio (Vs/Vb) (lower panel)
versus standard length for D. theta.
ð4Þ
where aL is the major axis of its swimbladder.
TS of three species of lanternfishes
S. californiensis and N. japonicus
In the absence of gas, S. californiensis and N. japonicus
were treated as bladderless fish. The models liquid-PSM
and liquid-DCM were applied to the body shape. Computations were done for three typical individuals of each
species, since the samples covered almost the same length
classes in both species. The aspect ratio was >5 for all
specimens.
Typical TS patterns for these two species are shown in
Figure 4. The peaks are quite narrow and pronounced,
suggesting that changes in fish orientation will have a major
effect on TS variance. Estimated maximum, mean, and
normalized TS are given in Table 3, where it can be noted
Figure 3. A plot of log(aL) versus log(SL) for D. theta. A
regression line and equation are shown. The b (in brackets)
indicates 95% confidence limits of the regression line.
588
H. Yasuma et al.
Figure 5. Typical TS patterns of D. theta as functions of tilt angle,
obtained from the DCM (solid line) and PSM (dotted line).
that the estimates from the PSM are 34 dB higher than
those from the DCM.
D. theta
Figure 4. Typical TS patterns of bladderless fish as functions of tilt
angle, obtained by the DCM (solid line) and PSM (dotted line):
Upper panel is the pattern of S. califormiensis and lower panel is
the pattern of N. japonicus.
All 23 specimens used for swimbladder measurement were
available for computation. A typical TS pattern is shown in
Figure 5 and the estimated TS in Table 4. As with the other
species, the TS values from the PSM were higher than those
from the DCM. The TS patterns are smooth, suggesting that
the effect of fish orientation is relatively small in this case.
The b20 values of D. theta varied widely among
individuals; the difference between largest and smallest
values was about 6 dB in both models. The b20 tends to
decrease with increasing body length (Table 4). This is due
to the non-allometric growth of the swimbladder and
implies that the scattering cross-section of D. theta is not
proportional to the square of body length. We obtained
Table 3. Estimated maximum, average, and normalized TS for bladderless fish (S. californiensis and N. japonicus) from DCM and PSM
calculations.
TS from PSM (dB)
TS from DCM (dB)
Specimen
number
SL
(mm)
Maximum
Average
b20
Maximum
Average
b20
S. californiensis
1
2
3
108.4
100.8
85.0
56.5
56.5
58.0
61.8
61.8
62.4
82.5
81.9
81.1
60.0
60.2
62.9
65.3
65.6
67.0
86.0
85.6
85.7
N. japonicus
1
2
3
126.0
133.1
131.9
55.6
55.0
56.5
61.9
61.4
62.7
83.9
84.0
85.1
58.4
58.1
64.5
64.3
86.6
86.9
Target strength of mesopelagic lanternfishes
589
Table 4. Estimated maximum, average, and normalized TS of D. theta from DCM and PSM calculations.
TS from PSM (dB)
Specimen
number
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
TS from DCM (dB)
SL (mm)
Maximum
Average
b20
Maximum
Average
b20
71.7
74.7
60.3
75.5
68.6
70.0
73.2
77.4
33.0
33.3
34.3
36.3
33.9
29.4
34.3
34.5
32.6
33.8
27.2
26.9
31.6
31.7
30.4
52.9
51.2
54.5
53.8
53.1
52.4
56.4
55.3
55.6
55.0
60.6
58.1
58.6
58.6
58.9
57.0
55.9
59.9
55.3
58.7
53.4
52.0
54.8
54.1
53.5
52.8
56.6
55.6
55.8
55.3
60.6
58.2
58.7
58.7
59.1
57.0
56.0
59.9
55.6
58.8
69.0
69.5
71.5
71.0
70.8
70.6
67.1
66.3
67.0
65.9
70.0
68.9
69.5
68.9
69.7
65.7
64.6
69.9
65.6
68.4
56.5
56.7
54.6
58.8
57.0
56.2
56.1
60.8
60.5
59.7
59.2
60.5
66.5
60.1
60.6
64.8
64.2
63.5
62.4
56.6
56.8
54.7
59.0
57.2
56.5
56.7
61.1
60.6
59.9
59.3
60.7
66.5
60.1
60.7
64.8
64.3
63.5
62.4
73.7
74.3
72.3
75.8
74.1
73.7
74.5
71.5
71.1
70.7
70.5
71.3
75.9
70.8
71.4
75.4
73.0
73.5
72.5
significant linear relationships ðp < 0:01Þ by regressing
the mean TS against the log of SL (cm) (Figure 6).
Discussion
TS ¼ 15:2 log SL 69:8 ðDCMÞ:
ð5Þ
TS ¼ 11:8 log SL 63:5 ðPSMÞ:
ð6Þ
The swimbladder shape of deepwater fishes might be
influenced by large pressure changes when the fish are
brought to the surface, with serious consequences for the
TS estimation. To avoid this problem, D. theta was fished at
shallow depths during the night. In the daytime, the fish
were in deeper layers, but the gear was towed as slowly
as possible. Additionally, only the best specimens, based
on the swimbladder appearance, were selected for the
computations.
The S. californiensis and N. japonicus specimens did not
have inflated swimbladders. However, it is important to
note that they were all adults. Although there are few
published reports on the swimbladders of S. californiensis
and N. japonicus, Butler and Pearcy (1972) found swimbladders in eight other species of juvenile and adult myctophids captured off Oregon and, of these, two had fully
inflated swimbladders, while six showed swimbladder
atrophy or loss of gas with growth. Similar changes related
to body size have been reported in many species (Marshall,
1960), suggesting that S. californiensis and N. japonicus
quite possibly have inflated swimbladders in their earlier
ontogenetic stages. Such growth patterns should be investigated through further studies covering all life stages.
D. theta, on the other hand, is widely distributed around
the subarctic region, with high abundance compared
with other myctophids (Yamamura and Inada, 2001).
While the slopes were not significantly different
(ANCOVA, p ¼ 0:16), the b20 intercepts were significantly different (ANCOVA, F ¼ 63:1, p < 0:001).
Figure 6. A plot of TS versus log(SL) from DCM (open circles)
and PSM (filled circles) for D. theta. Regression lines and
equations are shown for both cases.
Swimbladder conditions
590
H. Yasuma et al.
Consequently, more is known about its swimbladder
(Marshall, 1960; Neighbors, 1992). According to these
reports, all small fishes (<25 mm SL) have thin-walled,
gas-filled swimbladders in which the volume is relatively
high in proportion to the whole-body volume (67%). Indigenous changes can be seen when the fish exceed 25 mm;
these changes appear to have some morphological variations.
Butler and Pearcy (1972) observed various size classes of
D. theta (3462 mm SL) captured off Oregon and reported
that some of the specimens retained a high proportion
of swimbladder volume (67%), while in others, this
had fallen to 0.3% or less. In addition, Neighbors and
Nafpaktitis (1982) found no gas in atrophied swimbladders
of adult D. theta captured off southern California.
In the observations reported in this study, all the
measured specimens (26.977.4 mm SL) had a thin-walled,
gas-filled swimbladder, linear proportions and relative
volume which decreased with increasing body length.
Specifically, a negative allometric growth relationship was
derived between the swimbladder and body lengths of D.
theta. However, in this study, adequate data covering the
middle range (e.g. 36.360.3 mm SL) were not obtained. In
addition, it is known that metamorphosis of D. theta occurs
at an SL of 1114 mm (Moser and Ahlstrom, 1996) and
that they grow to a maximum length of 117 mm (Ivanov
and Lapko, 1994). Additional observations of swimbladders
covering the entire ranges of body length are still required
for a more precise understanding of the growth relationship
between body and swimbladder lengths in D. theta.
In many fish species, swimbladders grow proportionally
with increases in body size after metamorphosis (Kitajima
et al., 1985), but in lanternfishes, the swimbladder atrophies.
In species that have non-allometric swimbladder growth, the
TS does not follow the simple ð20 log L þ constantÞ relationship (McClatchie et al., 1996). The lanternfishes are
a case in point. This phenomenon is closely related to diel
vertical migration, the unique behaviour of micronektonic
mesopelagic fish. The maintenance of a constant swimbladder volume throughout their vertical range would require
considerable gas secretion or resorption. Alternatively, their
swimbladders may be inflated only while the fishes are in the
upper portions of their vertical ranges. Since swimbladder
morphology may reflect ontogenetic changes in vertical migratory behaviour, the presence of gas-filled swimbladders in
juveniles does not ensure that the same condition will persist
in the adults (Butler and Pearcy, 1972; Neighbors and
Nafpaktitis, 1982). Further studies on the relationships between timing or scale of vertical migration and morphological
changes in the swimbladders of lanternfish may allow a better
understanding of the mechanisms of ontogenetic changes.
Target strength
In the computations for the bladderless S. californiensis and
N. japonicus, both the maximum and averaged TS indicated
34 dB difference between the PSM and DCM estimates.
A plausible reason for this might be the difficulty in describing the complicated body shapes of bladderless fish.
The PSM is less likely to provide an accurate description,
as it approximates the body shape by a simple spheroid. It is
therefore concluded that the estimates using the DCM were
more accurate.
In results obtained by the DCM, the average b20 values
of S. californiensis and N. japonicus were 85.7 and
86.7 dB, respectively. Generally, the b20 of swimbladder
fish is in the range 72 to 65 dB, whereas these results are
substantially lower. Based on comparative experiments on
cod, Foote (1980a) concluded that a difference of 10 dB or
more in b20 might arise from the presence or absence of
a swimbladder. He also reported that the b20 of bladderless
Atlantic mackerel (Scomber scomber) is in the range 90
to 80 dB. These results are consistent with those studies.
In D. theta, as in the other two species, both the maximum
and averaged TS show a 34 dB difference between the
values from the two models. In the case of D. theta, the
theoretical limitation causing this difference might be in
the DCM, rather than the PSM, as applied in the cases of
S. californiensis and N. japonicus. Relatively simple swimbladder shapes for D. theta were observed that are easily
described with the PSM. On the other hand, aspect ratios of
the swimbladder are too small (<3) for the DCM, although
the swimbladder tilt angles were suitable (>20 ) (Table 2).
Consequently, the PSM is preferred for D. theta, and
TS ¼ 11:8 log SL 63:5 (Equation (6)) is recommended
for the TSlength dependence of this species.
From cage experiments at 25, 50, and 100 kHz,
Miyanohana et al. (1985) reported that the TS of Diaphus
sp. (40 mm SL) was about 60 dB at each frequency.
Hamano (1993) obtained the relationship, TS ¼ 17:4 log
L 69:6 (59.1 dB for 40 mm SL) at 88 kHz, for the
micronectonic swimbladder fish Maurolicus muelleri (Gonostomatidae) from a theoretical model calculation (Anderson, 1950; Love, 1977). Assuming an SL of 40 mm in
Equation (6), the TS estimate of D. theta found in this study
is higher (56.4 dB) than the cited reports.
A small, gas-filled organ has acoustical resonance at low
frequency, ka 1, where k is the wavenumber and a is the
equivalent spherical radius of the swimbladder (Love,
1978). This effect might be relevant, because the swimbladders of our D. theta were small. According to Furusawa
(1989), resonance frequency is about 10 kHz at 50 m depth
and 20 kHz at 250 m when a is 1 mm. In our samples, the
smallest a was 1.1 mm (29.4 mm SL). Resonance is therefore unimportant at 38 kHz, but it could be important for
a combination of lower frequencies and fish in deeper water
or either of these situations alone.
These results provide fundamental information on the TS
of mesopelagic lanternfishes, but this study is only a first
step. Further work is required, for example, comparing
measurements with theoretical predictions to improve our
understanding of the acoustic-scattering properties of these
species.
Target strength of mesopelagic lanternfishes
Acknowledgements
We thank the captains and crew of the RV ‘‘Kaiyo Maru-3’’
and the RV ‘‘Tansei Maru’’ and the scientists of the Ocean
Research Institute, University of Tokyo, for their cooperation and advice. We also thank the scientists of National
Research Institute of Fisheries Engineering, Japan, for their
help in various ways.
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