© 2005 International Council for the Exploration of the Sea
The influence of tilt angle on the acoustic target strength of the Japanese common squid (Todarodes pacificus)
a Marine Living Resources Research Division, Korea Ocean Research & Development Institute Ansan PO Box 29, Seoul, 425-600, Korea
b Hokkaido University, Graduate School of Fisheries Sciences 3-1-1 Minato-cho, Hakodate, Hokkaido 041-8611, Japan
c Yosu National University, Department of Fisheries & Technology Yosu 550-749, Korea
*Correspondence to D. H. Kang: tel: +82 31 400 6233; fax: +82 31 406 2882. e-mail: dhkang{at}kordi.re.kr.
To measure the influence of changes in tilt angle on the acoustic target strength (TS) of the Japanese common squid (Todarodes pacificus), we conducted a series of experiments to estimate TS in relation to tilt angle and swimming angle. Swimming angle was measured in a seawater tank using two infrared, underwater cameras under dark conditions. Ex situ measurements of TS in relation to tilt angle on live specimens using a fishhook and cage method were then conducted at 38 and 120 kHz; mantle length (ML) ranged from 21 to 27 cm (mean 24.75 cm). For the more precise TS measurement with tilt angle, another set of ex situ TS measurements relative to tilt angle was made at 38 and 120 kHz on tethered, anesthetized specimens in seawater. The mean swimming angle was –17.7° (±12.7° s.d.). The mean TS varied from –48.6 to –44.6 dB and was relatively higher at 120 kHz than at 38 kHz, in the order of 0.7 and 2.5 dB. The empirical relationship between TS (dB) and ML (cm) is given by TS = 20 log10(ML) – 75.4 (r = 0.81) at 38 kHz or TS = 20 log10(ML) – 73.5 (r = 0.64) at 120 kHz. Based on the tethered method for the anesthetized squid, the mean standardized TS values (b20) were found to be highly correlated with the tilt angle, and the resultant fitted equations for b20 were expressed as: b20 = –73.3 + 0.48 x 
+ 0.0122 x 2 + 0.00016 x 3 for 38 kHz and b20 = –72.6 + 0.53 x + 0.0134 x 2 + 0.00014 x 3 for 120 kHz, where is the negative tilt angle in degrees. The mean TS based on the measurements using live squid was higher than that of tethered measurements, i.e., 2.6 dB at 38 kHz and 4.0 dB at 120 kHz. The higher mean TS in the ex situ measurements for the live squid can be explained by the influence of the low tilt angle on the overall TS data. The results can be used to understand the influence of tilt angle on the TS of Todarodes pacificus and thus improve the accuracy of biomass estimates.
Keywords: Japanese common squid (Todarodes pacificus), swimming angle, target strength, tilt angle
Received 21 June 2004; accepted 10 January 2005.
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Introduction |
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 Top Introduction Material and methodsResultsDiscussionReferences |
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In acoustic-biomass estimates, the target strength (TS) of the surveyed fish or zooplankton is an important parameter that is needed to estimate stock abundance (MacLennan and Simmonds, 1992). In many cases, biomass estimates using acoustic-survey data incorporate mean TS equations that typically include only fish length (Foote and Traynor, 1988) as an independent variable. However, the variability in measured individual TS depends upon biological and physical factors such as tilt angle (Jørgensen and Olsen, 2002; McQuinn and Winger, 2003), length (Gauthier and Rose, 2001; Kang and Hwang, 2003), morphology (Clay and Horne, 1994; Jech et al., 1995), ontogeny (Horne, 2003), physiology (Ona, 1990), depth (Mukai and Iida, 1996; Francis and Foote, 2003), and carrier frequency (Foote, 1985). Among these factors, the tilt angle of the fish or zooplankton is the most important factor that influences variability in TS (Hazen and Horne, 2003).
Even though TS measurements have been conducted extensively for commercially important pelagic fish and zooplankton, some species, including cephalopods such as squid, have not been investigated thoroughly to date. In fish the swimbladder contributes 90–95% or more to the acoustic scatter of TS (Foote, 1980); however, it is very difficult to measure TS in cephalopods because they have no swimbladder and are composed entirely of soft tissue (Mukai et al., 2000).
Japanese common squid (Todarodes pacificus Steenstrup) is a commercially important species in the northwestern Pacific Ocean (Japan, Korea, Taiwan, and China) as a fisheries resource and on ecological grounds, and is distributed around the Kamchatka Peninsula of Russia to Taiwan. The annual catch of this species has fluctuated greatly since 1960 and increased markedly during the early 1990s (Sakurai et al., 2000). Although this squid is a very important species, methods to estimate its biomass have depended upon direct catch data throughout the fishing season. As this species has a short life cycle, recruitment is highly variable, and it is difficult to assess stocks only from direct catch data within a vast area. Hydroacoustic surveys, with their inherent advantages that include rapidity and repetition, may be a good alternative to catch data. However, a precondition for the stock assessment or ecological investigation of the Japanese common squid using this approach is for the TS of squid to be well verified in general and, in particular, the mean TS function to be based on the tilt-angle properties.
TS experiments on the Japanese common squid have been conducted under ex situ conditions. Arnaya et al. (1988, 1989a) made such measurements under freshwater conditions on dead and tethered squid and derived an equation for mantle-length (ML) dependency at 50 and 200 kHz. Using a single-beam transducer, Arnaya et al. (1989b) and Mukai et al. (2000) measured the TS relationship in various groups of live squid enclosed in a net cage and derived the mean dorsal-aspect TS (28.5 and 96.2 kHz) as well as an equation for mantle length. Additionally, measurements of the TS of stunned squid were conducted assuming a tilt angle of 0° using a 38-kHz split-beam sensor (Kawabata, 2001).
Because of the soft tissue and hollow mantle of the squid, it is difficult to conduct TS measurements on live individuals; indeed it is possible to increase measurement error in TS experiments because of absorbed air bubbles when the animals are exposed to air. Furthermore, TS estimates based on live squid enclosed in a net cage may be relatively poor as a result of the arbitrary tilt angle of the grouped squid. Given that each measurement used in previous experiments on squid TS has had one or other of these inherent problems, this exercise aimed at obtaining higher quality TS data. The goals of this paper were in fact, twofold: (i) to provide information on the swimming angle of Japanese common squid, and (ii) to report TS information on live and anesthetized squid in seawater, especially in relation to tilt angle, for application to biomass estimates in acoustic surveys.
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Material and methods |
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Three separate sets of measurements were made under different experimental conditions. The first considered the swimming angle of free-swimming Japanese common squid in an oval tank. The second focused on the relationship between target strength and tilt angle of live squid. The third involved anesthetized squid in seawater, to obtain more precisely target strength relative to tilt angle. The mantle-length (ML) range and number of squid used in each case are described in Table 1.
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Measurement of swimming angle
Swimming-angle data on free-swimming squid were collected from 12 to 14 July 2003 at the Usujiri Fisheries Station of Hokkaido University, Japan. The length, breadth, and depth of the oval seawater tank were 5.5, 2.5, and 1.1 m, respectively. The volume and recycling period of the filtered seawater in the tank were about 12 m3 and 1 cycle h–1, respectively. The seawater temperature was maintained at 12°C throughout the experiment using a cooler. Squid were captured by set-net near the experimental site and carefully transported in oxygenated water to the experimental tank. Eighteen live individuals, ranging from 14.8 to 19.3 cm in ML (mean, 16.45 cm) and judged to be in good condition, were selected for the experiments.
Because squid are attracted to visible light, only a red lamp (30-W incandescent bulb), installed on the side of the tank, was used to minimize stimulation of the experimental animals by visible light and thus to avoid any unnatural behaviour on their part. Light intensity (Lux) around the tank was measured with an illumination meter and ranged between 1.3 and 40 Lux. Because the light intensity of the lamp was not sufficient to clearly record squid behaviour with a normal underwater camera, an infrared version was used to measure the swimming angle. Two perpendicularly-orientated infrared cameras were installed at a depth of 0.7 m to allow continuous monitoring of the animals. Data from these two video cameras were stored separately on VHS videotape and a digital hard disk. The total recording time was approximately 27 h (13:30–20:00 on 12 July, 12:10–08:40 on 13–14 July), including videotape-replacement time.
From among all the video data, the captured-image data that were evaluated visually as being in a plane perpendicular to the optical axis (i.e. both fins overlapping) were selected for tilt-angle measurements (Figure 1). Video images were then "frame-grabbed" in Joint Photographic coding Experts Group (JPEG) format. All JPEG files of the images from the two cameras were imported into a tilt-angle measurement interface, which was designed to determine tilt angle from two separate points (head and fin) in the images. Tilt angle was defined as negative when the head (or tentacle) of a squid was at a lower position with respect to the horizontal axis, and the inverse was true for positive angles.
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Set-up of the acoustic system in the artificial-seawater tank
The TS measurements of the tilt angle of the live and anesthetized squid were carried out in a tank (5 m x 5 m x 5 m) filled with filtered seawater at the aquaculture centre of Yosu National University, Korea. Acoustic data were collected with a SIMRAD EK500 echosounder connected to both 38 kHz (ES 38B) and 120 kHz (ES 120-7) split-beam transducers. The two transducers were set to 30-cm depth under the surface. The transducers were calibrated with a standard-sphere target (Foote, 1987; SIMRAD, 1997) before and after measurement. Considering the shallow depth and narrow range, the medium pulse length was set to 1.0 ms for 38 kHz and 0.3 ms for 120 kHz. Each measurement continued for between 1 and 2 h, depending on the number of the detected samples. In all experiments, single-detection data (TS) were saved at ping intervals of 1.0 s.
Given both the shallow water depth (5 m) and the depth of the squid (3.5 m) from the acoustic transducers, we checked whether the depth could be satisfied with the acoustic far-field range (Rc), especially for 38 kHz. According to the transducer specification, the diameter of the split-beam transducer is 450 mm (SIMRAD, Norway, see http://www.simrad.com). The actual diameter in this instance was greater because 88 piezo-ceramic elements and an outer protective casing for them had been added. Hence, the actual diameter was not the "active diameter" for transmitting (Nielsen and Lundgren, 1999). For indirectly calculating the active diameter, we used two parameters of the sensor: the directivity index and the equivalent two-way beam angle (Urick, 1983). According to the manufacturer's brochure, directivity index (DI) is 28 ± 1 dB, and equivalent two-way angle (
) is –20.5 ± 1 dB. From DI and sound speed (1500 m s–1), the diameter of the transducer ranged from 28.2 to 35.4 cm. On the other hand, the diameter from ranged from 28.8 to 36.3 cm. From the equation for the far-field range using these values the maximum Rc was 261.4 cm (Urick, 1983), so the distance from the transducer face to the squid, 350 cm, met the far-field condition. The calibration exercise of the transducer carried out both before and after the TS measurement showed no obvious problems with variability in theoretical TS using the calibration sphere.
TS and tilt-angle measurement on live squid
The ex situ TS measurements with tilt angle for live squid were conducted in December 2002 and involved both individual and randomly distributed squid, respectively. Eleven specimens were captured off the coast of southern Korea using squid jigs and were carefully transported to the experimental site in oxygenated water.
Limited TS data can be obtained from individual squid because they have the ability to swim in any direction and can thus be out of beam width. Therefore, the measurements were made using two different methods:
- first, in order not to affect the TS measurement, a very small hook and thin fishing line was attached to the end of the fin of the individual live squid and this, in turn, was connected to a 0.4-m line offset laterally from a vertical line at a depth of 3.5 m. The vertical line was tied to a weight, and the distance between the connecting line and the weight was 1.1 m, which was sufficient to separate the echo of the squid from those of the weight and bottom. Three squid of different sizes (ML: 22.8, 25, and 27 cm) were used for this experiment.
- second, eight free-swimming squid were studied inside a cage (2 m in diameter x 5 m in depth), that was placed in the same tank used for the measurement of the individual squid. The ML of these animals ranged from 21 to 27 cm (mean, 24.20 cm), and their wet weight from 221 to 515 g (mean, 380.6 g). All squid used for these measurements were not exposed to air during the sampling.
Simultaneously with the acoustic measurements, three underwater video cameras were used to continuously monitor squid behaviour. Two perpendicularly-orientated, side-view cameras with complete remote control of zoom and focus were used to monitor squid orientation, and an upward-view camera was used to check squid position on the acoustic axis. The cameras did not produce any interfering echoes. Data from these three video cameras were stored separately on two VHS videotapes and a digital hard disk. Analogue data on the VHS tapes were transformed to digital files (AVI format) using a specialized A/D image board. The digital file was loaded onto a programme that was designed to capture image data, together with TS data and measurement environment (Figure 2).
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After synchronization between the TS data and the digital video data, all data were played at the same time with a 1.0-s ping interval. When a squid was visually evaluated as being in a plane perpendicular to the optical axis, corresponding TS data were available. To determine tilt angle, the pointed end of a fin was defined as the first reference position (x1, y1) and the pointed end of the head was at position (x2, y2). The tilt angle (
) was simply calculated from the relationship: = tan–1[(y2 – y1)/(x2 – x1)], which was positive in the head-up (fin-down) position with respect to the horizontal axis, and negative in the head-down (fin-up) position.
The mean TS from all selected TS data was calculated from the backscattering cross-section (
bs, m2) of the squid before logarithmic transformation.
TS and tilt-angle measurement on anesthetized squid
In addition to determining the TS and tilt angle of live squid, additional measurements using anesthetized squid were carried out in December 2003 to clarify more precisely the TS pattern relative to tilt angle. Live squid were anesthetized in seawater (FA100, 4-allyl-2-methoxyphenol), without exposure to air. Then they were tethered within the seawater tank (Figure 3). Squid used for this measurement were adult size and ranged between 24.5 and 25.5 cm in ML. Very small hooks were attached to the head and fin, and thin horizontal lines positioned the squid between the vertical suspension lines. By pulling or unfastening any vertical line, different tilt angles were created. Because the configuration was controlled by a hand-operated system, it was impossible to make changes on a fine scale. Therefore, the vertical line was pulled or unfastened at equal intervals, and the tilt angles ranged from –50° to 50° approximately. After being correlated with a specific tilt angle of a squid, the TS data were collected at two frequencies and the tilt angle recorded from a perpendicular, side-view camera.
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Results |
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Swimming-angle measurements
All the video data collected during a 27-h period were separated into intervals of 2.5–4.5 h to create observation data sets comprising 550 frame-grabbed images. For each of these, the mean swimming angles and standard deviations are summarized in Table 2. From the first to the sixth observation, the mean angles ranged from –16.5° to –18.4°; the mean angle for the seventh observation was –21.1°. Because there was no difference in mean angle from the first to the sixth observation (t-test, p < 0.05), these data were pooled. The overall mean swimming angle and standard deviation were –17.7 ± 12.7°. Figure 4 shows that overall the swimming angles tended to be negative, i.e. squid were in a head-down position for approximately 88% of the observations.
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TS in relation to tilt angle of live squid
Among the recorded data, some TS data were derived from unnatural behaviour, e.g. when squid were positioned at an unnaturally steep tilt angle. These data were excluded from the comparison between the TS and continuous video data based on the condition that tilt angle was between ±50°. Then, the normal tilt angle was estimated from Figure 4. The relationship between TS, ML (cm), and wet weight (g) of the Japanese common squid was established at 38 and 120 kHz. The results are summarized in Table 3. The mean TS varied from –48.6 to –45.1 dB and was relatively higher at 120 kHz than at 38 kHz, with a difference between 0.7 and 2.5 dB. The empirical relationships between TS (dB) and ML (cm) are given as:
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| (1) |
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Using the mean ML of 24.75 cm, the mean TS was –47.5 dB at 38 kHz and –45.6 dB at 120 kHz. The difference between the mean standardized TS values (b20) was about 1.9 dB. Figure 5 shows the relationship between TS and tilt angle based on frequency (38 and 120 kHz) and ML. The TS data corresponding to the tilt angle were extracted from the synchronization of all the detected TS data and the continuous video data. The maximum TS without respect to the ML and frequency appeared around a tilt angle of 0°. Because the number of TS measurements made at positive tilt angles was relatively low, our ability to express the entire trend of TS in relation to tilt angle is limited. However, there was a significant change in TS distribution with tilt angle in that, overall, TS showed a tendency to decrease with increasing tilt angle. The shaded area in Figure 5 represents the range (mean ± s.d.; –17.7 ± 12.7°) of the swimming angles that were calculated from the observation data in Figure 4. The mean TS at each ML (22.8, 25, and 27 cm) was –48.6, –48.2, and –45.9 dB at 38 kHz and –46.3, –46.5, and –44.6 dB at 120 kHz.
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TS in relation to tilt angle in anesthetized squid
As shown in Figure 5, TS varied with tilt angle of the live squid, although there was a general trend in tilt angle. Therefore, accurate measurements of squid TS at various tilt angles need to be made under the precisely controlled conditions of the tethered method.
After an anesthetized squid was suspended at the lowest negative tilt angle, approximately –50°, its posture was changed from negative to positive in stages. At the same time, TS data at each tilt angle were continuously recorded. Figure 6 shows the TS variation at different tilt angles and represents the apparent TS distribution relative to tilt angle. The maximum TS at a 0° tilt angle is in good agreement with that calculated for a similar mantle length (ML = 25 cm) in Figure 5, approximately –43.5 dB.
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Figure 7 shows the relationship between the TS and tilt angle of the controlled squid. Even though the TS at negative and positive tilt angles did not show an exactly symmetrical distribution, the results indicate a general trend of squid TS relative to tilt angle. The maximum TS had a 0° tilt angle for both frequencies. Considering that the TS pattern at a negative tilt angle reflected the general swimming angle of the squid (see Figure 4), the TS has a tendency to decrease rapidly between 0° and –10°, by approximately 3–8 dB. However, between –10° and –30° the TS decreased gradually from –49 to –54 dB. No frequency dependence at a given tilt angle was apparent.
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In Equations (1) and (2), b20 was derived using the ML of the squid, without considering the specific tilt angle. However, because the values were calculated from the mean TS at both frequencies, the mean tilt angle was being inherently taken into account. The ML values of all the squid used for the measurements were within a similar range, so the observed TS data obtained from a negative tilt angle were pooled for each frequency. As this study focused on the TS and tilt angle of the squid, we empirically fitted the best regression to determine the influence of tilt angle on b20 (Figure 8). The resultant fitted equations for b20 were expressed as follows:
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is a negative tilt angle in degrees.
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To compare tilt-angle effects on the mean TS between ex situ live squid and anesthetized squid, b20 in Equations (1) and (2) was replaced with b20 derived from Equations (3) and (4). For the mean TS, we made two reasonable assumptions; the first was that b20 values showed a symmetrical distribution with reference to 0°, and the other that mean tilt angle of the squid was –17.7° ± 12.7°. After calculating TS between –50° and +50°, the mean TS was averaged for a normal distribution of tilt angle and standard deviation, based on a method described by Foote (1980). The mean ML in the anesthetized squid was 25.1 cm, and the mean TS values were –50.1 dB at 38 kHz and –49.6 dB at 120 kHz, respectively. The mean TS values in the live squid (mean ML = 24.75 cm) were higher than those in the anesthetized squid: 2.6 dB at 38 kHz and 4.0 dB at 120 kHz. The higher mean TS from the live squid may be explained by the influence of the low tilt angle on the overall TS data during the measurements.
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Discussion |
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TopIntroductionMaterial and methodsResultsDiscussionReferences |
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In evaluating the effects of biological factors on the target strength of marine organisms, tilt angle is the most important factor influencing TS, compared with other factors such as frequency, length, and depth (Hazen and Horne, 2003). The tilt-angle effect on TS also applies to squid, which have no swimbladder. If squid are distributed at mid-depths during the daytime and near the surface at night, then the mean TS will be affected mainly by the horizontal tilt angle or hovering angle; whereas before and after sunrise and sunset, TS will be influenced by a higher tilt angle. To minimize error in converting volume-backscattering strength into squid biomass, the influence of tilt angle on squid TS must be verified.
In the ocean, squid adopt various tilt angles, especially while capturing prey, avoiding external stimuli, and maintaining position within currents. Furthermore, given the response of squid to external light, it is difficult to measure normal tilt angle using equipment such as remotely controlled vehicles (ROV) and underwater cameras with attached light systems. As a proposed method, average tilt angle can be introduced in the processing of acoustic data obtained from mid-depths during the daytime and near the surface at night. The average tilt angle can be expressed as a normal distribution with a standard deviation. If squid change position to capture prey, tilt angle is decreased, while the tilt angle employed to maintain a neutral position against currents is increased and may be related to the hovering angle under natural conditions.
For Japanese common squid, the mean tilt angle has been reported as –4.0 ± 11° (Arnaya et al., 1989a) and about –25° to –30° (Mukai and Iida, 2002). These tilt angles were measured in an aquarium and were based on hooked squid under daylight conditions. Our results were intermediate between those two results. Unlike these two studies, however, we attempted to darken the conditions and to protect squid from external stimuli. We did not feed the animals, and thus their tilt angle should have been close to the hovering angle. With the assumption that the mean tilt angle in this study was similar to the hovering angle, the mean TS was calculated from b20 with a normally distributed tilt angle. The mean TS calculated using the mean tilt angle reported by Arnaya et al. (1989a) was –48.2 dB at 38 kHz and –47.8 dB at 120 kHz. The difference in the mean TS for both mean tilt angles was about 2 dB. Based on these results, the measurement of the appropriate tilt angle for the mean TS should be examined in situ.
In situ measurements of squid TS have been reported for Loligo opalescens (Jefferts et al., 1987), Loligo edulis (Lee et al., 1992), and Todarodes pacificus (Kawabata, 1999). Unfortunately, these measurements have the inherent problem of identifying squid from overall acoustic data and are limited in their ability to verify the influence of tilt angle on individual TS. The tethering method using dead specimens and the ex situ measurements using live samples were used to overcome these problems.
For Todarodes pacificus, ex situ measurements using live specimens in small cages were conducted by Arnaya et al. (1989b) and Mukai et al. (2000). In measurements using multiple squid of several size classes in a small cage, the mean TS was relatively low compared with those of individual squid obtained from our measurements. A possible reason is that squid in a cage change their tilt angles, and the mean TS was calculated by dividing the echo energy by the number of squid being sampled. This method is limited in its ability to explain the influence of tilt angle on squid TS. Arnaya et al. (1988) and Kawabata (2001) conducted tethered TS measurements using dead squids. Then, Arnaya et al. (1988) reported that squid TS at a 0° tilt angle was –51 dB to –36 dB at 50 kHz and –43 dB to –29 dB at 200 kHz in animals that ranged from 14.8 to 29.2 cm ML. In the meantime, Kawabata (2001) reported mean TS values of –47.9 dB to –44.7 dB at 38 kHz (split-beam type) and a b20 of –73.7 dB in animals with ML from 20.4 to 28.4 cm. Thus, previous mean TS values are about 1–2 dB higher than those determined in our experiments. This difference could be a result of the measuring methods used; Kawabata (2001) considered only TS data at a 0° tilt angle, whereas we considered TS data at various tilt angles. Because these two studies considered only a 0° dorsal-aspect angle, the effects of the tilt angle on TS could not be explained in more detail. Given the difference in TS estimates using only a 0° tilt angle and those using various tilt angles, a more reasonable mean TS must be estimated using various tilt angles.
Although a few studies have been carried out on squid TS in relation to tilt angle, TS measurements at various tilt angles have not previously been conducted precisely. In this study, we focused on the relationship between TS and tilt angle for both free-swimming and anesthetized Japanese common squid. Unlike previous studies, we used an acoustic system operating at 38 kHz and 120 kHz for the measurements. These are the preferred frequencies in several acoustic surveys, allowing the present squid TS data with tilt angles to be applied to interpret in situ TS data for squid-biomass estimations. Although this study provided limited TS information based on only a few squid, the results suggest that TS varies largely with tilt angle and that the mean TS must be calculated based on a consideration of squid behaviour to get reasonable results. Further investigations, including additional tilt-angle observations, use of different size classes of squid, and physiological properties, are necessary to understand the variable nature of TS.
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Acknowledgements |
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This study was mainly supported by the programme of "The Japan Society for the Promotion of Science (JSPS) postdoctoral fellowship for foreign researchers". Some funding came from the "Core University Programme on Fisheries Sciences between Japan and Korea" and "the Ministry of Maritime Affairs and Fisheries of Korea through the Marine Ranching Programme in Korea". We are grateful to Mr Sadayasu for his invaluable help in tethered measurement and to Professor Sakurai for permitting the use of the seawater tank. We are also grateful for the constructive comments of an earlier draft of the manuscript by anonymous reviewers.
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References |
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TopIntroductionMaterial and methodsResultsDiscussionReferences |
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Arnaya I.N., Sano N., Iida K. (1988) Studies on acoustic target strength of squid. I. Intensity and energy target strength. Bulletin of the Faculty of Fisheries Hokkaido University 39:187–200.
Arnaya I.N., Sano N., Iida K. (1989) Studies on acoustic target strength of squid. II. Effect of behaviour on averaged dorsal-aspect target strength. Bulletin of the Faculty of Fisheries Hokkaido University 40:83–99.
Arnaya I.N., Sano N., Iida K. (1989) Studies on acoustic target strength of squid. III. Measurement of the mean target strength of small, live squid. Bulletin of the Faculty of Fisheries Hokkaido University 40:2100–115.
Clay C.S. and Horne J.K. (1994) Acoustic models of fish: the Atlantic cod (Gadus morhua). Journal of the Acoustical Society of America 96:1661–1668.[CrossRef][Web of Science]
Foote K.G. (1980) Averaging of fish target-strength functions. Journal of the Acoustical Society of America 67:504–515.[CrossRef][Web of Science]
Foote K.G. (1985) Rather-high-frequency sound scattering by swimbladdered fish. Journal of the Acoustical Society of America 78:688–700.[CrossRef][Web of Science]
Foote K.G. (1987) Fish target strengths for use in echo-integrator surveys. Journal of the Acoustical Society of America 82:981–987.[CrossRef][Web of Science]
Foote K.G. and Traynor J.J. (1988) Comparison of walleye pollock target-strength estimates determined from in situ measurements and calculations based on swimbladder form. Journal of the Acoustical Society of America 83:9–17.[CrossRef][Web of Science]
Francis D. and Foote K.G. (2003) Depth-dependent target strengths of gadoids by the boundary-element method. Journal of the Acoustical Society of America 114:3136–3146.[CrossRef][Web of Science][Medline]
Gauthier S. and Rose G.A. (2001) Target strength of encaged Atlantic redfish (Sebastes spp.). ICES Journal of Marine Science 58:562–568.
Hazen E.L. and Horne J.K. (2003) A method for evaluating the effects of biological factors on fish target strength. ICES Journal of Marine Science 60:555–562.
Horne H.J. (2003) The influence of ontogeny, physiology, and behaviour on the target strength of walleye pollock (Theragra chalcogramma). ICES Journal of Marine Science 60:1063–1074.
Jech J.M., Schael D.M., Clay C.S. (1995) Application of three sound-scattering models to threadfin shad (Dorosoma petenense). Journal of the Acoustical Society of America 98:2262–2269.[CrossRef][Web of Science]
Jefferts K., Burczynski J., Pearcy W.G. (1987) Acoustical assessment of squid (Loligo opalescens) off the central Oregon coast. Canadian Journal of Fisheries and Aquatic Sciences 44:1261–1267.
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.
Kang D. and Hwang D. (2003) Ex situ target strength of rockfish (Sebastes schlegeli) and red seabream (Pagrus major) in the Northwest Pacific. ICES Journal of Marine Science 60:538–543.
Kawabata A. (1999) Measurement of the target strength of live squid, Todarodes pacificus Steenstrup. Bulletin of the Tohoku National Fisheries Research Institute 61:29–40.
Kawabata A. (2001) Measurement of the target strength of live squid, Todarodes pacificus Steenstrup, in controlled body tilt angle. Bulletin of the Tohoku National Fisheries Research Institute 64:61–67.
Lee K.T., Liao C.H., Shih W.H., Chyn S.S. (1992) Application of dual-beam, acoustic-survey techniques to assess the size distribution of squid, Loligo edulis. Journal of the Fisheries Society of Taiwan 19:25–34.
MacLennan D.N. and Simmonds E.J. (1992) Fisheries Acoustics(Chapman and Hall, London) 325 pp.
McQuinn I.H. and Winger P.D. (2003) Tilt angle and target strength: target tracking of Atlantic cod (Gadus morhua) during trawling. ICES Journal of Marine Science 60:575–583.
Mukai T. and Iida K. (1996) Depth dependence of target strength of live kokanee salmon in accordance with Boyle's law. ICES Journal of Marine Science 53:245–248.
Mukai T. and Iida K. (2002) Simultaneous measurement of swimming angle and target strength of live squid (Todarodes pacificus) using synchronized video images and audio signals. Proceedings of meeting of the Marine Acoustics Society of Japan pp. 29–32.
Mukai T., Iida K., Sakaguchi K., Abe K. (2000) Estimations of squid target strength using a small cage and theoretical scattering models. The Proceedings of the JSPS-DGHE International Symposium on Fisheries Science in Tropical Area pp. 135–140.
Nielsen J.R. and Lundgren B. (1999) Hydroacoustic ex situ target-strength measurements on juvenile cod (Gadus morhua L.). ICES Journal of Marine Science 56:627–639.
Ona E. (1990) Physiological factors causing natural variations in acoustic target strength of fish. Journal of the Marine Biological Association of the United Kingdom 70:107–127.[Web of Science]
Sakurai Y., Kiyofuji H., Saitoh S., Goto T., Hiyama Y. (2000) Changes in inferred spawning areas of Todarodes pacificus (Cephalopoda: Ommastrephidae) due to changing environmental conditions. ICES Journal of Marine Science 57:24–30.
SIMRAD. (1997) SIMRAD EK500 Scientific Echo Sounder. Operator Manual, P.2170.
Urick R.J. (1983) Principles of Underwater Sound(McGraw Hill, New York).
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