Masahiko Furusawa

(Tokyo University of Fisheries, 4-5-7 Kounan, Shinagawa Tokyo, 108-8477 Japan)

Quantitative echo sounders (QES) are widely and effectively used for the purposes of fisheries resources surveys, ecological studies, and selective fishing. Some up-to-date and sophisticated QESs, such as SIMRAD EK500 and Kaijo KFC 3000, have been developed. Their prominent functions are split-beam method, multiple frequency observation, small cell echo integration, and accompanying post processing soft-wares.

There are, however, still inherent problems which seem not easy to resolve: dead zones near sea surface and near bottom, fish avoidance to approaching vessel, low sampling density due to sharp vertical beam, difficulty in species identification, difficulty in target strength (TS) measurement for relatively dense school, TS variation due to swim-bladder and tilt variation, and so on.

In this presentation some methods or techniques, having been tried by present author and his colleagues, which will resolve these problems in some extent are introduced. Details on some topics are seen in References. Some of these are almost developed, but others are under test or will be possible in near future.

Followings list up the problems (upper rows) and possible countermeasures (lower rows) of which underlined ones are referred to in this presentation.

• Dead zones ( near surface, near bottom )

horizontal sonar, upward transducer, good observation base (towed body, ROV, AUV)

• Fish avoidance to vessel

silenter ship, dual beam echo integration, horizontal sonar

• Low sampling density

sonar, low frequency, ship of opportunity

• Species identification

multi frequencies, school feature extraction, behavior observation, acoustically derived quantities (TS, size, volume backscattering strength, density)

• TS measurement for relatively dense school

multiple echo indexes, high resolution, body motion Doppler, good observation base

• Swim-bladder and tilt variation

in situ bladder and tilt observation, echo trace analysis

Sonars have some merits over echo sounders using vertical beam; they can observe shallow schools, broad area, and movement of fish. There have been several effort to utilize fisheries purpose sonars for scientific researches [eg 1], but such sonars have some restrictions for quantitative use because of their original purpose, fisheries. Difficulties to be resolved for the sonar are: TS variation by fish aspect, reverberations by surface and bottom, reverberation by wakes, refraction of sound ray, quantification of transmitting and receiving process, calibration, beam direction change by ship motion, difficulty to make reasonable display, and reasonable range compensation (TVG), and so on.

Despite these difficulties, we should develop a quantitative sonar, expecting to realize the above merits. As the first step to making up a genuine quantitative sonar, we are trying to make a fisheries 360-degree scanning sonar quantitative. Sensitivity calibration could be successfully conducted; we searched the echo of a calibration sphere suspended from a buoy changing the tilt angle and measured the average echo level to give a transmitting and receiving sensitivities product. The echoes from fish are directly fed onto a computer. Compensation for range dependence by 20 log r law and coefficient manipulation by using the above calibration data will give absolute volume backscattering strength. Position adjustment of each PPI echogram using the information from GPS and gyro makes a composite display showing successive absolute movement of fish schools.

It is impossible and expensive to observe broad areas only by research vessels. Therefore, we should consider to utilize other vessels especially fishing vessels. The instrument or quantitative echo sounder for the purpose must be full-automatic, robust, and standardized. A concept of an automatic quantitative echo sounder was discussed in [2]. The main specifications are: high acoustic power (2.4kW), rather low frequency (38kHz), large pulse width (3ms), small bandwidth (1.5kHz), and standardized data format.

On the other hand, QES installed onboard research vessels should give more accurate and precise results. Most of today's quantitative echo sounders generate their outputs without any sign, even if they include errors inevitably introduced by, for example, fish avoidance to vessel noise, transducer motion, and single echo extraction error. The solution to make the sounders reliable should be found in two ways: to resolve errors themselves and to show error indexes along with the outputs. Several indexes are considered for the latter: number of fish in reverberation volume in in situ TS measurements [3], comparison of echo integration and echo counting outputs, comparison between narrow and wide beams for detection of avoidance and transducer motion error [4], and comparison between several frequencies [5]. The proposed concept of a self-checking QES is described in [6] and some functions have already been realized in our sounder.

Despite sophisticated instrumentation and processing, nowadays QES does not necessarily succeeded in the single echo measurement. The most important problem is that for relatively dense school it is not possible to extract single echoes. It is ordinarily impossible to see objects simultaneously broadly and in detail by one method or instrument. Therefore, sometimes separate methods are necessary. For example, the requirements for echo integration method and in situ TS measurements are different in some aspect, especially in resolution.

One challenge is to develop an extremely high resolution sounder to measure exclusively single echoes. The purposes are to expand range to observe single echoes and to get more detailed ecological information; they will serve for sizing and species identification. The important point will be how to realize high resolution (sharp beam and small pulse width) under transducer motion and under noisy condition. One candidate method to reduce the transducer motion error [7] is to send and receive pulse at slow speed phase of the swaying.

The single echo trace analysis [8] to measure behavior and in situ TS pattern is now practically usable under some good condition [9]. As Fig.1 shows, fish swimming vector and in situ TS pattern can be observed. Through the maximum TS value in the pattern, individual (not average) fish size can be derived. Adopting the above proposition of the extremely high resolution sounder or a stable observation base such as ROV, the single echo trace analysis will be used more extensively.

Acoustic fish species identification has been one of the most important efforts in the fisheries acoustics community. There should not be a decisive method and the key is to increase the number of useful information and to increase the accuracy and precision of each information. In this sense, the above single echo trace analysis method to give precise behavior and TS information will be a powerful method.

Observing frequency characteristics difference of scattering has also been a good method for species identification. Our recent study [5] introduces a more precise and effective approach for this type of method. The frequency comparison must be done in the same detection range [10] of frequency channels with the same beam width for small cell integration results or volume backscattering strength (SV). Figure 2 shows one of the results from our study showing the difference (120kHz SV - 38kHz SV) in small cell (0.1 nautical mile x 1m). Redder color shows mainly euphausiid and bluer walleye Pollock.

Other methods than our ordinary methods may be used for special purpose. Utilization of Doppler effect by a continuous wave or a long pulse may be used for dense school behavior or size observation. The earlier trials [eg 11] only showed the possibility, but under the present technical condition and the strong need for observation of dense school which could not be attained by the traditional pulse-echo method may afford us to built up a Doppler system. The Doppler effect can detect tail beat, moving speed and other movement of marine lives. Specifically the sizing method which can be used for dense schools may be possible using the relation between body length, swimming speed, and tail beat frequency. The problems to be solved are how to compensate for the transducer motion and what signal is the best among long pulse, continuous wave, CTFM, or several signals combined.

Observation technique of in situ bladder shape and orientation of fish are the most important factors for nowadays high precision measurements of fish abundance. It is, however, very difficult to observe them in situ especially the bladder because of its invisibility. Candidate techniques may be an underwater X-ray system and a high frequency ultrasonic system as used in medical diagnosis fields. It is necessary to overview present and potential techniques to realize the purpose.

[1] O. A.Misund, "Abundance estimation of fish schools based on a relationship between school area and school biomass," Aquat. Living Resour., 6, 235-241 (1993).

[2] M.Furusawa, "On acoustic techniques for census of fishes," Census of fish workshop (Scripps Institution of Oceanography, Oct 16-17, 1997) (mimeo).

[3] K.Sawada, M.Furusawa, and N.J.Williamson, "Conditions for the precise measurement of fish target strength in situ," J.Mar.Acoust.Soc.Jpn., 20(2), 73-79 (1993).

[4] Y.Takao and M.Furusawa, "Dual-beam echo integration method for precise acoustic surveys," ICES J.Marine Sci., 53(2), 351-358 (1996).

[5] M.Kang, M.Furusawa, K.Miyashita, "Accurate difference of mean volume backscattering strength for fish speices information," Proceedings of International Symposium on Advanced Techniques of Sampling Gear and Acoustical Surveys for Estimation of Fish Abundance and Behavior (2001) (submitted).

[6] M.Furusawa and T.Asami, "A proposal of a self-checking quantitative echo sounder," International Workshop on Acoustic Surveys of North Pacific Fisheries Resources (25 Oct., 1997, Pusan Korea), Paper No.A1(1997).

[7] M.Furusawa and K.Sawada, "Effects of transducer motion on quantifying single fish echoes," Nippon Suisan Gakkaishi, 57, 857-864 (1991).

[8] M.Furusawa and Y.Miyanohana, "Behaviour and target strength observation through echo trace of individual fish," Rapp.P.-v.Reun.Cons.int.Explor.Mer, 189, 283-294 (1990).

[9] K.Sawada, "Study on precise estimations of fish target strength," Tokyo University of Fisheries, Doctoral thesis (2000).

[10] M.Furusawa, T.Asami, and E.Hamada, "Detection range of echo sounder," Proc. 3rd JSPS Int. Seminar, 207-213 (1999).

[11] F.J.Hester, "Identification of biological sonar targets from body-motion Doppler shifts," In Marine Bio-Acoustics Vol.2, Pergamon Press (1967).

Fig.1 Single echo trace analysis. Fish swimming vector and in situ TS pattern are shown [9].

Fig.2 Frequency difference of volume backscattering strength (120kHz - 38kHz) is shown for small cell echo integration data [5].

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