研
究
Conditions for the precise measurement of fish target strength in situ *
Kouichi SAWADA, Masahiko FURUSAWA**
(National Research Institute of Fisheries Engineering)
Neal J. WILLIAMSON***
(Alaska Fisheries Science Center, National Marine
Fisheries Service, National Oceanic and Atmospheric
Administration,
U.S.A.)
(Received
October 7, 1992)
Conditions for precise measurement of in situ fish target strength (TS) are empirically studied and two indexes are introduced for this purpose. One is the number of fish in the effective reverberation volume which contributes echo tormation at a certain instant and the other is the percentage of the multiple echoes which is derived from a residual of the single echo extraction. With the decrease of both indexes measured target strength approach a cer-tain asymptotic value which is admitted as reliable from the past study. This shows the exist-ence of some threshold values and below these threshold values TS measurement will be successful. The effectiveness of both indexes is confirmed by the data set obtained from one large same fish school in the eastern shelf of Bering sea during the intership calibration be-tween Japanese and U.S. vessels on 15 and 16 August 1991.
1. Introduction
It is important to measure the in situ target strength (TS) of fish, because the TS is a scale factor to estimate fish abundance acoustically. Recently, the dual beam method or the split beam method has become popular in measuring in situ fish TS. We have been conducting acoustic surveys of walleye pollock (Theragra chalcogramma) in the Bering Sea since 1988 and have measured TS of the walleye pollock in the surveys using the dual beam method.
A simulation study1) and measurement results2) show that it is difficult to get precise TS data when the spatial density of fish is high or when a fish school is deep. Therefore, usually TS measurements have been conducted for low fish density or for shallower fish. But actual fish density varies widely and fish attitude changes with distribution density, so TS should be measured for each variable situation. Then the TS
measurement is expected to be conducted even in a high density condition and, consequently, an index which indicates the possibility of reliable TS measurements in several situations is needed.
Two indexes are introduced for this purpose; one is the number of fish in the effective reverberation volume and the other is the percentage of the multiple echoes. The threshold values of the two indexes are derived to get accurate TS data. By using these indexes, reliability of TS measurement will advance.
2. Theory
2.1 Fish length and average TS
Observed fish TS is considered to be a function of mainly fish length and orientation or tilt angle of fish, if the single echo isolation is done successfully. Assuming that the fish length and the tilt angle of fish are independent, observed average TS is shown as follows:
*自 然 状態 で の魚の ターゲ ッ トス トレ ングス測定 のため の条件 **澤 田浩一,古 澤 昌彦(水 産 庁水 産工学 研究 所)
(1)
where L is fish length, θ is tilt angle of fish, fa (θ) is probability
density function (PDF) of the tilt angle, and fb(L) is PDF of the fish length.
It is known that there is a relationship between the fish length and the TS averaged with respect to the tilt angle of fish as follows:
(2) where Tscm is the normalized TS with respect to fish length squared. From past studiee, TScm=10 log Tscm=-66.0 dB was obtained for the walleye pollock. (In this paper a decibel variable for a corresponding linear one is indicated by making the first two letters of the linear variable upper case as above.) The inner integration of Eq. (1) is this TS(AVG) of fish with a length L. Then Eq. (1) is reduced as follows by using Eq (2):
(3)
where L AvG is avemge fish length and σL is standard deviation
of fish length. Then we can calculate TScm from the average of measured TS and from the average fish length and the standard deviation obtained from, for example, trawl results. Assuming that the standard deviation of the fish length is small enough, Eq. (3) coincides formally with Eq. (2). In this situation the measured average TS is considered to be the same as the TS averaged with respect to the tilt angle of fish. 2.2 Indexes showing TS measurement condition
Single echo isolation is the first stage of TS measurement. The common method for isolating such echoes uses a measured width of the received echo pulse. If the measured pulse width is in the range of a pulse width criterion, the echo is considered to be a single echo. The window of pulse width should be chosen large enough to allow for the noise-induced bias and standard deviation. But a wide pulse window is less effective in removing multiple echoes.6) In the second stage, TS is calculated precisely by correcting for the directivity of the beam using the dual beam method or split beam method. The higher the noise and the fish density, the more difficult is single echo isolation.
The number of fish in the effective reveberation volume and the percentage of multiple echoes are introduced as indexes to judge a condition for estimating TS precisely.
(1) Number of fish in effective reverberation volume The echo from fish becomes so called multiple echo when the number of fish in the effective reverberation volume (Nv) for one ping increases (Fig. 1). Therefore, Nv is considered to show the probability of the occurrence of multiple echoes.
The number Nv is shown by
(4)
where c is the sound speed, T is the effective pulse width,￠is
the equivalent beam angle, r is the range and n is the distribution density of fish. The larger the range, density, pulse width and beam width, the larger the Nv becomes and the larger the possibility of multiple echoes.
To estimate the density of fish, echo integration results, that is average volume scattering strengths (S v) are used:
(5) Here S v values are obtained for the same analysis period and layer as TS analysis. TS is calculated by Eq. (3) using the fish length distribution derived from trawl data.
(2) Percentage of multiple echoes
If the single echo isolation procedure works well for the homogeneous fish distribution, the echo number (NE), which is discriminated as single echoes and counted, is proportional
to a cutoff solid angle(Ω)as
(6)
where ns is density of echoes discriminated as single, r1 and r2 are start and end depths of analysis respectively, and P is the number of pings. From the slope read from the graph showing the relationship between the cutoff angle and the echo number, the density of single echo, ns, is obtained.
Assuming that real fish density is n, the percentage of multiple echoes, M, can be defined as follows:
(7) If the distribution density of fish becomes high and the fish depth becomes deep, multiple echoes occur frequently and single echoes decrease in number. Then M value, which shows the difference between the real density and the single echo density, indicates the percentage of occurence of multiple echoes.
3. Method
The analysis flow is shown in Fig. 2. As shown in the flow, the following analysis uses data from three sources, that is the biological data by trawl, the echo integration data, and the data from the dual-beam processor (DBP).
The present analysis method is applied to the data obtained during our survey of walleye pollock (Theragra chalcogramma) in the Bering Sea in 1991 summer. The survey was conducted cooperatively by Japan and U.S. vessels. A fisheries vessel Shoyo-maru (50 m, 349 t) and Continuity (41 m) were chartered for the Japanese and U.S. survey, respectively.
The Japanese acoustic system is a versatile echo sounding system4) (VESS) which has functions of the echo integration and the dual beam TS measurement. The block diagram is shown in Fig. 3. A towed body with a 38 kHz transducer was used. Pulse width was 0.6 ms. Single echoes were isolated by a pulse width comparison method or a variable threshold method in which the echo pulse widths are compared with a given criterion at half and base echo levels. 5),6)
The U.S. system is SIMRAD EK-5007); which has functions of echo integration and the split beam TS measurement.
In order to know the deviation of measured TS caused purely by the multipleness of echoes, we need to select the same fish school which has different density parts. A data set obtained for a large school of walleye pollock during the operation of an intership calibration cooperatively conducted between two vessels in the eastern Bering sea shelf on 15 and 16 August fulfills the requirement to some extent and was analysed.
In the intership calibration two vessels closely tracked the lines (about 11 nmi long) aiming at a same fish school many times at relatively low speeds (about 4 - 6 kt) for one and a half days. It is assumed that the same fish school was observed on each line.
Prior to the intership calibration, a midwater-trawl was conducted by U.S. vessel on 14 August at 13:19 - 14:03 (Alaska daylight time). The mesh sizes of the net ranged from 10.2 cm forward to 8.9 cm in the codend and 3.2 cm in the codend liner. The vertical mouth opening of the trawl was 16 m and the average foot rope depth was 97 m. The sea bottom depth was 119 m at the trawl site. By the trawl, 255 walleye pollock and 3 squid were caught. Fig. 4 shows the fork length distribution. The average length was 18.3 cm and the standard deviation was 2.7 cm.
Several acoustic data sets obtained by Japanese system were selected from the view point of observing the differences of density and distribution pattern. Table 1 shows the summary of TS analysis. The selected positions for the analysis and the trawl position are shown in Fig. 5 by two circles exhibiting start and end points connected by a line. The number in Table 1 is indicated near the start position of TS analysis. The analysis layer was set from 72 m to 88 m from the transducer (the depth of the towed body was about 9 m) to select only the walleye pollock echoes and to avoid inclusion of large fish echoes near the sea bottom.
Nv values are calculated for this whole layer (72 m-88 m),
Fig. 2 Calculation flow of Nv and M.
and also for four 4 m thick layers in 72 m - 88 m in order to decrease the effect of biased distribution in the layer. The echo integration period was 1 min by time base. The fish density n is calculated from the average Sv corresponding to TS analysis time using Eq. (5).
Figure 6 shows an example (No. 15 of Table 1) of the relationship between the cutoff angle and the echo number. The echo number is proportional to the cutoff angle up to
about 3.4°(about 0.01 sr). The half beam width of the tran sducer is 3.2°.
From the study of the cutoff angle,1)TS estimation error is
minimized when the cutoff angle is a half beam width. Therefore 3.20 is chosen as the cutoff angle to calculate average TS. The slope of NOfS ) is calculately by the least mean square methOd for the data in the range of less than 3.2。 in this case.
The acoustic data collected by the Japanese system are mainly analysed, but for comparison purpose the data by U.S. system were also used. The pulse width was 0.6 ms, the equivalent beam angle was 0.00794 sr, the calculation layer was 50-100 m, and analysis period was 0.5 nmi.
Fig. 4 Fork length distribution obtained by trawl. Fig. 5 Selected positions for target strength analysis and trawl position.
4. Results and discussion
Table 1 also shows all the results of TS analysis made for Japanese data.
The fish distribution was scattered and separated from the sea bottom in nighttime (Fig. 7). With the day light, the fish schools became patchy as seen like vertical strips from the bottom (Fig. 8). Figure 7 corresponds to No. 7 of Table 1, and Fig. 8 corresponds to No. 11. These are the typical echograms observed at about the same position in the nighttime and
daytime by the color printer of VESS. As can be seen in Table 1, the ratios of the echo number to the ping number for the nighttime scattered schools are much larger than those for daytime patchy schools.
Figure 9 shows the relationship between Nv and measured average TS for all the results with the analysis layer of 72
m-88 m. In this figure standard deviations are also shown. From Fig. 9 the tendency is clearly observed that with the increase of Nv the average TS increases. On the other hand, as Nv Fig. 6 Relationship between cutoff angle and echo
number.
Fig. 7 Typical echogram observed in nighttime.
Fig. 8 Typical echogram observed in daytime.
decreases, the average TS approaches near -40.7 dB which is calculated by putting TScm=-66.0 dB, LAvG=18.3 cm, and
σL=2.7 cm in Eq.(3). This fact suppOrts the effectiveness of Nv as an index.
In Fig. 9 two data points (at Nv =0.06 and 0.17) are far deviated from other data. These data corresponds to No. 11 and No. 13 of Table 1 and both data were obtained for patchy schools. Nv value may be underestimated if there is a partial high dense portion within one analysis layer of integration. Therefore, the layers are divided into narrower (4 m) layers to realize the homogeneous density in the layers and result is shown in Fig. 10. The large white squares correspond to No. 11 and large black squares correspond to No. 13. However, even in Fig. 10 the measured average TS obtained for patchy schools in daytime are higher than the one for the scattered schools in nighttime. This might come from a difference of the fish attitude in daytime and in nighttime. Another plausible reason is that, although in this analysis an unique body length distribution (Fig. 4) is assumed, there might be some difference in the length distribution, because some TS analysis positions are a little far from the trawl station (see Fig. 5).
Figure 11 shows the relationship between M and the measured average TS. With the decrease of M, average TS approaches an asymptotic value as was the tendency for Nv. All data except one near M=65% show a smooth trend. This
exception corresponds to No. 12 of Table 1. This might be caused by the small number of echo data.
Figure 12 shows another relationship between Nv and measured TS obtained by using the U.S. system. The data were selected to correspond to the Japanese data (see Table 2). Some sounder and analysis parameters are different from Japanese ones. The trend of U.S. data is lower than Japanese one by about 2 dB, and this is considered to come from the difference in the isolation methods of single echoes between
Table2 Results of TS analysis and condition index (Nv) for U.S. data.
both systems. In EK-500 system phase information is used to discriminate single echoes from multiple echoes in addition to the pulse width Criterion. But the trend of Fig. 12 is similar to Fig. 9 and we can confirm the generality of the present method.
5. Conclusion
The number of fish in the effective reverberation volume, Nv, and the percentage of the multiple echoes, M, are effective indexes to know the limit of in situ TS measurments.
We could not show the conclusive threshold values for Nv and M, because the data number is small especially where Nv and M are small. But roughly speaking, Nv value of 0.04 and M value of 70% are thresholds to ensure accurate TS measurements. The present analysis includes several experimental data and some uncertainty is inevitable. Therefore, in future we need to make a computer simulation
and determine more clear threshold values.
Since Nv value does not depend on the single echo isolation procedure, it can be a standard index between different systems. On the other hand, M value does not depend directly on fish distribution, so it is a good index for data obtained by the same system. Since M depends strongly on the single echo isolation process of a measurement system, echo number (NE of Eq. (6)) may be different in different systems even for the same fish distribution. But if the single echo isolation technique greatly advance, M value can be used as a standard index between different systems.
Calculations of Nv and M value were done by the off-line process this time, but it will be possible to make them an on-line process. Reffering to these indexes and using real time TS values, more precise and accurate fish abundance will be estimated.
6. Acknowledgment
The authors would thank Y. Takao of National Research Institute of Fisheries Engineering and J. J. Traynor of Alaska Fisheries Science Center, NOAA, USA, for their useful advice. T. Fukaya, formerly a student of Sibaura Institute of Technology is thanked for his cooperation in analyses. References
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4) M. Furusawa and Y. Takao, K. Sawada, T. Oukubo, and K. Yamatani, "Versatile echo sounding system using dual beams," Nippon Suisan Gakkaishi (submitted) (1992). 5) K. Sawada, Y. Takao, Y. Miyanohana, M. Furusawa, and
K. Nemoto, "Improvements of the dual beam fish target strength measurement method," Proc. Meet. Marine acoust. Soc. Jpn., 47-50 (in Japanese) (1989.5). 6) J. E. Ehrenberg, "Analysis of split beam backscattering
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7) H. Bodholt, H. Nes, and H. Solli, "A new echo sounder system for fish abundance estimation and fishery research," Counc. Meet. Int. Counc. Explor. Sea 1988/ B:11, Copenhagen, Denmark.
Fig. 11 Relationship between M and average TS (72 m-88 m).
