Fast acoustic fluctuations caused by fish

Journal of Sound and Vibration (1975) 39(3), 287-292

FAST ACOUSTIC FLUCTUATIONS CAUSED BY FISH P. A. CHING AND D. E. WESTON

Admiralty Research Laboratory, Teddington, Atiddlesex, England (Receh'ed 14 June 1974) Acoustic fluctuations of sound have been studied in the shallow waters of the Bristol Channel for a number ofyears and several different types have been identified. One of these, occurring in the summer day-time only, is a fluctuation of about I0 dB with a period of about 80 seconds. It has been observed at transmission frequencies between 312 Hz and 4.44 kHz but was most frequently recorded between I and 2 kHz. Because of its timing the phenomenon is attributed to fish, and, although the mechanism has not been positively identified, it is though.t to involve fish clustering round the hydrophone.

1. INTRODUCTION One of the difficulties in making acoustic measurements underwater is that the results may be confused due to the activities of fish; perhaps parvenu man.should not grumble about this! For example in the shallow waters of the Bristol Channel there can be very large attenuations due to the fish distributed along the transmission path, especially at night, as fully reported already [1]. Another effect is described here, with large fluctuations of typical period 80 seconds, thought to be due to fish clustering round the hydrophone. A firm attribution to fish comes from its day-time occurrence, but the detailed mechanism is uncertain. In the course of extensive experiments in the Bristol Channel many different mechanisms causing fluctuation have been identified (see, e.g., references [I]-[3]), and the 80 second fluctuation was first noticed during the day-time in summer 1964 [1]. In the series of multifrequency amplitude fluctuation experiments which took place at monthly intervals between 1967 and 1969 [2] many further examples ofthis fast fluctuation were recorded. Note that the adjective fast is used to distinguish it from the longer-period fish attenuations mentioned above. 2~-EX PER1MENTS In the early experiments in June 1964 [1], 2083.3 Hz CW transmissions were received on hydrophones 7.7 and 10 km from the projector, and displayed on logarithmic level recorders. Fluctuations of a few minutes period and up to 15 dB in amplitude were noticed, commencing near dawn and ceasing near dusk. In the later 1967-69 experiments a sequence of up to fourteen 4-second duration pulses, covering a frequency range from 312 Hz to 4.44 kHz, was transmitted every 100 seconds. The path length for frequencies of 870 Hz and above was 23 km, but frequencies below 870 Hz were measured over only 17 km of this path. The levels received were displayed on Sanborn logarithmic level recorders. Measurements were also made over a path length of 137 km but the day-time fast fluctuation effect described here was not identified over this range, which is not necessarily to say that it did not happen. Reference [2] gives a general description of all this work, as well as a chronological listing of the experiments. 287

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Additional 2 kHz CW transmissions were made in August 1969, the levels being received over the 23 km path as well as at hydrophones situated 4, 1.5 and 1.2 km from the projector. All these hydrophones are between 3 and 7 m from the sea-bed. The fast fluctuation was seen over only the 23 km path. 3. D E S C R I P T I O N

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The fast fluctuation effect is illustrated in Figures I and 2. Figure 1 shows part of a recording made in September 1969 when pulsed transmissions were being used.The record is divided into night-time and day-time, one mounied under the other. It shows the comparatively smooth envelope ofthe pulses at night, with slowly varying modal interference patterns due to the tidal changes of water depth, and the pulse to pulse variation of about 10 dB present during the day-time when the modal interference patterns are almost completely destroyed. (In general such destruction is most evident at the height of the season for fast fluctuations, specified below.) Figure 2 shows two sections of the 2 kHz CW recording made in August 1969, the first taken during the night and the second during the day. The period of the day-time fluctuation is of the order orS0 seconds, but the spectrum is fairly broad and

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The transmission frequencies showing an effect are indicated in Figure 3 as well. (The irregularity of the times of making the recordings affects the appearance, but should be ignored.) The percentage of recordings having a fluctuation at a particular frequency has been calculated for each )'ear and plotted in Figure 4, records with no flnctuation at any frequency having been rejected. Most of the relevant records show the fluctuation effect at

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Figure 4. Percentage occurrence of fluctuations as a function of transmission frequency, 1967-1969. (a) 1967, July to September; (b) 1968, September to October; (c) 1969. 1.44 kHz, but the percentage falls offabove and below this frequency. Both the frequency of peak occurrence and the range of frequencies tend to increase slightly throughout the season. The fast fluctuation is often associated with a reduction of a few dB in signal level (note that such an attenuation in the day-time is the opposite of the usual case reported in reference [3]) but sometimes the associated level is significantly lower at night; this whole picture may be explained if there is one attenuation effect due to the night-time dispersion of fish along the whole path [3] and another separate effect associated with the fast fluctuation (see section 4). There is no obvious indication of dependence on other parameters such as the phase o f the m o o n - - t h e latter might conceivably have had an influence through its control of the tidal streaming and consequent disturbance o f the fish. 4. EXPLANATION OF THE FLUCTUATION The prompt dawn start and dusk finish to the effect point positively to its biological origin, and this is reinforced by the seasonal occurrence. Its magnitude and frequency dependence suggest bladder fish of length a few tens o f c m . But are the fish distributed along the path or local, are they pelagic or bottom-living, and how are they grouped? Consider first the distributed fish possibility, and in particular the pelagic fish such as pilchard and sprat which cause an attenuation when they are dispersed at night [3]. Extensive echo-ranging observations [4] show that the most common pelagic fish are pilchards, and that during the day these congregate into shoals, about 2 per km 2. Typical shoal size is 13 m across by 3 m deep, with target strength +5 dB r e 1 m 2 (i.e., reference distance is 1 m). Calculations show that the mean attenuation along the path is insignificant during the day-time, which

FAST FLUCTUATIONS CAUSED BY FISH

291

alone is sufficient to rule out the pelagic fish as the explanation. To pursue this a little further note that a shoal passing reasonably close in front of a transducer might have a resolved velocity of about 0.2 m/s, and so would cast an acoustic shadow for the reasonable time of 65 seconds. But one needs to know what proportion of time there will be a shoal reasonably close in front, and this depends on the shoars shadow area, a generous measure o f which is the Rayleigh distance times the width. Rayleigh distance is (shoal width) 2 divided by wavelength: i.e., 169 m at 1.5 kHz, where the wavelength is l m. Thus shadow area is about 169 x 13 m 2, or 2197 m 2, which is only 0.4 ~o of the mean area available per shoal. Thus the fluctuations or dips would be far too infrequent, even allowing for effects near both source and receiver. It is difficult to see how the effects offish that are distributed along the path, but living near the sea-bed, could compete with the scattering effects due to the surface waves. There remains only the possibility oflocal fish: i.e., fish very close to one ofthe transducers. Consider a single fish at 1 m distance, having a target strength o f - 2 0 dB r e 1 m 2 (compare, e.g., reference [3]). If the direct signal amplitude at a hydrophone is taken as unity the addition ofthe scattered signal will make the sum vary between 0.9 and l.l, depending on the position of the fish. This variation comes to nearly 2 dB, so that about 25 randomly spaced fish would produce the observed 10 dB effect. At the same time they might decouple or shield the transducer from the medium and so cause an attenuation. If they were closer to the hydrophone many fewer fish would be needed, and ifnearly touching one would suffice (see also references [5] and [6]). The lack o f any fluctuation at night is explained by the quiescence or the absence of the fish. The discovery that a few fish can interfere with an acoust!c experiment is not a new one, but the magnitude and continuing character of the present effect add a new dimension. There are several suitable types of bottom-living fish such as pollack or gurnard that do live in the area. It is well known that fish like to live near objects such as rocks and wrecks, and divers have actually seen many fish near some of the transducer assemblies. One point against this idea is the lack of any effect for the hydrophones between 1 and 4 km from the source. This indicates, first, that there is no clustering around the projector--perhaps they are driven away by the high sound levels. It suggests secondly a lack of fish around the closer hydrophones, where the sound levels are unlikely to be too high. The explanation may lie in the uneven distribution o f the fish, and the echo-ranging studies [4] do show very few fish out to these ranges although it is basically the pelagic fish which are being monitored. Clustering can explain the order of magnitude of the fluctuation period. Thus a purposeful fish at 0.2 m/s and I m range would take about 30 seconds to circumnavigate a transducer. In addition if the fish were sufficiently spread in depth the scattered signals would tend to smear out the modal interference pattern. 5. SIGNIFICANCE Ifthe present fast fluctuation is due to fish clustering around a transducer, it may be regarded from one viewpoint as an artefact, because man has installed the transducer. Nevertheless it is a fluctuation that really happens in the open sea, not just a theory or a laboratory demonstration. For complete understanding o f the propagation problem it must be sorted out from all the other fluctuations. One question is how much it may have affected or spoilt other people's experiments, on propagation or related quantities. This point is reinforced by the possibility of an associated systematic decoupling of the transducer from the medium. Tripod-mounted transducers may be most at risk, but there could also be an effect for transducers laid directly on the seabed. (For sea-bed transducers there is the related problem of losses due to gas trapped near

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the interface.) It is thought that even transducers suspended from a stationary or slowly moving ship could rapidly gather a circle of curious fish. I f countermeasures were thought necessary it might be sutiicient to tow at a moderate speed, though repulsion by chemical or ultrasonic means might also be possible. The characteristics of the effect do give information on the behaviour and size of the fish participating, and although this might be of some passing interest in fisheries research it is a very indirect method of study. 6. CONCLUSIONS (1) There is a fluctuation during the sunamer day-time of about 10 dB amplitude and about 80 seconds period. (2) It has been recorded at transmission frequencies between 312 Hz and 4.44 kHz. The peak occurrence increa?ses in frequency from June to October, but 1.5 kHz is typical. (3) It has been recorded at hydrophones 8, I0, 17 and 23 km from the projector but not at closer hydrophones, xfhich may be due to the distribution offish. (4) The fluctuation is attributed to fish which change their behaviour between night and day, and is probably due to their clustering round the hydrophone during the day. ACKNOWLEDGMENTS The authors wish to acknowledge the help of J. Revie and S. J. L. Thomas. REFERENCES 1. D. E. WESTOS, A. A. HORRIGAN, S. J. L. THOMASand J. REVIE 1969 Philosophical Transactions of the Royal Society A265, 567-606. Studies of sound transmission fluctuations in shallow coastal waters. 2. D. E. WESTON,K. J. STEVENS,J. REVIEand M. PENGELLY1971 Journal of Sound and Vibration 18, 487--497. Multiple frequency studies of sound transmission fluctuations in shallow water. 3. P.A. CmNG and D. E. WESTON1971JournalofSoundand Vibration 18,499-510. Wideband studies of shallow-water acoustic attenuation due to fish. 4. D. E. WESTONand J. REVIE1971 Journal of Sound and Vibration 17, 105-112. Fish echoes on a long-range sonar display. 5. W. F. HUNTER 1968 Proceedings of the 6th hzternational Congress on Acoustics, Paper P-0-2.2, pp. L33-L36. The effect of gas bubbles near monopole and dipole underwater sound sources. 6. J. E. FFowcs WmLIAMSand W. F. Hur,rrER 1970 Proceedings of the Royal Society A314, 363-385. The scattering of multipole near-field sound by a gas bubble.