Acoustic characteristics of soniferous fishes in shallow waters off Cochin

Transcription

1 Acoustic characteristics of soniferous fishes in shallow waters off Cochin M M Mahanty, G. Latha and M.Ashokan National Institute of Ocean Technology Pallikaranai, Chennai , India mmmahanty@niot.res.in, latha@niot.res.in, ashokan@niot.res.in Abstract: Identification of marine soniferous species from passive ambient noise measurements in shallow South Eastern Arabian Sea is attempted here. Earlier studies have reported the use of passive acoustics to detect specific signatures of soniferous fishes for identification of species, quantification of fish population at that location, distinguishing spawning grounds and understanding the communication between members of the species etc. This also gives information on bioacoustics noise contribution to the overall ambient noise at the site. Soniferous fishes have been known to produce sounds associated with courtship and agonistic social interactions. The biological noise due to soniferous fishes has high seasonal/diurnal dependence, and can greatly affect the ambient noise level. The acoustic characteristics of soniferous fishes have been studied in different parts of the world, but this is the first attempt to characterize fish signatures in shallow South Eastern Arabian Sea. Ambient noise measurements were made in shallow waters off Cochin using an autonomous system, during January to May, The time series measurement of ambient noise continuously at a particular site enables us to study the seasonal behavior of soniferous fishes. All records were analyzed using time/frequency characteristic spectrograms. Acoustic characteristics like fundamental frequency, dominant frequency, sound duration, pulse period and pulse duration are used to identify the occurrence of fishes. Two species could be identified; Oyster toadfish Opsanus tau and Cusk-eel fish Ophidion marginatum. It is seen that the oyster toadfish produces acoustic emission, often in choruses which is comparable to an intermittent foghorn advertisement call. During the spawning season, male oyster toadfish Opsanus tau occupy well defined areas on the seabed and emit foghorn sounds that attract females, which is generally observed as high in sound pressure level before dawn and after dusk. Number of foghorn choruses were maximal during the beginning of January to end of March, but ceased during dawn in the month of May. The Cusk-eels bury themselves under the sand all day and produce sounds similar to jackhammer during the night. Sounds made by Cusk-eels have been recorded only during the month of May. These studies suggest that passive acoustics is an important tool to understand sound generated by soniferous fishes, their vocal behavior, spawning grounds, acoustic communications and the impact on local environmental noise. ACOUSTIS2013NEWDELHI, New Delhi, India, November 10-15,

2 1. Introduction Fishes produce sounds during a variety of ecological relationships such as spawning and mating, feeding, or being aggressive and other behaviours [1] [2] [3] [4].There are many recreational and commercial significant fishery species that make sounds; over 800 species are currently described as soniferous [5]. Sound characteristic depends largely on the source of sound production such as vibration of the swimbladder, contraction of rapid intrinsic or extrinsic sonic muscles and stridulating from rubbing bony elements against each other [6]. The vibration of the swimbladder results in the emission of low-frequency sounds made up of pulses that can be repeated at different rates [7] [6]. Fish can produce different sound types and changing the characteristics of one sound type according to activity for their communication [8].The temporal features within a sound, including pulse rate can be important to its communicative values and behavioural studies [7]. Passive acoustics is a technique that enables recording of underwater sounds of marine fishes [9]. These methods are rapid, inexpensive, non-invasive and useful in low-visibility waters. Using this technique, low frequency sounds produced by certain species of sound producing specific fishes are identified [10] [11] [12] and differentiated [13]. It is used to identify the particular habitats and the locations of spawning and feeding grounds [14]. Several passive acoustic studies have focused on daily and seasonal patterns of the soniferous species which can be correlated with daily and seasonal patterns of chorus spawning behaviour [15] [16] [17]. Biological choruses can be assumed by the rise of noise intensity where species sounds are most dominant [18] which may extend over significant areas [19] [20].Variations in choruses can occur over time scales of hours as the weather conditions change. Fish vocalizations have been examined for the diurnal dependence of the day and can greatly affect the ambient noise level. Male toadfish calling alone do not change the sound duration whereas, a chorus significantly lengthens the call duration [21]. Numerous studies have examined the acoustic characteristics, geographical variation and seasonality of foghorn production in the oyster toadfish [22] [23]. The Cusk eels would hide by staying buried under the sand all day and slowly poke their heads out of the sand, produce sounds like a jackhammer during the night [24]. Male Cusk eel use calls as part of their courtship behavior and attract females for spawning.this analysis can give the species distinction by their vocal behavior, spawning grounds, acoustic communications and the impact on local environmental noise. Two kinds of soniferous fishes have been identified; Oyster toadfish Opsanus tau and Cuskeel fish Ophidion marginatum in shallow South Eastern Arabian Sea (off Cochin). 2. Materials and Methods 2.1. Moored hydrophone survey An autonomous passive acoustic noise measurement system comprising of a vertical linear array (VLA) of omnidirectional hydrophones, was moored in the shallow waters off Cochin at 30 m ocean depth during January to May 2011 (Fig. 1). Figure 1. Location of moored hydrophone in shallow waters off Cochin. VLA is shown as. Hydrophone with the frequency range 0.1 khz to 10 khz is used with a data acquisition system and a sampling interval of 3hours with data recording duration for 30 seconds is set to acquire eight ACOUSTIS2013NEWDELHI,NewDelhi, India, November 10-15,

3 samples per day. The hydrophone is placed at the middle of the vertical column to measure the noise. This enables the detection of individual events and great variability in spectral structure Spectral analysis In-situ time series data records have been analyzed using time/frequency characteristics spectrograms. The acoustic characteristics have been found useful for identifying soniferous fish species; the fundamental frequency (khz), dominant frequency (khz), sound duration (ms), pulse period (ms) and pulse duration (ms) via oscillograms, spectrograms and averaged power spectra. Oscillograms and spectrograms have been drrived using MATLAB. Oscillograms (waveforms) of each sound recording have been plotted as the relative pressure measured by the hydrophone. The signal processing program of MATLAB has been used to calculate spectrograms based on 4096-point fast Fourier transform (FFT), windowed with a Hamming window with 90% overlap. The frequency resolution, determined by the sampling frequency and the number of points in the FFTs in each power spectrum, is 12.2 Hz. In each of our spectrograms, the frequencies ranging from 0.1 khz to 3 khz are shown. The dominant frequency is the frequency at which the average power spectral density (PSD) is greatest during the period examined and is indicated by the peak of the average power spectrum graph of frequency. Welch s averaging periodgram method has been used to calculate the noise spectrum. Multiple spectra have been obtained first by segmenting the data into smaller portions, windowed with a Hamming window, and a 2048 point FFT with 50% overlap. Spectra are then averaged to obtain the final spectrum. By knowing the acoustic characteristics via oscillograms, spectrograms and averaged power spectra, identification of the occurrence of soniferous fishes in shallow waters off Cochin has been done. 3. Results Time/frequency spectrograms have been created from the recorded data and have been converted into audio files (.wav). Acoustic characteristics like pulse period, pulse duration, fundamental frequency and dominant frequency components have been used to identify the occurrence of fishes. Two common sounds have been categorized as biological sound (Fish sound) and have been identified as originating from toadfish Opsanus tau and Cusk eel Ophidion marginatum characterizing choruses of foghorn sound and jackhammer sound respectively Oyster toadfish Opsanus tau A total of 160 sounds of the oyster toadfish Opsanus tau, have been recorded, often choruses in nature. It is observed that the oyster toadfish produces acoustic emission, often in choruses which is comparable to an intermittent foghorn advertisement call. These sounds are used to attract females to nests that the male has prepared. Spectrogram and oscillogram (waveform) of toadfish recorded during this study is shown in Fig. 2. Figure 2. Waveform and spectrogram of chorus sound emitted by the oyster toadfish off Cochin. There are trains of pulses (Fig. 3A) in a single sound produced by toadfish was similar in duration about 350 ms and presented a harmonic structure (Fig. 3B). The fundamental frequency of each ACOUSTIS2013NEWDELHI, New Delhi, India, November 10-15,

4 sound has been about khz with a typical dominant frequency around khz and peak sound level of about 100 db re 1 µpa (Fig. 3C). Pulse duration having ~3 ms (Fig. 3D).The foghorn sound production has exhibited a strong daily pattern across the study period. The Chorus sounds have been generally observed as peaks in sound level before dawn (between 5 AM to 6 AM) and after dusk (between 6 PM to 9 PM) (Fig 3E). The sounds produced during the dusk hours have been higher than those produced during the hours of dawn. Chorus sound has been maximal at the beginning of January to end of March, but ceased during dawn in the month of May. Figure 3. Oscillogram (A), spectrogram (B) and average power spectra (C) of a single sound emitted by the toadfish. Pulse duration of the toadfish (D), Peaks in sound level before dawn and after dusk (E) Cusk eel Ophidion marginatum A total of four sounds have been recorded which belong to the cusk-eel Ophidion marginatum only during the month of May. Spectrogram and oscillogram (waveform) of cusk-eel sounds recorded during this study is shown in Fig. 4. Figure 4. Waveform and spectrogram of the Cusk-eel sounds recorded over the study period. The acoustic characteristics of single Cusk-eel sound consists of a number of pulses (Fig.5A). The average power spectra of the sound indicates that the dominant frequency of the Cusk-eel has been 1.21 khz with the maximum sound level of about 89 db re 1 µpa (Fig.5C.) and pulse period 43.7 ms (Fig.5D.). The Cusk-eel produces sound like a jackhammer during the night (between 10 PM to 11 PM) (Fig.5E). ACOUSTIS2013NEWDELHI,NewDelhi, India, November 10-15,

5 Figure 5. Oscillogram (A), spectrogram (B) and average power spectra (C) of a single sound emitted by the Eel fish. Pulse duration of the Eel fish (D), Eel fish sounds recorded during night. (E). 4. Conclusion This work shows the presence of soniferous fishes in the shallow waters off Cochin. It has been shown that fish vocalizations are described by aural and different spectrographic structures. It is seen that the oyster toadfish produces acoustic emission, often in choruses which is generally seen as daily pattern and peaks in sound pressure level before dawn and after dusk. This determines the time of day the fish spawns. The Cusk-eels produce sounds like jackhammer individually during nocturnal. Identification of spawning habitat can be achieved by knowing the behaviour associated with a particular fish sound. These studies suggest that passive acoustics is an important tool to understand sound generated by soniferous fishes, their vocal behaviour, spawning habitat, acoustic communications and the impact on local environmental noise. Acknowledgement The authors thank Director, NIOT for his support in carrying out this work. Thanks are also due to A.Thirunavukkarasu, G.Raguraman, P.Edwards Durai, K. Nithyanandam, and C. Dhanaraj of Ocean acoustics group of NIOT who participated in all phases of field deployment and retrieval operations. The authors are thankful to Mrs. A. Malarkodi, Dr. M.C. Sanjana and Mrs. M. Dhanalakshmi for their support in testing and calibration of hydrophones. References [1] H.E. Winn., J.A. Marshall., B.Hazlett., Behavior and diel activities and stimuli that elicit Sound production and reactions to sounds in the longspine squirrelfish. Copeia. 1964: [2] A. A. Myrberg., Sound communication and interception in fishes. In: Reese S, Lighter F, eds. Hearing and Sound Communication in Fishes. New York, NY: Springer, [3] J.D.Crawford., A.P.Cook., A.S.Heberlein., Bioacoustic behavior of African Fishes (Mormyridae): potential cues for species and individual recognition in Pollimyrus. J Acoust Soc America. 102: [4] J.R.McKibben., A.H.Bass., Behavioral assessment of acoustic parameters relevant to signal recognition and preference in a vocal fish. J Acoust Soc Am. 104: [5] I. M Kaatz., Multiple sound-producing mechanisms in teleost fishes and hypotheses regarding their behavioural significance. Bioacoustics 12: [6] F.Ladich and M.L.Fine., Sound-generating mechanisms in fishes: a unique diversity in vertebrates. In Ladich F., Collin S.P., Moller P. and Kapoor B.G. (eds) Communication in fishes. Enfield, NH: Science Publishers, pp [7] F. Ladich., Sound production and acoustic communication. In Van der Emde G., Mogdans J. and Kapoor B.G. (eds) The senses of fishes. Adaptations for the reception of natural stimuli. New Delhi: Narosa Publishing House, pp ACOUSTIS2013NEWDELHI, New Delhi, India, November 10-15,

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