Sounds produced by herring (Clupea harengus) bubble release
Introduction
The herring (Clupea harengus) is quantitatively the most important fish species of northern Europe. During decades of intense research, we have learned about its migratory, diurnal and foraging behavior, and about its role in the marine food web (Klinkhardt, 1996). In spite of this, little is known about herring sound production and communication. In this study, we focus on acoustic signals produced by herring.
Compared to most other fish species, herring has excellent hearing abilities (Enger, 1967). In clupeid fish, the swim bladder connects to the inner ear, which facilitates the perception of sound. This hydro-acoustic detection system is in close contact to the lateral line canal system on the head of the fish (Blaxter et al., 1981) and works well for the perception of both acoustic pressure as well as hydrodynamic displacement signals.
Another feature of the herring anatomy is the connection between swim bladder both to the stomach and to the anal opening (Fig. 1; Bennett, 1879–1880). Herring does not seem to produce gas in the swim bladder as many other fish species do (Fahlén, 1967, Blaxter and Batty, 1984). Air is inhaled at the surface, swallowed and transported into the swim bladder through a small canal, ductus pneumaticus (Klinkhardt, 1996; Fig. 1).
It is well known among fishermen that herring schools can release air, producing clouds of bubbles that may be observed at the surface (Muus and Dahlstrøm, 1974, Thorne and Thomas, 1990, Nøttestad, 1998). Bubbles are usually released through the anal opening, connected to the swim bladder (Fig. 1). Sometimes, bubbles are released through the mouth (personal observation). Such bubbles may originate from air that has not yet been transferred to the swim bladder. Gas may be released both during ascent and decent and as a response to distress (Nøttestad, 1998). It has been suggested that the air release may work as an optic and acoustic screen that confuses predators (Nøttestad, 1998).
Herring is known to produce sounds Murray, 1831, Marshall, 1962, Hering, 1964, Fish and Mowbray, 1970, Freytag, 1971, Schwarz and Greer, 1984). Previous studies performed to investigate sound production have, to our knowledge, not reported the source level of such sounds, nor have they suggested how the sounds are produced. Both these pieces of information are important to evaluate the possible function of the sound.
In this study, we report on the sound production during herring gas release. We estimate source level of herring sounds and investigate a potential sound-production mechanism.
Section snippets
Materials and methods
Herring sound production was studied with controlled experiments and field observations. The recording system consisted of a BK 8101 (sensitivity –184 dB re 1 V/μPa) and an Atlantic Research LC32 hydrophone (–202 dB re 1 V/μPa) connected via a custom-built amplifier to a Sony TCD-D7 DAT recorder (recording band width 0.1–22 kHz). Data were digitally transferred to the computer via a U2A digital interface (Egosys Inc.) and subsequently analyzed using Matlab (MathWorks, Inc.) and Cool Edit 2000 (
Results
During the recordings with the LPC, in the fish trap and in the spawning bay, sounds were recorded from the herring. The most prominent signal consisted of a series of pulses, where each pulse had a lower frequency emphasis than the previous one (Fig. 2). This sound, we call the pulsed chirp.
In the LPC experiments, 19 out of 20 herrings produced sounds that could be analyzed. The source level of the chirps ranged from 55 to 90 dB re 1 μPa rms (root mean square) at 1 m (73 ± 8 dB, mean ± 1 S.D.;
The biological significance of the pulsed chirp
The pulsed chirp (Fig. 2) has previously been observed in recordings of herring (Schwarz and Greer, 1984). To our knowledge, there have been no reports of any other marine animal producing this kind of sound. Eels (Anguilla anguilla; J. P, Lagardère, personal communication) and some other fish species (e.g. weakfish, Cynoscion regalis; Sprague, 2000; see Fish and Mowbray, 1970 for other examples) are known to produce transient sounds of similar frequency content as the herring pulses, but these
Acknowledgements
The Swedish Navy provided funding, logistics and support. Fernando Ugarte kindly loaned us underwater video uptakes of herring schools.
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Present address: Center for Sound Communication, Department of Zoophysiology, University of Aarhus, C. F. Møllers Allé Building 131, 8000 Aarhus C, Denmark.