Anthony D. Hawkins

A field study of hearing in the cod,Gadus morhua L.

Summary1.Field measurements of hearing in the cod,Gadus morhua L., have shown that these fish are sensitive to pure tones in the frequency range from 30 to 470 Hz with greatest sensitivity in the range 60 to 310 Hz. At the most sensitive frequencies the mean thresholds varied between -18 and -26 dB/μbar (Fig. 4).2.Variation in the thresholds at most frequencies was related to changes in the level of ambient sea noise (Figs. 5–7). Only in calm sea conditions were unmasked thresholds obtained. The masking effect of noise was confirmed by raising the level artificially. The thresholds increased in proportion to the increase in noise level.3.The thresholds were largely independent of the distance of the sound source over the range from 1.7 to 50m, suggesting that cod are sensitive to acoustic pressure. However, a changeover to particle displacement sensitivity was noted at frequencies below 50 Hz when the sound source was moved to within l m of the fish (Fig. 9).4.It is concluded that the swimbladder plays an accessory role in hearing. Differences obtained in the thresholds at different sound source distances may be explained in terms of the displacement sensitivity of the otolith organs. These respond to displacements re-radiated from the swimbladder in the far-field, and to the greater incident displacement in the near-field at very low frequencies.

Underwater Sound and Fish Behaviour

It has long been known that sounds are important to fish. Isaac Walton advised anglers ‘to be patient and forbear swearing, lest they be heard’. A wide range of species, including many that are commercially valuable, emit sounds as part of their social behaviour (Tavolga 1976), and several species have been shown to be acutely sensitive to underwater sounds. However, before we consider the acoustical behaviour of fish in more detail, we need to understand what sound is, how sounds are created, and how they are transmitted through water.

Listening to Fish

Abstract Passive acoustics is a rapidly emerging field of marine biology that until recently has received little attention from fisheries scientists and managers. In its simplest form, it is the act of listening to the sounds made by fishes and using that information as an aid in locating fish so that their habitat requirements and behaviors can be studied. We believe that with the advent of new acoustic technologies, passive acoustics will become one of the most important and exciting areas of fisheries research in the next decade. However, a widespread lack of familiarity with the technology, methodologies, and potential of passive acoustics has hampered the growth of the field and limited funding opportunities. Herein, we provide an overview of important new developments in passive acoustics together with a summary of research, hardware, and software needs to advance the field.

The Hearing Abilities of Fish

Earlier this century, von Frisch and his pupils paved the way to the objective measurement of the hearing abilities of fish. They trained fish to respond in an unambiguous way to sounds, presenting them with tones of differing frequency and amplitude to explore the limits of their sensitivity. Such techniques form the basis for most modem studies of the hearing capacities of fish.

Information gaps in understanding the effects of noise on fishes and invertebrates

The expansion of shipping and aquatic industrial activities in recent years has led to growing concern about the effects of man-made sounds on aquatic life. Sources include (but are not limited to) pleasure boating, fishing, the shipping of goods, offshore exploration for oil and gas, dredging, construction of bridges, harbors, oil and gas platforms, wind farms and other renewable energy devices, and the use of sonar by commercial and military vessels. There are very substantial gaps in our understanding of the effects of these sounds, especially for fishes and invertebrates. Currently, it is almost impossible to come to clear conclusions on the nature and levels of man-made sound that have potential to cause effects upon these animals. In order to develop a better understanding of effects of man-made sound, this paper identifies the most critical information needs and data gaps on the effects of various sounds on fishes, fisheries, and invertebrates resulting from the use of sound-generating devices. It highlights the major issues and discusses the information currently available on each of the information needs and data gaps. The paper then identifies the critical questions concerning the effects of man-made sounds on aquatic life for which answers are not readily available and articulates the types of information needed to fulfill each of these drivers for information—the key information gaps. Finally, a list of priorities for research and development is presented.

Responses of free-living coastal pelagic fish to impulsive sounds.

The behavior of wild, pelagic fish in response to sound playback was observed with a sonar/echo sounder. Schools of sprat Sprattus sprattus and mackerel Scomber scombrus were examined at a quiet coastal location. The fish were exposed to a short sequence of repeated impulsive sounds, simulating the strikes from a pile driver, at different sound pressure levels. The incidence of behavioral responses increased with increasing sound level. Sprat schools were more likely to disperse and mackerel schools more likely to change depth. The sound pressure levels to which the fish schools responded on 50% of presentations were 163.2 and 163.3 dB re 1 μPa peak-to-peak, and the single strike sound exposure levels were 135.0 and 142.0 dB re 1 μPa(2) s, for sprat and mackerel, respectively, estimated from dose response curves. For sounds leading to mackerel responses, particle velocity levels were also estimated. The method of observation by means of a sonar/echo sounder proved successful in examining the behavior of unrestrained fish exposed to different sound levels. The technique may allow further testing of the relationship between responsiveness, sound level, and sound characteristics for different types of man-made sound, for a variety of fish species under varied conditions.

Parvulescu Revisited: Small Tank Acoustics for Bioacousticians

Researchers often perform hearing studies on fish in small tanks. The acoustic field in such a tank is considerably different from the acoustic field that occurs in the animals natural environment. The significance of these differences is magnified by the nature of the fishs auditory system where either acoustic pressure (a scalar), acoustic particle velocity (a vector), or both may serve as the stimulus. It is essential for the underwater acoustician to understand the acoustics of small tanks to be able to carry out valid auditory research in the laboratory and to properly compare and interpret the results of others.

A sound approach to assessing the impact of underwater noise on marine fishes and invertebrates

Increasing attention is being paid to the ecological consequences of underwater noise generated by human activities such as shipping and maritime industries including, but not limited to, oil and gas exploration and extraction, sonar systems, dredging and the construction of offshore renewable energy devices. There is particular concern over the extension of these activities into previously undeveloped areas of the oceans, including Polar Regions and areas of coral reef habitat. Most of the concern by regulators and others has focussed upon effects upon marine mammals and other protected species. However, examining the impacts upon the overall ecology of affected habitats is also important as it may be dominated by effects upon the far larger biomasses of fishes and invertebrates, which do not have the same degree of legal protection. Many of these assessments of the impact of noise on fishes and invertebrates have overlooked important issues, including the sensitivity of a substantial proportion of these species to particle motion rather than sound pressure. Attempts have been made to establish sound exposure criteria setting regulatory limits to the levels of noise in terms of effects upon mortality levels, injury to tissues, hearing abilities, behaviour, and physiology. However, such criteria have almost exclusively been developed for marine mammals. Criteria for fishes and invertebrates have often had to be assumed, or they have been derived from poorly designed and controlled studies. Moreover, the metrics employed to describe sounds from different sources have often been inappropriate, especially for fishes, and invertebrates, as they have been based on sound pressure rather than particle motion. In addition, the sound propagation models employed to assess the distances over which effects might occur have seldom been validated by actual measurements and are especially poor at dealing with transmission under shallow water conditions, close to or within the seabed, or at the surface. Finally, impacts on fish and invertebrate populations are often unknown and remain unassessed. This paper considers the problems of assessing the impact of noise upon fishes and invertebrates and the assessment procedures that need to be implemented to protect these animals and the marine ecosystems of which they form an integral part. The paper also suggests directions for future research and planning that, if implemented, will provide for a far better scientific and regulatory basis for dealing with effects of noise on aquatic life.

Active and Passive Acoustics to Locate and Study Fish

Two important goals in studying the biology of fishes are to detect and enumerate the fish and to define where the fish are to be found. Locating and counting fish is difficult, but defining and mapping a fish’s habitat can be even more daunting. A fish’s habitat is the physical, chemical, geological and biological environment in which it resides or migrates through and includes the pelagic (open water), benthic (on or in the sea floor), and demersal (on or near the sea floor) realms. With the continuing loss of estuarine and coastal habitats it is especially critical to seek out the waters and substrates that are necessary as spawning, nursery and feeding areas for fishes. In the United States, the Magnuson-Stevens Fishery Conservation and Management Act, Public Law 94-265, as amended through October 11, 1996 calls for direct action to stop or reverse the loss of fish habitat and requires the identification of “essential fish habitat” (Section 305 of the Act). In a wider context, the wish to promote conservation through the establishment of marine protected areas also requires the identification of habitats of managed, threatened, and endangered species. Investigating the distribution of fish is especially difficult because fish can rarely be seen and counted underwater. Fisheries trawl or net surveys can provide an overall picture of fish distribution, but are destructive of the species being surveyed. One of the greatest challenges to the study of fish populations is the ability to collect data over large spatial scales and to study behavior for long periods of time, without intruding upon the lives of these animals. Two uses of acoustics have been developed for studying fish populations and behavior. Active acoustics uses sound generated actively by transducers and the acoustic scattering properties of fish to image individual fishes and populations of fishes. Passive acoustics relies on listening to the sounds produced by fishes with a hydrophone to infer their distribution and behavior. For passive acoustics to be useful a fish must make a sound, thus this technique is limited to species that produce sounds and to the times and places where they produce them. These techniques have typically been used independently, depending on the situation and goals of the study. This chapter reviews each of these technologies and

PASSIVE HYDROPHONE CENSUS OF SCIAENA UMBRA (SCIAENIDAE) IN THE GULF OF TRIESTE (NORTHERN ADRIATIC SEA, ITALY)

Sciaena umbra belongs to Sciaenids, often called croakers because most of tropical Sciaenids produce sounds by compressing the swimbladder with sonic muscles (S0rensen 1984). S. umbra has been known to emit sounds since 1947, when Dijkgraaf observed and heard them in the Naples Aquarium. It is distributed along the coasts of Mediterranean Sea and Black Sea and along the Atlantic coast from Senegal to the English Channel. Since 1995, UNEP has included S. umbra in the annex 3 of RAC-SPA protocol, i.e. the list of the species of fish whose exploitation has to be regulated (AA. VV. 1995). In Miramare Marine Reserve, Trieste, where the species is very abundant from May to September, it has been monitored by visual census and acoustic recordings since 1999 (Bonacito 2000). Other sites along the coast of Trieste gulf were acoustically mapped to determine where the species can be found. This has enabled the real distribution of the species to be assessed.