Examining the hearing abilities of fishes

Abstract

Fishes obtain substantial information about their environment by listening to the sounds around them. Indeed, because sound propagates rapidly and over great distances in water as compared to in air, it provides fishes with information from far greater distances than do other sensory stimuli. Thus, any interference with detection of sounds has the potential of reducing fitness and impacting the lives of fishes (e.g., Popper and Hawkins, 2019). Although the sounds that fishes hear are confined to low frequencies (often to no more than 800-1000 Hz, but this is very species dependent) in comparison with many terrestrial vertebrates and aquatic mammals, fishes are able to discriminate between sounds of different amplitude and frequency , and between calls that differ in their temporal characteristics (e.g., Fay, 1988; Fay and Megela Simmons, 1999). Fishes are also able to use auditory cues to seek out the location of a sound source (Sand and Bleckmann, 2008; Hawkins and Popper, 2018). Sounds may play a role in navigation, foraging for prey, detection of predators, and communication of reproductive state, and some marine species may use sound for habitat selection. Detailed discussions of the role of sound in the lives of fishes can be found in several recent reviews (e.g., Popper and Hawkins, 2019; Putland et al., 2019). II. PURPOSE OF THIS PAPER Because sound is so important to fishes, knowledge of their hearing capabilities is imperative for determining whether human activities, particularly in terms of noise pollution, have an impact on hearing and thus on fish behavior. It is important, therefore, to determine those levels of different sounds that particular species are able to respond to, and those levels that they cannot detect, in order to evaluate the significance of different sounds to fishes and to determine the distances over which sounds can be detected. It is also important to have a far better understanding of how fishes detect and process sounds. A key point that led to our thinking for this paper derives from the observation that many investigators (including the authors) have measured hearing by fishes using a wide range of techniques and approaches. Most of this work has focused on measuring hearing sensitivity by determining hearing thresholds-defined as the lowest sound levels an animal can detect and respond to at particular frequencies. There have been far fewer studies of other, albeit very important, questions, such as whether, how, and how well, fishes can discriminate between sounds (e.g., frequency, intensity, temporal patterns), detect signals in the presence of sounds that mask them, and determine the direction to a sound source. While there are few data for anything but hearing thresholds and band-width of hearing, it was recognized a number of years ago that there is very wide variation in thresholds determined for even a single species (Fig. 1). This variation has been attributed to differences in experimental techniques and approach rather than reflecting actual differences in hearing between species (e.g., Hawkins, 1973; Ladich and Fay, 2013; Sisneros et al., 2016). Hence, we are not able to reliably examine and compare the hearing abilities of different species, nor can we even be fully confident of most of the data we have on hearing in species that have been studied in a single lab. This unreliability of data hamper our understanding of what fishes can hear, and thus reduces our understanding of fish bioacoustics. Moreover, uncertain data on fish bioacoustics affects our understanding of the potential impacts of anthropogenic sounds on fishes. The purpose of this paper is twofold. First, we discuss the basis for the variation in data, and point out what we see as the major issues resulting from having unreliable data. Second, we present, as investigators who have focused on fish hearing for many decades, some initial thoughts on what and how future scientific work should be carried out to investigate fish hearing for both basic science and applied purposes. Our hope is that these suggestions may provide a basis for future discussions and approaches on how experiments should be done. We do want to point out that this paper is not meant to be a full or comprehensive review. While we do cite some literature, our intent is to just illustrate points with some basic (and often "historic") literature and focus on our ideas. Readers interested in more depth on various parts of this paper should refer to the reviews we cite throughout. III. BACKGROUND Before getting to the heart of our arguments, it is important to provide a few ideas and terms that help understanding of various issues. In each case, we provide a number of references that will provide more detailed background for those needing additional information. A. Underwater acoustics One of the fundamental issues with regard to any experiments on fish hearing is the nature of sound in water, and, in particular, sound in tanks. As a reminder, sound originates as a local mechanical disturbance generated by the movement or vibration of any immersed object, and results from the inherent elasticity of the surrounding medium. Sound consists of a traveling energy wave, within which the component particles of the water are alternately forced together and then apart. The to-and-fro motion that constitutes a) This paper is part of a special issue on The Effects of Noise on Aquatic Life. b) Electronic mail: apopper@umd.edu 948