Knowing How Well Fish Can Hear
At the Mechanosensory Physiology Laboratory of University of Kentucky, United States, Hong Young Yan has been learning how well marine and freshwater fish can listen to sounds in their natural habitats
With recent rapid advancement in technology in manufacturing underwater sound listening devices (e.g., hydrophone) and analysis of these signals, passive listening to sounds produced by fishes has been used extensively to map out the distribution of fish in their natural habitats. In particular, the mapping of spawning grounds of sonic fish and the use of these information to prevent man-made noise to disturb the spawning activities is crucial in protecting the spawning stock of fish. However, it is important to know that most fish of commercial fishery values are mute yet are sensitive to all sorts of underwater sounds, including man-made noises from boat engines, offshore drilling activities, etc.
To understand how well fish can respond to either natural or man-made sounds, it is imperative that a non-invasive yet very simple and quick electrophysiological method be used to study how well fish can hear. It is found that animals can generate a series of very specific brainwaves in response to acoustic stimuli received. Hong Young Yan has taken advantage of this unique physiological responses to gauge how well fish can hear underwater sound by using a simple setup (Figure 1, ) which consists of two micromanipulators to hold recording electrodes to record brainwaves from fish when it responds to sound signals generated from a speaker (as seen on the top of Fig. 1). The recorded signals are first amplified by a pre-amplifier before sending to a computer for further processing. A close-up view of Figure 2 shows in order to record brainwaves from the test subject, fish has to be sedated and fastened to a holding device. A respirator tube which provides well-oxygenated water is used to irrigate the gill during recording with an aim to keep fish alive throughout the recording session. Two silver-wire electrodes are placed and in close contact with dorsal part of the fish skull in order to record brainwaves in response to sound stimuli given. As illustrated in Figure 2, this recording method is non-invasive and the same fish can be used repeated for various treatment uses.
The brainwaves in response to acoustic stimuli can be visualized easily on a computer screen as seen in Figure 3. A typical brainwave consists of a series of up and down peaks that can be viewed on the computer screen. By applying various frequencies and intensity of sounds to tested subject, hearing ability of a species fish can be easily obtained within 2-3 hours.
So far hearing ability of more than 30 species of fish has been investigated with the use of this electrophysiological recording technique. Since its inception, the technique has been adopted in many laboratories around the world. The information obtained by such method can supplement the information that can not be yielded by the passive listening method. In addition, another direct implication of this research method is the understanding of how man-made sound such as boat engine noise can impact overall change of hearing ability of fish. Hong Young Yans laboratory has demonstrated that exposure to high intensity noise as short as 2 hours can alter hearing ability of some fishes and the recovery of hearing ability takes up to 5-7 days to return to normal.