MARIANA ISLANDS RANGE COMPLEX ... - Guam Legislature

MARIANA ISLANDS RANGE COMPLEX FEIS/OEIS MAY 2010
able to discriminate sounds of different intensities and frequencies (reviewed in Fay and Megela-
Simmons 1999; Popper et al. 2003) and perform sound source localization (reviewed in Popper et al.
2003; Fay 2005).
Fish are also able to detect signals in the presence of background noise (reviewed in Fay and Megela-
Simmons 1999; Popper et al. 2003). The results of these studies show that fish hearing is affected by the
presence of background noise that is in the same general frequency band as the biologically relevant
signal. In other words, if a fish has a particular threshold for a biologically relevant sound in a quiet
environment, and a background noise that contains energy in the same frequency range is introduced, this
will decrease the ability of the fish to detect the biologically relevant signal. In effect, the threshold for the
biologically relevant signal will become poorer.
The significance of this finding is that if background noise is increased, such as a result of humangenerated
sources, it may be harder for a fish to detect the biologically relevant sounds that it needs to
survive.
Sound Detection Mechanisms. While bony and cartilaginous fish have no external structures for
hearing, such as the human pinna (outer ear), they do have an inner ear which is similar in structure and
function to the inner ear of terrestrial vertebrates (i.e., inner ear with sensory hair cells). The outer and
middle ears of terrestrial vertebrates serve to change the impedance of sound traveling in air to that of the
fluids of the inner ear. However, since fish already live in a fluid environment, there is no need for
impedance matching to stimulate the inner ear. At the same time, since the fish ear and body are the same
density as water, they will move along with the sound field. While this might result in the fish not
detecting the sound, the ear also contains very dense calcareous structures, the otoliths, which move at a
different amplitude and phase from the rest of the body. This provides the mechanism by which fish hear.
The ear of a fish has three semicircular canals that are involved in determining the angular movements of
the fish. The ear also has three otolith organs, the saccule, lagena, and utricle, that are involved in both
determining the position of the fish relative to gravity and detection of sound and information about such
sounds. Each of the otolith organs contains an otolith that lies in close proximity to a sensory epithelium.
The sensory epithelium (or macula) in each otolith organ of fish contains mechanoreceptive sensory hair
cells that are virtually the same as found in the mechanoreceptive cells of the lateral line and in the inner
ear of terrestrial vertebrates. All parts of the ear have the same kind of cell to detect movement, whether it
be movement caused by sound or movements of the head relative to gravity.
Hearing Generalists and Specialists. Very often, fish are referred to as “hearing generalists” (or
nonspecialists) or “hearing specialists” (e.g., Fay 1988; Popper et al. 2003; Ladich and Popper 2004).
Hearing generalists generally detect sound to no more than 1 to 1.5 kHz, whereas specialists are generally
able to detect sounds to above 1.5 kHz (see Figure 3.9-1). And, in the frequency range of hearing that the
specialists and generalists overlap, the specialists generally have lower thresholds than generalists,
meaning that they can detect quieter (lower intensity) sounds. Furthermore, it has often been suggested
that generalists only detect the particle motion component of the sound field, whereas the specialists
detect both particle motion and pressure (see Popper et al. 2003).
However, while the terms hearing generalist and specialist have been useful, it is now becoming clear that
the dichotomy between generalists and specialists is not very distinct. Instead, investigators are now
coming to the realization that many species that do not hear particularly well still detect pressure as well
as particle motion and pressure. However, these species often have poorer pressure detection than those
fish that have a wider hearing bandwidth and greater sensitivity (see Popper and Schilt 2008).
FISH AND ESSENTIAL FISH HABITAT 3.9-27