Harmful effects of underwater sound on fish.

The acoustic impedance of aquatic animals nearly matches that of water; consequently, most of the sound energy from an underwater source will enter and interact with their bodies. Very intense sounds may even stun or kill fish. Several studies [see summary by K. S. Norris and B. Mohl, Am. Nat. 122, 85–104 (1983)] have reported easily observable damage such as stunning and rupturing of internal organs, caused by underwater sound. Available data indicate, however, that fish may suffer internal morphological and physiological damage as well as apparent physical damage when exposed to sufficient sound pressure levels for relatively short periods of time. Results of experiments relating underwater sound pressure level of pure tones at frequencies from 100 to 500 Hz with different types of damage to three species of freshwater fish are presented. The results show that fish may have internal morphological damage (i.e., destruction of the apical ciliary bundles of hair cells in the inner ear) in the absence of external physical or behavioral changes. Thus, harmful sound pressure levels may be much lower than those commonly reported.