Temporary Hearing Threshold Shift in California Sea Lions (Zalophus californianus) Due to One-Sixth-Octave Noise Bands Centered at 0.6 and 1 kHz

Abstract

To determine the frequency-dependent susceptibility of California sea lions (Zalophus californianus) to noise-induced temporary hearing threshold shift (TTS), one of two subjects were exposed for 60 minutes to two continuous one-sixth-octave noise bands (NBs) as fatiguing sounds: one centered at 0.6 kHz, at sound pressure levels (SPLs) of 168 to 174 dB re 1 µPa (sound exposure levels [SELs] of 204 to 210 dB re 1 µPa2s), or one centered at 1 kHz, at SPLs of 144 to 159 dB re 1 µPa (SELs of 180 to 195 dB re 1 µPa2s). Using a psychoacoustic technique, TTSs were quantified at 0.6, 0.85, 1, 1.2, 1.4, and 2 kHz (at the center frequency of each NB, half an octave higher, and one octave higher). When significant TTS occurred, higher SELs resulted in greater TTSs. In the sea lion that was tested 1 to 4 minutes after exposure to the fatiguing sounds, the largest TTSs occurred when the hearing test frequency was half an octave higher than the center frequency of the two fatiguing sounds. The highest TTS levels elicited were 8.7 dB at 0.85 kHz and 9.6 dB at 1.4 kHz. When their hearing was tested at the same time after the fatiguing sounds stopped, initial TTSs and hearing recovery patterns were similar in both sea lions. These findings will contribute to the protection of hearing of species in the Otariidae family from anthropogenic noise by facilitating the development of an evidence-based underwater sound weighting function.

Temporary Hearing Threshold Shift in California Sea Lions (Zalophus californianus) Due to a Noise Band Centered at 32 kHz

“…(1) 0.6 and 1 kHz (Kastelein et al, 2022b), (2) 2 and 4 kHz (Kastelein et al, 2021b), (3) 8 and 16 kHz (Kastelein et al, 2022a), (4) 32 kHz (present study), and (5) 40 kHz (ongoing).…”

“…Even when their heads were completely out of the water during occasional surfacings (low jumps), the California sea lions were exposed to the fatiguing sound transmitted from the water, as demonstrated by the SPLs measured in air during sound exposure (Table 1). In addition, the building around the pool had hard inner surfaces, which caused the SPL in air 30 cm above the water surface to be relatively homogeneous (Kastelein et al, 2022b), 2 and 4 kHz (Kastelein et al, 2021b), 8 and 16 kHz (Kastelein et al, 2022a), and 32 kHz (present study) that caused 6 dB TTS1-4 in F01 () and 6 dB TTS12-16 in M02 (). No TTS12-16 could be elicited in M02 after exposure to the NB at 0.6 kHz.…”

“…Short breaks in the fatiguing sound can allow hearing to recover and may result in significantly smaller TTSs than occur after exposure to fatiguing sounds without breaks (Kastelein et al, 2022a(Kastelein et al, , 2022b. Fatiguing sounds in the present study were continuous (100% duty cycle), but breaks in exposure may have occurred when the California sea lions lifted their heads out of the water to take breaths.…”

“…The subjects had hearing thresholds that were similar to those of other California sea lions (Reichmuth et al, 2013;Kastelein et al, 2023) and, thus, are representative of the species. Both sea lions had participated in previous TTS studies and a basic audiogram study (Kastelein et al, 2021b(Kastelein et al, , 2022a(Kastelein et al, , 2022b(Kastelein et al, , 2023.…”

“…Understanding the parameters of sounds that cause TTS and the consequences for California sea lions of experiencing TTS will help regulatory agencies determine safe and acceptable noise exposure levels for this species, and perhaps for other species of the Otariidae family (as suggested by Houser et al, 2017, andSouthall et al, 2019). TTS in California sea lions has previously been studied by Kastak et al (1999Kastak et al ( , 2005, Finneran et al (2003), and Kastelein et al (2021bKastelein et al ( , 2022aKastelein et al ( , 2022b. Comparison with studies on other taxa will help elucidate general principles of TTS in marine mammals (e.g., harbor seals [Phoca vitulina] and harbor porpoises [Phocoena phocoena]; Kastelein et al, 2012aKastelein et al, , 2012b.…”