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Pinniped hearing in a changing acoustic environment


Increasing levels of anthropogenic noise in the world’s oceans are a matter of concern for the conservation of pinnipeds (seals, sea lions, and walruses). Sound from noise-generating human activities—such as marine construction, oil exploration, and shipping traffic—may alter the lives of pinnipeds in a variety of ways. For example, anthropogenic noise can interfere with an individual’s ability to detect calls from conspecifics or the sound of an approaching predator; and loud, aversive sounds can cause animals to abandon preferred habitats. Laboratory studies of hearing in pinnipeds over the past 50 years have expanded knowledge of the basic auditory capabilities of certain species, and data from these studies are currently being used to predict how specific noise-generating activities may affect wild animals. However, predicting the effects of anthropogenic noise is a complex process, and certain knowledge gaps must be addressed before accurate predictions can be made. First, it is unclear how well standard hearing data—which are generated using simple, synthetic sounds—can predict pinniped auditory sensitivity to spectrally and temporally complex anthropogenic sounds. Second, hearing sensitivity data above 80 kHz are scarce for most species, which is problematic given the recent proliferation of high-frequency, high-energy acoustic marine technologies such as commercial sonar and recreational fish finders. The research described in this dissertation addresses these two knowledge gaps using behavioral hearing experiments with trained pinniped subjects combined with mathematical models of hearing and sound propagation.

The data presented here improve the ability of regulators to predict the effects of anthropogenic noise on pinnipeds and indicate future research directions. The reported sensitivity data for complex sounds show that certain spectral and temporal features can significantly alter detectability. Comparison of detection thresholds for complex stimuli to predictions from hearing models based on standard hearing metrics showed differences of up to 8 dB in both quiet and noisy conditions. Such discrepancies indicate the need for future research into how complex features influence the detectability of underwater sound. High-frequency sensitivity data for two seals and one sea lion show that these animals can detect underwater sound at frequencies well above their traditional high-frequency hearing limits, with all three subjects able to detect tonal signals centered at 180 kHz. The shapes of the sensitivity profiles for all subjects indicate that the idea of a traditional high-frequency hearing limit is problematic for underwater pinniped listeners, a fact that regulators must take into account.

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