Anthropogenic noise has been identified as a major threat for the recovery of the endangered Cook Inlet beluga, Delphinapterus leucas, population, but little is known about its occurrence in critical habitat. We analyzed 8,756 h of acoustic recordings from different locations and months in Cook Inlet, Alaska, to describe anthropogenic noise, evaluate the potential for acoustic impact to Cook Inlet belugas, and provide management recommendations. Nine total sources of noise were identified: commercial ship, dredging, helicopter, jet Aircrafts (commercial or military non-fighter), fighter jet, propeller Aircrafts, outboard motor, pile driving, sub-bottom profiler, as well as four repetitive unidentified noise sources. Several noise metrics were compared across noise sources, months, and locations. A total of 6,263 anthropogenic acoustic events were detected and classified, for a total duration of 1,025 h representing 11.7% of the sound recordings analyzed. Anthropogenic noise was present in every day sampled. The natural background noise (i.e., ambient noise without anthropogenic contribution), in areas where currents and vegetative debris did cause minimal self-noise, was below 120 dBrms re. 1μPa for 94% to 100% of the time; much lower than previously reported and unsupportive of the current application of the elevated behavioral harassment threshold for non-impulsive noise of 125 dBrms. Based on spectral received levels and spatial and temporal prevalence, several anthropogenic noise sources in Cook Inlet have the potential to chronically mask beluga communication and hearing in most of the locations and periods sampled for this study. Current activities (e.g., shipping, dredging) exceed behavioral harassment levels on a daily basis in a significant portion of the critical habitat. Ship noise was identified as the top priority focus for noise mitigation management actions. A high concentration of noise sources was identified in the lower region of Knik Arm. This area is recommended for further research to evaluate the potential for beluga displacement and the basis to start considering cumulative impact effects in the permitting process. The amount of detected unidentified machinery noise or unclassifiable noises highlights the need for further research in coordination with industry, in particular in areas of oil and gas production (i.e., Trading Bay) or intense shipping related activities (i.e., facilities). Finally, unpermitted activities were also detected suggesting the need for further enforcement and outreach to increase the awareness towards this endangered population of belugas and the negative consequences of underwater noise in their protected habitat.
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