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Evaluating threats to marine mammals from vessel traffic in an urbanized estuary
Vessels pose a number of threats to marine mammals, especially where areas of heavy traffic overlap with their habitat. One such location is San Francisco Bay where large shipping vessels, high-speed ferries, and recreational craft transit a highly developed urban estuary with a growing number o f species. Vessel presence has the potential to disturb marine mammal activity, high vessel speed can increase the risk of lethal collisions, and underwater vessel noise can inhibit the biological uses o f sound for these animals. To evaluate these potential risks, we utilized vessel data from the Marine Monitor, a vessel tracking system that integrates data from the Automatic Identification System and a marine-radar sensor linked to a high-defmition camera to include both commercial and recreational vessels. Acoustic data were collected from a hydrophone near the Marine Monitor. We found that commercial traffic traveled in distinct paths while recreational traffic was more dispersed, and some traffic traveled at speeds known to increase collision risk. We also found that large ships had the highest source levels in both low- and high-frequencies, but ferries and motorized recreational craft contributed the highest daily cumulative sound exposure levels to the largest area. Incorporating data from recreational vessels allowed for a complete assessment o f all relevant vessel types which revealed the pervasiveness of recreational traffic. Results of this research car inform risk assessments and habitat suitability models for marine mammals in San Francisco Bay that seek to identify locations and times of likely vessel and habitat overlap.