Seal bomb explosions disrupt harbor porpoise (Phocoena phocoena) echolocation in Monterey Bay

Small explosions (“seal bombs”) have been used for decades in some California purse seine fisheries to deter depredation by pinnipeds or to aggregate catch. Nearby cetaceans can also be exposed to high intensity impulses from these explosions. Harbor porpoises (Phocoena phocoena) are known to be particularly sensitive to anthropogenic sound compared to other odontocetes, and the small, localized population in Monterey Bay, California may be vulnerable to adverse impacts associated with displacement or interruption of foraging. To investigate the response of harbor porpoises to explosions, we collected acoustic recordings at five sites around Monterey Bay, California from August 2019 to February 2020. Explosions were detected in clusters over consecutive hours and days, with 83% of detections at night, and 99% on weekdays (Monday – Friday). We report spectral and temporal characteristics of explosions, including noise metrics relevant to management. Explosion sound exposure levels at the instruments were at or near the threshold for temporary hearing loss in porpoises (140 dB re: 1 uPa2s), although the actual received levels for exposed porpoises are unknown. High levels of porpoise acoustic activity were documented at Sunset State Beach, where porpoises were present every day during the study. Permutation tests show significantly reduced porpoise activity following explosions, with porpoise activity returning to normal levels after 24-36 hours, depending on the site. Random forest models indicate both vessel noise and explosions were important predictors of the absence of porpoise acoustic detection, although the time of night and lunar phase may be more important. Specific behavioral response studies would be useful to understand behavioral drivers. Further effort is needed to investigate the energetic and population level consequences of disturbance for harbor porpoises.