batch_auto_20260713 202607131256465730 BOEM ESP geoESPIS ESPIS@boem.gov Bureau of Ocean Energy Management Low-frequency Hearing in Pinnipeds University of California Santa Cruz University of California Santa Cruz The objective of the present study was to obtain underwater hearing and masking data for seals and sea lions, filling existing data gaps regarding pinnipeds and low-frequency noise exposures. This work was an opportunistic expansion of scheduled research originally funded under a related award from the International Association of Oil and Gas Producers’ (IOGP) E&P Joint Industry Programme (JIP) on Sound and Marine Life. This effort involved direct evaluation of the auditory sensitivity of representative seals and sea lions at low frequencies between 200 Hz and 40 Hz using a behavior-based psychoacoustic approach. Two bearded seals (Erignathus barbatus) and one California sea lion (Zalophus californianus) living in human care were trained to cooperatively participate in behavioral hearing tests using operant conditioning and positive reinforcement methods. Hearing tests were conducted in highly controlled acoustic conditions under water, both in quiet conditions (to characterize absolute hearing sensitivity) and in the presence of controlled noise (to determine key masking parameters). The resulting auditory sensitivity thresholds, critical ratios, and critical bandwidths are presented in this report, along with supplemental data describing temporal processing capabilities at low frequencies. 199937 U.S. Department of the Interior, Bureau of Ocean Energy Management, Environmental Studies Program https://doi.org/10.13039/100012475 M22AC00005 10.67297/9A1EC1 https://boem.gov/awardpages/100417