| Summary: | Most physical oceanographic data and some biological data used in this study are available through servers supported by federal and state agencies, as specified in the Methods section. Glider data are available from UCSD: https://spraydata.ucsd.edu/ projects/CUGN/. All other data presented have been deposited in a repository and are available at: https://doi.org/10.6084/m9.figshare.c.4584026.v1. The article of record as published may be found at https://doi.org/10.1371/journal.pone.0222456 This study examines the occurrence of humpback whale (Megaptera novaeangliae) song in the northeast Pacific from three years of continuous recordings off central California (36.713˚N, 122.186˚W). Song is prevalent in this feeding and migratory habitat, spanning nine months of the year (September–May), peaking in winter (November–January), and reaching a maximum of 86% temporal coverage (during November 2017). From the rise of song in fall through the end of peak occurrence in winter, song length increases significantly from month to month. The seasonal peak in song coincides with the seasonal trough in day length and sighting-based evidence of whales leaving Monterey Bay, consistent with seasonal migration. During the seasonal song peak, diel variation shows maximum occurrence at night (69% of the time), decreasing during dawn and dusk (52%), and further decreasing with increasing solar elevation during the day, reaching a minimum near solar noon (30%). Song occurrence increased 44% and 55% between successive years. Sighting data within the acoustic detection range of the hydrophone indicate that variation in local population density was an unlikely cause of this large interannual variation. Hydrographic data and modeling of acoustic transmission indicate that changes in neither habitat occupancy nor acoustic transmission were probable causes. Conversely, the positive interannual trend in song paralleled major ecosystem variations, including similarly large positive trends in winddriven upwelling, primary productivity, and ...
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