Silencing the Shore: The Effects of Onshore Anthropogenic Noise on Marine Mammal Abundances Informed by Mixed Methods

Abstract We undertook a mixed-methods study to investigate the recent decline in abundance of a colony of Harbor Seals ( Phoca vitulina ) carefully documented by a community science group in Pacific Grove, California, USA. Combining a generalized additive modeling (GAM) analysis with qualitative analysis of data collected through semi-structured interviews, we analyzed abundance trends between 2003 and 2023, documented Local Ecological Knowledge (LEK) about noisy anthropogenic disturbances and their impact on Harbor Seal behavior and reproductive success, and investigated hypotheses about Harbor Seal abundance trends at the study site. We found that LEK identified a range of potential anthropogenic disturbances whose cumulative effect might explain the decline of the Harbor Seal abundance in recent years. We also found that, in 2022 and 2023, Harbor Seal abundance was 29.81% lower than predicted given the probability of occurrence of forage fish, an important ecological determinant of Harbor Seal abundance. Roadwork construction that occurred in 2022 near the harbor seal colony during the early pupping season may have had significant adverse effects on reproductive success and, consequently, on Harbor Seal abundance at the study site as the “final straw” in an environment of increasing disturbance. This study demonstrates (1) the potential demographic effects of anthropogenic disturbance on marine mammals, (2) the value of mixed-methods design, qualitative interview methods, and community-based science in understanding ecological dynamics in complex social-ecological coastal systems, and (3) the important of mitigating anthropogenic disturbance to sensitive marine mammal colonies.