Reconstructing the Historical Abundance and Spatial Distribution of Pacific Harbor Seals (Phoca vitulina) in Monterey Bay, California

Abstract

Marine mammal species were heavily targeted by hunters during the 1900s, decimating populations along the West Coast of the United States. Many have since recovered, aided by the Marine Mammal Protection Act (MMPA) of 1972. The Pacific harbor seal (Phoca vitulina) is among those recovered and is now commonly seen hauled out on beaches, particularly in Monterey Bay. However, local monitoring of this recovery has been limited, with scarce regional abundance data.

This study draws on 28 previously unexamined journals authored by Alan Baldridge, librarian at Stanford’s Hopkins Marine Station, which contain detailed seal counts from 1966 onward. Using these records and supplemental census data, this project reconstructs the historical abundance and spatial distribution of Pacific harbor seals in Monterey Bay and analyzes the influence of environmental and anthropogenic factors on their haul-out behavior.

Results indicate that seal counts have increased from an average of ~45 in the late 1960s to over 150 in recent years, with notable spikes in 1969, 1974, 1982, 1989, and the early 2000s. Spatial analysis revealed a shift in seal density from the western to the northern Peninsula during the late 1970s. Key environmental drivers included storm intensity, prey availability, and ENSO events. Anthropogenic variables showed no significant correlation, likely due to data limitations.

These findings underscore the sensitivity of harbor seals to localized environmental variability. As climate change accelerates shifts in upwelling, prey dynamics, and storm regimes, understanding these fine-scale behavioral responses will be essential for effective conservation planning.