Relative influence of climate variability and direct anthropogenic impact on a sub-tropical Pacific top predator, the Hawaiian monk seal

Abstract

Abundance trends of Hawaiian monk seals Monachus schauinslandi at 6 subpopulations throughout the Northwestern Hawaiian Islands (NWHI) have fluctuated considerably during the past 5 decades. Direct human impacts have been widely accepted as the primary cause of previous monk seal population declines and, consequently, much conservation effort has focused on minimizing these threats. Yet climate variability has been increasingly identified as a factor influencing monk seal demography. We endeavored to evaluate the relative influence of climate versus direct anthropogenic impacts on historic trends in monk seal populations. Periods of growth and decline in monk seal abundance at 4 subpopulations were associated with positive and negative phases, respectively, of the Pacific Decadal Oscillation (PDO). Moreover, PDO regime shifts were followed, after approximately 2 yr lags, by corresponding changes in monk seal abundance trajectories, especially in the absence confounding human impacts. We propose that the PDO is a proxy for varying productivity in the northern NWHI, the effects of which propagate vertically through the food web and are reflected in top predators such as the monk seal. Our results suggest that long-term dynamics of monk seal populations may have been driven as much, if not more, by climate-ocean variability as by direct human activity. In recent years, direct human impacts on monk seals have been virtually eliminated from the NWHI. As such, previous and continued detailed monitoring of Hawaiian monk seal demographics offer a unique opportunity to document and understand how future anticipated global climate change will affect marine ecosystems in the central North Pacific.