Examining the Potential Effects of the North Atlantic Oscillation (NAO) on Humpback Whale Site Fidelity and Residence on Jeffreys Ledge

Photo identification methods have allowed researchers to monitor humpback whale (Megaptera novaeangliae) populations and site fidelity in the Gulf of Maine (GOM) for over 35 years, and have demonstrated that humpback whales commonly exhibit strong annual philopatry when returning to feeding grounds such as Jeffreys Ledge, a region characterized as a rocky submarine ridge close to a basin edge in the Gulf of Maine. Recent studies of site fidelity using sightings data collected between 1996 and 2005 have shown large fluctuations in the number of animals observed at these two sites.

The distribution and presence of humpback whales on feeding grounds are intimately linked to a complexity of oceanographic variables that influence annual productivity. In the Gulf of Maine, the interaction between many of these variables can be described by one simple index termed the North Atlantic Oscillation (NAO). In this study, we hypothesized that: (1) Humpback whale residency on Jeffreys Ledge may shift due to fluctuations in the NAO; (2) Humpback whale resightings may change annually due to shifts in the NAO; (3) Humpback whale resightings may be influenced by length of residency that occurs on Jeffreys Ledge in previous years.

Different trends and correlations between residency and the NAO index for Jeffreys Ledge were demonstrated though the visual examination of locational data for re-identified humpback distributions and statistical regression analyses. The number of resighted animals were strongly associated with an NAO index delayed by 3 years (R2=0.8676), implying that oceanographic variables may influence an ecosystem incrementally and accumulatively in subsequent years as a trophic "cascade" effect.