Distribution, timing of aggregation, and foraging processes in an ecological hotspot: aggregation of humpback whales at the southern flank of Stellwagen Bank

Jesús Pineda, Victoria Starczak and José da Silva, Biology

Abstract

Cetaceans, sand lance, and elasmobranchs are major components of the Massachusetts Bay pelagic
community. Even though the distributions of each group have changed over the decades, these
organisms converge in relatively high abundance at Stellwagen Bank’s (SB) southwest corner. This
location is arguably one of the most ecologically important areas within the Stellwagen Bank National Marine Sanctuary (SBNMS). The southern flank of SB lies within the Northern Right Whale Critical Habitat Area, and is heavily fished commercially and recreationally. It is not clear what processes makes SB’s southern flank so important ecologically, but it may be related to the shallow topography and potential trapping of zooplankton and the frequent and predictable non-linear internal waves (NLIWs) that shoal onto the bank’s southern flank. Thus, SB’s southern flank is a hotspot in the sense that density of key species is high, and abundant phenomena take place.

Humpback whales tend to concentrate at the southern and northern flanks of Stellwagen Bank, although distribution patterns vary from daily to inter-annually. The dynamic distribution of humpback whales at Massachusetts Bay may be related to foraging, but understanding of this process is lacking, and the ecological processes at the sites where whales and prey aggregate are not well understood. Stellwagen Bank humpback whales feed primarily on sand lance, a small fish found in sandy bottoms at night that emerges to open waters during the day to feed on zooplankton (mostly copepods). Whales feed on sand lance at the bottom and in the water column, and the dominant foraging mode of whales, benthic or pelagic, varies interannually (D. Wiley, pers. obs.). Humpback whales also feed near the surface, sometimes when NLIWs are present.

In this proposal, we focus on humpback whales Megaptera novaeangliae and their prey, sand lance
Ammodytes spp. to further understand how they interact, as well as their response to physical processes that makes SB’s southern flank so rich in resources and phenomena. We hypothesize that whale concentration is related to (1) zooplankton and sand lance aggregation by predictable shoaling internal waves, and (2) by availability of sand lance. Our study will continue observations documenting interannual variability in humpback whales at SB south flank, and these observations will address whether interannual variability in distribution in Massachusetts Bay is related to foraging. In situ observations will be completed from a vessel anchored at SB southern flank. Internal waves will be measured using acoustic backscatter, temperature, circulation, and will also be measured from satellites with Synthetic Aperture Radar. Whale occurrence and behavior will be assessed by expert observers, and fish will be measured with acoustic backscatter and live video.
Whale distribution at Massachusetts Bay is an issue of societal concern, with implications for large ship navigation, and a source of revenues for the whale watching fleet. By investigating specific processes (e.g. foraging response), and at the patterns of interannual variability in distribution, our study will contribute to untangling the complex web of processes influencing the ecology and distribution of humpback whales and their preferred prey.