Vessel-Based Habitat Studies

During the winter months the wide, shallow bay, protected by the arm of Cape Cod, creates the perfect habitat for blooms of phytoplankton, the basis of a complicated and poorly understood food web. Right whales, searching for food, become part of this habitat. For over a decade, PCCS has looked into the factors that make Cape Cod Bay a critical habitat for right whales. It is hoped that a clearer understanding of the habitat, and threfore the whale, will inform conservation efforts.

Cape Cod Bay is not the only known feeding ground of right whales but it is the only known habitat where the whales are commonly found feeding at the surface (in the Bay of Fundy, right whales generally feed at depth, making observations of this behavior almost impossible). The unique situation of Cape Cod Bay affords researchers with the opportunity to do fine scale studies of right whale habitat requirements. Using our research vessel Shearwater, regular stations within Cape Cod Bay are monitored for a series of environmental characteristics, including: salinity, ambient light, temperature, phytoplankton and zooplankton. Coupled with sightings of right whales (or the lack of sightings) these environmental factors create an image of how right whales use the Bay.

What has been found is that right whales feed upon a host of zooplankton species, most notably, the crustaceans called copepods. The occurrence of different copepods is tied to such environmental factors as temperature, salinity and availability of their prey, including phytoplankton and other zooplankton. Since all of these factors change through the season and throughout the Bay, the diversity and density of copepods differs over time and space. This patchiness of copepods affects the movements of feeding right whales. The habitat studies team has begun to define the composition of these patches (their size and density), and their occurrence throughout the Bay.

This long term sampling of the Bay environment has created some surprisingly precise results. For example, right whales may begin feeding when the density of zooplankton reaches 3,750 organisms per cubic meter of water. In other words, right whales may not bother to open their mouths to filter plankton until that threshold is reached. Further, by using energetics models, right whales may not “break even” (where the whale is gaining more energy from the food than is lost to getting that food) until the density of copepods reaches 6600 organisms per cubic meter. And density is not the only factor. Right whales seem to show a preference for certain species of zooplankton, especially copepods. In particular, certain species of Calanus are favored. Thick patches containing Calanus may sustain aggregations of right whales for days or weeks. When Calanus is not available, right whales may target Centropges and Pseudocalanus or even Oithona and Acartia. Any one of these species may be numerous at different times and different areas over the Bay. The composition can even change throughout the 24 hour cycle (see these abstracts).

How do these patches form? How do whales find these patches? Why does the composition of zooplankton change from year to year? How would changes in the health of the Bay affect zooplankton and therefor the population of right whales? What is the carrying capacity of the Bay (how many whales can the habitat support)? Only through long term research can we hope to answer such questions. Certainly, these questions are not purely academic; they may go a long way to saving one of the rarest animals on earth.