For a few weeks in early fall, Georges Bank--a vast North Atlantic fishery off the coast of Cape Cod--teems with masses of herring that take over the region to spawn. The seasonal arrival of the herring also attracts predators to the shallow banks, including many species of whales.
Researchers from the Massachusetts Institute of Technology, Northeastern University, the National Oceanic and Atmospheric Administration, and Norway's Institute of Marine Research have found that, as multiple species of whales feast on herring, they tend to stick with their own kind, establishing species-specific feeding centers along the 150-mile length of Georges Bank.
Based on acoustic data they collected in the region, the researchers identified and mapped the calls of various whales, and discovered a clear grouping of species within the dense herring shoals: humpback whales congregated in two main clusters, at either end of the spawning grounds, while minke, fin, sei, and blue whales set up feeding territories in the space in between.
In general, calls from each whale species increase dramatically at night, when herring tend to form extremely dense shoals. During the day, these whale calls dissipate, as herring scattered throughout the seafloor.
"It's known that different marine mammal species will eat fish, but no one has mapped their simultaneous feeding distributions over these huge scales," says Purnima Ratilal, associate professor of electrical and computer engineering at Northeastern University. "Maybe there is some territorialism going on, or maybe they are preferentially selecting these locations based on their different foraging mechanisms. That's material for new research."
Ratilal and her husband, Nicholas Makris, professor of mechanical engineering at MIT, along with their students, are coauthors of the study.
Ratilal and Makris led a two-week cruise to Georges Bank, initially to track and study the behavior of populations of herring, which can number in the billions within a single shoal. The team had developed a remote-sensing system that uses acoustics to instantaneously image and continuously monitor fish populations over tens of thousands of square kilometers. Unlike conventional technologies, their system uses the ocean as a waveguide through which acoustic waves can travel over much greater distances, to sense the marine environment.
To get a much wider, more-detailed view of the herring populations, Ratilal and Makris deployed 160...