Researchers mapping several species of whales and dolphins converging towards their fish prey reveal the existence of species-specific foraging zones in the Atlantic Ocean. The findings, published in Nature this week, help us better understand marine mammal distribution, behavior, and interaction with prey species.
Observing marine mammal populations continuously over the expansive ocean areas they inhabit has been challenging. Northeastern University’s Purnima Ratilal and colleagues turned to a technique called passive ocean acoustic waveguide remote sensing to map the vocalizations and spatial distributions of diverse dolphin and whale species – including blue, fin, humpback, sei, minke, sperm, pilot, and killer whales – over a 100,000-kilometer-square herring feeding ground in the Gulf of Maine. The acoustic recordings were obtained using a 160-hydrophone array that was towed by a research vessel along designated tracks north of Georges Bank from September 19 to October 6 of 2006. That coincided with the annual herring spawning period.
By instantaneously detecting, localizing, and classifying marine mammal vocalizations from multiple species, the team found that predators divide the shoal into overlapping foraging sectors – which are species-specific but with varying degrees of spatial overlap. And these sectors are maintained for at least two weeks of the herring spawning period. The vocalization rates of all the marine mammals studied here follow a 24-hour cycle, with some being more vocal at night and others during the daytime.
With so many marine mammals, mix-ups can happen – especially for the smaller toothed whales. “But our study shows that there is much higher likelihood that individuals from the same species will tend to group together,” Ratilal tells IFLScience in an email. “That could be the reason for their focused species-specific feeding areas.”
These findings add to our knowledge of the dynamics (in both time and space) of the combined foraging activities of multiple marine mammal species in the vicinity of an extensive fish prey field – which turned out to be a massive ecological hotspot. The researchers hope that their findings will improve the understanding of our impacts on marine protected species and help with the management of their ecosystems.
A humpback whale photographed during the Ocean Acoustic Waveguide Remote Sensing Gulf of Maine Fall 2006 Experiment. Michael T. Einhorn