Right whales feed on zooplankton, primarily copepods, tiny drifting animals approximately the size of a small grain of rice. Because of this specialized diet, right whales must locate feeding areas where copepods are concentrated into high-density patches. URI oceanographer Robert D. Kenney is studying how right whales find their feeding grounds and, once there, how they locate dense zooplankton patches. He conducted his study with Charles A. “Stormy” Mayo of the Center for Coastal Studies in Provincetown, Massachusetts, and the late Howard Winn of the URI Graduate School of Oceanography.
Kenney has estimated that zooplankton patches where right whales feed must reach concentrations on the order of tens to hundreds of thousands per cubic meter in order for the whales to obtain a net energy benefit from feeding. As reported in a recent special issue of the Journal of Cetacean Research and Management devoted entirely to right whales, copepod densities of that magnitude have rarely been measured in the North Atlantic. Although the actual extent of such high-density zooplankton patches in the western North Atlantic is very poorly known, some of the highest densities have been measured near feeding right whales.
“It is clearly of interest to determine how right whales locate the dense zooplankton patches within their feeding grounds,” said Kenney. “It may provide insight into how and why right whales become entangled in fishing gear and how they may cope with potential changes in prey distribution caused by climate change.”
Right whales use a number of strategies to locate feeding grounds and prey concentrations. Some have to do with long-distance seasonal migrations over weeks and months, and others depend on minute-by-minute selection of the optimal prey patches within a particular feeding area. Kenney has compiled and presented a variety of hypotheses resulting in a conceptual model of the mechanisms and strategies that may be involved in the annual cycle of distribution, migration, movements and foraging of the whales.
Over long-distance migrations of 1,000 kilometers or more, the right whales’ navigation mechanisms are probably more related to geography than to prey distributions. Kenney and other scientists have speculated that right whales simply follow the “lay of the land,” such as the coastline, the continental shelf edge, or undersea landmarks such as seamounts, during their migration between their wintering grounds and their feeding grounds in the Gulf of Maine region. Studies have also suggested that whales use the sun or the Earth’s magnetic field for migratory cues. Right whales are probably capable of hearing and localizing the direction of very low-frequency sounds, such as those produced by surf. Right whales could also use ocean current patterns, either for directional cues or for a locomotory assist in one or both directions.
On a regional scale of ten to hundreds of kilometers, the primary hypothesis regarding how a right whales find the location of feeding grounds once they arrive in the vicinity of the Gulf of Maine from their winter habitat is that they return to particular feeding grounds based largely on prior experience, indicating that learning is important. Environmental cues, including temperature, currents, salinity, water stratification and chemistry, could assist whales in finding previous feeding grounds.
Once whales are in the general area of their Gulf of Maine feeding grounds, the mechanisms used to navigate longer distances will no longer be of value. Chemical cues given off by the dense concentrations of zooplankton in certain areas may attract whales to their feeding grounds. Salinity or other water mass properties might indicate the location of appropriate conditions for the development of dense copepod patches, while organic compounds produced by zooplankton may be direct indicators of dense patches.
Within a feeding ground, right whales feed in the patch or layer of zooplankton which provides the maximum net energy benefit. The behavior of feeding right whales suggests that they are capable of detecting fine-scale variations in zooplankton density in both the horizontal and vertical dimensions and adjusting their behavior accordingly.
In the horizontal dimension, the path of a feeding whale is sinuous, with many turns, as it attempts to remain within the area of maximum copepod density. In the vertical dimension, whales feeding on zooplankton layers in the upper two meters of the water column regularly adjusted their swimming depth, apparently in response to changes in the depth of the densest parts of the layer. Vertical adjustments of as little as 20 centimeters could increase the whale’s energy intake by as much as 20% above that predicted if the animal simply swam at a constant depth.
Tactile clues are probably the most reliable indicator of zooplankton density at this scale. A right whale could use the whiskers on the front of its head for detection of individual zooplankton organisms.
“There is much that remains unknown about the migratory and foraging strategies of right whales,” said Kenney. “However, it is clear that right whales are capable of detecting the dense patches of zooplankton which constitute their preferred prey resource. An understanding of these mechanisms may help scientists address the issue of why the population of right whales is not recovering.
“The best hope for the long-term recovery of the western North Atlantic right whales may be those females taking their calves on ‘the tour’ of the foraging grounds, someday leading to re-occupation of the historic right whale habitat off eastern Canada.”
Journal
IWC Journal of Cetacean Research and Management