"The California gray whale has recently been removed from the endangered species list, largely as a result of a prohibition against their harvesting since the turn of the century. Their estimated population of 22,000 now may face increased pressures from population stress and the need for construction materials, such as sand and gravel, in Alaska. Using sonar devices, U.S. Geological Survey scientists are able to distinguish feeding traces made by gray whales and walruses from other physical features on the seafloor. Careful measurement of these features allows geologists to develop the numerical basis for models that define the feeding ecology of gray whales and walruses. In turn, biologists are better able to understand the geologic aspects of the gray whale and walrus ecosystems."
- Dr. Hans Nelson,
U.S. Geological Survey
During the late 1970's, the U.S. Geological Survey (USGS) surveyed the seafloor of the Northeast Bering Sea and the Chukchi Sea for evidence of hazards to petroleum development.
USGS scientists studied the Alaskan continental shelf for evidence of hazards that would interfere with possible offshore oil drilling activities and pipelines. The waters of the Northeast Bering Sea and the Chukchi Sea average 30 meters in depth and cover an area the size of California. Half the region is overlain by a clean fine-grained sand about one meter thick, and the other half is covered by muds and gravel. Many potential hazards were identified and mapped including active faults, active bedform fields, ice gouges, gas collapse features, and current erosion. In addition, the seafloor was found to be marked by pits and furrows that could not be explained by known physical geological processes.
USGS scientists conducted extensive cooperative studies in order to determine if the unexplained seafloor marks were biological in origin.
USGS scientists used sidescan sonar to collect images of the seafloor in areas where gray whales and walruses were observed feeding. These data were collected on USGS cruises in cooperation with biologists from the National Oceanic and Atmospheric Administration's (NOAA) Pacific Marine and Environmental Laboratory, the Moss Landing Marine Laboratory of the California State University system, the University of Alaska, and the Alaska Fish and Game Department. Funding was provided by the Minerals Management Service as part of their environmental assessment responsibilities. USGS scientists and cooperating biologists suspected, based on circumstantial evidence and diving observations, that the pits and furrows on the seafloor were caused by whales and walruses.
Left: Map shows areas of the northeast Chukchi Sea containing gray whale feeding pits and walrus feeding furrows.
An area south of the Bering Strait known as the Chirikov Basin contained numerous unexplained seafloor features that coincided with the distribution of a small shrimp-like crustacean. These crustaceans, called amphipods, are a principal source of food for California gray whales. Aerial observations of sediment plumes coupled with the seafloor distribution of a fine-grained sand layer, ideal for amphipod habitat and all directly associated with the presence of gray whales, provided strong evidence that whales were pitting the seafloor as they fed on the bottom-dwelling crustaceans. Likewise, the muddy areas surrounding the sand sheets are characterized by furrows. These muddy areas also coincide with the clam-rich feeding grounds of the Pacific walrus.
USGS researchers carefully measured the physical dimensions of the seafloor pits and furrows to calculate volume of sediment involved.
The size of pits caused by feeding whales varies somewhat, but typically ranges between 1 and 5 square meters. Fresh feeding pits are modified by subsequent feeding on the pit margins and by scouring from strong bottom currents. A conservative estimate of the area disturbed annually by whales is about 1,200 square kilometers. The area of the Chirikov Basin is about 22,000 square kilometers, therefore whales disturb a minimum of 6 percent of the Basin. If each pit averages 10 centimeters in depth, then the amount of sediment resuspended in the water column is calculated to be about 172 million tons each year. By comparison, the Yukon River (the third largest river in North America in terms of sediment discharge) introduces about 60 million tons of sediment into the Chirikov Basin annually.
Sidescan images of the sea floor show typical dendritic walrus furrows (top) and whale feeding pits (bottom). Vertical and horizontal scale bars equal 10 meters.
As walruses feed off clams and snails on the seafloor, they leave a furrow of disturbed sediment littered with clam and snail shells. These marks, like the whale feeding pits, can be seen on sidescan sonar images. The size of these walrus-feeding furrows also varies but is typically a half-meter wide, a few centimeters deep, and up to 50 meters long. Up to 36 percent of the seafloor in the Chukchi Sea is marked by walrus furrows, suggesting that several billion tons of sediment are reworked annually by more than 200,000 walruses feeding there. However, much of this sediment is not resuspended in the water column as the disturbance is restricted to the vicinity of the seafloor.
Available evidence suggests that certain environmental stresses in the Chirikov Basin would negatively impact gray whales.
Drawing shows the California Gray Whale and Pacific Walrus feeding methods. Whales suck a sediment mat of amphipod tubes from the sea floor into one side of their mouths. They then strain amphipods from the mat with baleen and expel suspended sediment out the other side of their mouths into the sea water. The walruses leave furrows full of clam and snail shells throughout the muddy areas where they feed. Drawing also shows sidescan sonar data collection in the furrowed and pitted areas.
Knowledge of the feeding habits of gray whales and the geological framework on which the habitat of amphipods depends suggests that any disturbance to the ecosystem could significantly reduce the gray whale population within a few years. Calculations suggest that the Chirikov Basin provides a minimum of six percent of the food supply for the California gray whale. Gray whales feed here for about 5 months before migrating south as Arctic sea ice advances; loss of the amphipod ecosystem would substantially reduce the whales' food supply. Such a loss of amphipod habitat could occur, for example, if sand is removed to support construction in Alaska or if the sand sheet is contaminated by petroleum spills. Monitoring these activities may help avoid placing the gray whale on the endangered species list once again. Similar activities could also affect the habitat of the walruses' preferred food. Although the walrus population exceeds 200,000, it is showing signs of stress. Further stress could significantly reduce their population as well. Geological analysis has helped define the feeding ecology of the California gray whale and the Pacific walrus and provides a method to monitor changes in their food supply, whether that change is of natural or human cause.
For more information please contact:
Dr. Hans Nelson
USGS 3475 Deer Creek Rd.
Palo Alto, CA 94304
Phone: (415) 354-3057