Why do bowhead whales linger off Pt. Barrow at certain times of the year? An 11-year project looking at the bowhead feeding zones near Utqiagvik asks: “Why do the whales stop there?”
“It’s called a ‘krill trap’ and it’s driven by local winds,” said Dr. Carin Ashljan, scientist with the Biology Department at Woods Hole Oceanographic Institution in Massachusetts. She spoke on this topic Tuesday evening, Sept. 19, as part of the Strait Science lecture series. Dr. Ashljan had just completed a 23-day scientific voyage aboard the Sikuliaq, UAF’s ice-capable research vessel.
The giant mammals winter in the Bering Sea and migrate up the Alaska coast and across to the Canadian Arctic, feeding as they go. When fall comes they journey back to the Chukchi Sea. During this migration they will spend time feeding on krill on the Beaufort Shelf in the vicinity of Pt. Barrow.
For efficient feeding the whales need dense concentrations of krill and copepods. The upwelling of deep water from the Arctic Basin onto the Beaufort Shelf brings high-nutrient water and offshore zooplankton to the shallow waters. The zooplankton, particularly krill and copepods, provide higher quality food to the upper trophic level animals on the shelf, including bowhead whales and Arctic cod. As beluga feed on Arctic cod the smaller white whales also feed at the shelf break, the area where the Beaufort Shelf drops off into the deeper waters of the Arctic Basin.
“When the wind blows from the east you get upwelling,” said Dr. Ashljan. The wind pushes surface water north, allowing the colder, deeper waters to rise to the surface. The Alaska Current flows up a submarine feature, the Barrow Canyon, bringing the krill to the surface. When the east wind stops the krill drift around the corner of Pt. Barrow and get trapped, producing very compact, dense clouds of krill. Scientists call it a krill trap. “The trap is driven by coastal winds,” said Dr. Ashjian. “It occurs when winds exceed a certain strength.” The researches found that a 12-knot wind was needed to generate the upwelling.
Temperature and salinity define different water masses. Vertical distribution of salinity shows upwelling of water at shelf break. Because there’s no light in the depths the phytoplankton doesn’t use the nutrients to grow. So the upwelled water is richer in nutrients than the water from the shallows. With the colder deeper water up comes nutrients and phytoplankton.
“The reason we started doing this work in Barrow is because we wanted to know why the whales stop there,” said Dr. Ashljan. The concentrations of whales provide hunting opportunities for villages along the coast. The villages are located where they are because those are good spots to hunt marine mammals.
Strait Science is an evening presentation series hosted by University of Alaska Fairbanks NW Campus and Alaska Sea Grant. The presentations are at 6:30 p.m. at the Nome Campus and are admission free. There is no fixed schedule but they do happen when scientists come through Nome.
— By James Mason/The Nome Nugget
Reprinted with permission from The Nome Nugget, September 28, 2017