Bowhead whales ( Balaena mysticetus ) are baleen whales uniquely adapted to life in the icy waters of the Arctic and subarctic regions. As one of the largest whale species, second only to the blue whale, understanding their diet and feeding habits is crucial to understanding their role in the Arctic ecosystem. This article explores the bowhead whale's diet, feeding mechanisms, and how these relate to their survival and the health of their environment.
Bowhead Whale Biology and Distribution
Bowhead whales reside almost exclusively in Arctic and subarctic waters experiencing seasonal sea ice coverage, primarily between 60° and 75° north latitude. Of all large whales, the bowhead is the most adapted to life in icy water. They inhabit the Bering, Chukchi, and Beaufort Seas, Hudson Bay and Foxe Basin, Baffin Bay and Davis Strait, the Sea of Okhotsk, and in waters from eastern Greenland and Spitsbergen to eastern Siberia. The majority spend the winter near the southern limit of the pack ice and move north as the sea ice breaks up and recedes during spring.
Four stocks of bowhead whales have been recognized worldwide by the International Whaling Commission. Small stocks of only a few hundred individuals occur in the Sea of Okhotsk and the offshore waters of Spitsbergen. Genetic, aerial survey, and tagging data suggests that bowheads from western Greenland (Hudson Bay and Foxe Basin) and eastern Canada (Baffin Bay and Davis Strait) should be considered one stock that may number more than a thousand individuals. waters is the Western Arctic stock, also known as the Bering-Chukchi-Beaufort stock.
Bowhead whales have a dark body with a distinctive white chin and unlike most cetaceans, do not have a dorsal fin. Bowheads have extremely large heads and stocky bodies. The bow-shaped skull can be over 16.5 feet long-about a third of a bowhead’s body length. The bowhead whale also has a 17- to 19-inch thick blubber layer-thicker than that of any other whale. The bowhead’s large, thick skull allows them to break through 8-inch-thick sea ice. Some Alaska Native whalers have even reported whales surfacing through 2 feet of ice. Bowhead whales often accumulate scars on their bodies from breaking ice, killer whale encounters, entanglement in fishing gear, and propellers.
Filter-Feeding Mechanism
Bowhead whales are baleen whales, so they filter their food by straining huge volumes of ocean water through their baleen plates (like the teeth of a comb). Bowhead whales have the longest baleen plates of all whales, with a maximum length of 4 m (13 ft). These plates, numbering between 230 and 360, hang from the upper jaw and are made of keratin. The mouth has a large, upturning lip on the lower jaw that helps to reinforce and hold the baleen plates within the mouth. This also prevents buckling or breakage of the plates from the pressure of the water passing through them as the whale advances. They feed by swimming forward with its mouth wide open.
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Diet Composition
Bowhead whales feed almost exclusively on marine invertebrates, including small to moderately sized crustaceans, such as shrimp-like euphausiids (i.e., krill) and copepods. They also ingest other invertebrates and small fish. Scientists estimate that a bowhead whale needs to eat about 100 metric tons (over 220,000 pounds) of crustaceans per year.
The Role of Sound in Foraging
Sound is critical to the survival of bowhead whales. They rely on keen hearing abilities to detect, recognize, and localize biologically important sounds for navigation, predator avoidance, foraging, and communication in the marine environment. Bowhead whales are highly vocal and have a large variety of calls. The echoes of some of their calls are used to help the whales find food and navigate through the ice as they migrate. Although direct measurements of hearing ability in baleen whales are lacking, scientists predict, based on anatomy and vocalizations of other closely related whales, that bowheads hear best at low-frequencies.
Life History and Reproduction
Historically, age determination in bowhead whales has been difficult, and life history parameters are better known in terms of body length than age. Based on the recovery of stone harpoon tips from harvested bowheads, it is evident that bowhead whales live well over 100 years. However, new techniques allow for more precise estimation of bowhead whale age, and studies suggest they may live to be over 200 years old. Genes that allow for repair of damaged DNA may be responsible for their longevity.
Bowhead whales reach sexual maturity at approximately 25 years of age, when their total body length is about 35 to 45 feet. Mating behavior has been observed year-round, though most conceptions are believed to occur during late winter or spring. Most calves are born between April and early June during spring migration. Females typically have one calf every 3 to 4 years after a gestation period of around 13 to 14 months. Calves are usually about 13 feet long, weigh about 2,000 pounds, and can swim at birth.
Threats to Bowhead Whales
Bowhead whale populations are exposed to a variety of human-caused stressors and threats, including:
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- Pollution (e.g., spilled oil, heavy metals, chemicals, debris)
- Vessel strikes and disturbance
- Entanglement in fishing gear
- Climate change
- Ocean acidification that can affect their prey
- Noise pollution that may affect their feeding, navigation, communication, and ability to detect and avoid predators
Entanglement in Fishing Gear
About 12 percent of the Western Arctic stock show scars from entanglement in fishing gear, mostly from commercial pot-fishing gear. Entangled whales either swim off with the gear attached or may become anchored in place. Once entangled, whales may drag gear or lines for long distances, ultimately resulting in fatigue, compromised feeding ability, or severe injury, which may lead to reduced reproductive success and/or death. An unknown number of whales die from entanglement, as some entanglements likely go undetected. In cases where a carcass is still entangled or bears entanglement scars, it is not always possible to determine if entanglement was the cause of death.
Contaminants
Contaminants enter ocean waters from municipal wastewater discharges, runoff, accidental spills, atmospheric deposition of airborne contaminants, discharges from commercial operations, such as fishing, shipping, and oil and gas development, and other sources. Many contaminants move up the food chain and accumulate in top predators. Bioaccumulating contaminants are present in bowhead prey and in their environment. Bowheads accumulate these contaminants because of their long lifespan, position at the top of the food chain, and large blubber stores. These pollutants may harm bowheads’ immune and reproductive systems.
Ocean Noise
Underwater noise may threaten bowhead whales by interrupting their normal behavior and driving them away from areas important to their survival. Noise from seismic exploration for petroleum reserves was found to drive bowheads from waters within about 12 miles of the sound source, although avoidance behavior is likely related to the activity that the bowhead is engaged in at the time of exposure. For instance, feeding whales may be more reluctant to abandon food concentrations due to noise. In addition, evidence suggests that bowheads’ prey, primarily small marine invertebrates, may be negatively affected by noise from seismic exploration.
Vessel Strikes
Vessel strikes can injure or kill bowhead whales. About 2 percent of subsistence-hunted bowheads show signs of scars from vessel strikes. However, as seasonal sea ice continues to retreat due to climate change, vessel traffic in Arctic waters is increasing and could increase the risk of future collisions.
Predation
Transient killer whales are known to prey on bowhead whales. Scars consistent with killer whale attacks were found on approximately 8 percent of subsistence hunted whales, and rates have increased each decade. This may be due to better reporting and/or sampling bias, an increase in killer whale population size, an increase in occurrence of killer whales at high latitudes, or a longer open water period offering more opportunities to attack bowhead whales. Bowhead whale carcasses that had injuries consistent with killer whale predation have been observed during aerial surveys since 2009.
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Conservation Efforts
The United States listed all bowhead whales as endangered under the Endangered Species Conservation Act in 1970 and the Endangered Species Act in 1973. Bowhead whales are also listed as depleted under the Marine Mammal Protection Act. NOAA monitors the health of the Western Arctic stock of bowheads and co-manages the bowhead stock with the AEWC. We also seek to minimize the impact of activities that are authorized, funded, or carried out by Federal agencies. Through ESA section 7 consultations we help Federal agencies in fulfilling their duty to ensure that their actions do not jeopardize the continued existence of a species, or destroy or adversely modify designated critical habitat. We do this by collaborating with the action agency to develop project modifications intended to decrease or eliminate the effects of noise, pollution, and other project-related threats on bowheads.
Co-managing Bowhead Subsistence Harvest
The IWC conserves and manages bowhead whale populations worldwide under the International Convention for the Regulation of Whaling (ICRW). In the United States, the Whaling Convention Act is the enabling act for the ICRW. This act allows whaling by aboriginal peoples to the extent it does not conflict with the ICRW. Government and the Alaska Eskimo Whaling Commission (AEWC) have co-managed the traditional subsistence harvest of bowhead whales under the terms of a cooperative agreement between NOAA and the AEWC. The purposes of this agreement are to “protect the bowhead whale and the Eskimo culture, to promote scientific investigation of the bowhead whale, and to effectuate the other purposes of the MMPA, the Whaling Convention Act, and the ESA as these acts relate to aboriginal subsistence whaling.” As part of this co-management agreement, the AEWC is responsible for enforcing the legal harvesting of bowhead whales, reporting on all strikes and landings of bowhead whales, and providing data on each whale landed. Only those whaling captains registered with the AEWC, along with their crew, may participate in subsistence harvest of bowhead whales.
The IWC establishes subsistence harvest quotas for the Western Arctic stock of bowhead whales based on an extensively tested strike limit algorithm. This bowhead quota is allocated by the AEWC among 11 Alaska Native communities that traditionally hunt bowhead whales and comprise the AEWC (Gambell, Savoonga, Wales, Little Diomede, Kivalina, Point Hope, Pt. Lay, Wainwright, Utqiaġvik (formerly Barrow), Nuiqsut and Kaktovik). A small portion of the IWC’s bowhead subsistence harvest quota is also allocated to whaling villages in Russia.
The authorization of the bowhead whale subsistence harvest quota includes the following bowhead catch limits, which were approved at the 2018 IWC meeting. From 2019 through 2025, the number of bowhead whales landed shall not exceed 392 total. The number of bowhead whales struck shall not exceed 67 per year, except that any unused portion of a strike quota from the three prior quota blocks shall be carried forward and added to the strike quotas of subsequent years, provided that no more than 50 percent of the annual strike limit shall be added to the strike quota for any one year. For 2020, the quota was 100 bowhead whales with the AEWC allocated 93 whales and the Russian Federation 7 whales. During 2020, 69 whales were struck, 54 of which were landed. Subsistence landings in Alaska have averaged about 45 bowhead whales per year since 2011.
NOAA Fisheries is investigating all aspects of acoustic communication and hearing in marine animals, as well as the effects of sound on whale behavior and hearing. In 2018, we issued revised technical guidance for assessing the effects of anthropogenic (human-caused) sound on marine mammal hearing.