Week 5: Summer Feeding Grounds of Southeast Alaska
Although some humpback whales have been sighted during the winter in Southeast Alaska, most leave for warmer breeding grounds like Hawaii and generally begin returning to the feeding grounds at the beginning of each spring. During the next 6 - 9 months, North Pacific humpbacks will roam from the Central California coast up through the Aleutian Islands and even into Russia looking for food.
Although there are always exceptions, there are some general patterns to the movements of whales within the North Pacific Ocean in the summer and winter. For example, most of the whales that feed from California throughWashington tend to migrate to Mexico and Costa Rica during the winter months, whereas most of the whales feeding in British Columbia and Southeast Alaska migrate to Hawaii. Each summer about 3,000 - 5,000 humpbacks visit the waters around Southeast Alaska and Northern British Columbia for the purposes of feeding.
Why do Humpbacks Travel to Alaska?
During the summer months, humpback whales spend the majority of their time, day and night, feeding on krill or other small schooling fish such as herring, capelin, sand lance, or mackerel. Arguably, the summer feeding season is the most critical and important time of year for whales.Without adequate food, both in terms of quality and quantity, there is little hope for successful reproduction to occur in the winter.Whales depend upon productive ocean conditions in summer to build enough fat reserves that they can survive the long winter months when food is scarce.
With the exception of short rest periods or periods of travel, humpbacks devote nearly all of their time and energy to feeding (as opposed to reproduction) during these months in higher latitudes. Occasionally, humpbacks are seen logging or resting at the surface between feeding bouts.
What DoThey Eat?
As mentioned above, humpback whales have a varied diet that includes krill as well as a variety of small, schooling fishes including herring, capelin, sand lance and mackerel.While humpback whales in the Southern hemisphere rely mostly on huge swarms of krill, the primary prey for humpback whales in Southeast Alaska is Pacific herring (Clupea harengus pallasi).
Patches of prey, however, are widely distributed throughout Southeast Alaska, and as a result, humpbacks feed on different types of fish, including krill within the waters of Southeast Alaska. For example, more reliable and higher densities of herring are found within Chatham Strait, whereas krill is more common in Frederick Sound.This type of varied prey distribution has led humpbacks to develop a number of different strategies to hunt and capture prey.
Feeding Behavior in Southeast Alaska
Humpback whales rely on different strategies to find and secure food. Sometimes they feed alone, but when large amounts of schooling fish are available (such as herring), the whales may also join together to feed in larger groups (see next section).
The type of prey determines the strategy used to capture the prey.When feeding on krill or slower moving prey, for example, humpbacks often feed alone.Whales feeding alone and on surface prey can be seen skimming the surface, or lunging through the water, with their mouths wide open. At other times, humpbacks feed on prey beneath the surface. In these situations, the whales spend most of their time underwater coming to the surface only to breathe every 5-8 minutes or so before diving and repeating the feeding cycle again.
Blowing Bubble Nets to Catch Fish
On the feeding grounds of Southeast Alaska, humpback whales occasionally work together to herd balls of Pacific herring toward the surface for feeding. In these groups, whales will use a net of bubbles, often about 10-25 feet wide, to encircle the herring so the whales can emerge as one communal group through the center of the bubble-net to feed on masses of herring at one time.This unusual but exciting technique is often described as bubble-net feeding. Feeding groups observed in Alaska using this technique range in size from 2 to over 30 individuals.
There is some evidence that there are specific roles for individual whales feeding in this coordinated manner. For example, one whale may be assigned the task of blowing the curtain of bubbles that traps the fish, while another initiates the feeding calls that cause the herring to move away from the sounds and towards the bubbles. How the whales determine who performs these functions, or even if the same whales do it all the time, is unknown.
For more information on bubble-net feeding, visit Alaska Whale Foundation.
The Bubble-Netters of Southeast Alaska are Unique
While humpback whales in other locations around the world (e.g., Northeast United States and South America) use bubbles to capture prey, the humpbacks in Southeast Alaska are the only population known to produce a series of loud, trumpet-like sounds while engaging in this spectacular bubble-net feeding behavior.
To hear vocalizations made when bubble-net feeding, please visit the Whale Trust Sound Gallery here.
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