Full-grown humpback whales can weigh more than 88,000 pounds (40,000 kilograms) and reach up to 50 feet (15 meters) long. Their mouths gape 15 feet (5 meters) across, but they can only fit prey the size of a human fist down their narrow esophagus.
Most commonly, humpbacks are solitary diners, eating a diet of krilla shrimp-like crustaceanand plankton, which they filter through hundreds of sieve-like plates called a baleen. The whales in Chatham Straight, however, eat herring, which are larger and much harder to catch.
Sharpe said it is not clear why these humpbacks have developed a taste for herring, but "they have a high fidelity for that prey and they stay on it all season long." The humpbacks spend the winter months breeding in waters near Hawaii and return each summer to feed in southeast Alaska.
Like most whales, humpbacks were hunted up until the mid-1960s for their oil, meat, and bone. Today, just 10,000 remain in the north Atlantic and even fewer in the north Pacific. The population Sharpe studies in southeast Alaska numbers no more than 1,000.
Calambokidis said humpback recovery looks strong, but threats such as boat traffic, entanglement in fishing gear, sonar noise, and disruption of habitat and food supply remain. "Long-term, the greatest threat is probably to their prey base as a result of climate and environmental change," he said.
For the humpbacks in Chatham Straight to succeed in catching herring, they work together, carefully coordinating their maneuvers to herd the fish into bubble nets. The herring won't cross the wall of bubbles.
"Herring spend their whole life in a school and are used to a normal, rhythmic beat of the tails of their school mates. Bubbles represent a threatening, chaotic, domain," said Sharpe.
Trapped in the bubble net, Sharpe and his colleagues had assumed the herring were an easy target, filling the humpbacks up for their 2,500-mile (4,000-kilometer) journey to Hawaii in the fall.
But the Crittercam footage yielded a few surprises. "The whales are really not eating as much as we had anticipated," said Sharpe. "We know sometimes it's a total bonanza, but it doesn't look like a slaughter is going on."
In fact, the herring routinely escape attack by the whales. They maintain personal space even when inside the bubble net, making them more elusive targets. They can also swim faster than the whales and some are quick enough to dive to safety if the whales' timing is off.
"The biggest hero in all this is the herring," said Sharpe.
Since one of the greatest future threats to humpback whales could be a dwindling food supply, understanding what it takes for them to get a meal may allow for better management of their food supply.
This is particularly important as the humpback whale recovery continues, said Sharpe. Humpbacks tend to re-colonize former parts of their range. Many of their former haunts are now heavily populated by humans, such as the Puget Sound in the Pacific Northwest.
Increased knowledge about the whales and their feeding requirements will allow for effective management of human behavior in terms of fishing quotas, shipping lanes, and even whale watching tours, said Sharpe.