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The Anchovy vs. The Whale

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by Brendan Bane

Sept. 22, 2013—A sudden surge in anchovy activity has lured hundreds of humpback whales into Monterey Bay. Ocean-going tourists have enjoyed watching the whales engage in this past week’s feeding frenzy. No one knows how long it will last, but the anchovy bloom is likely just a product of ecological happenstance, says one marine biologist—a lucky break for whales and whale watchers alike.

Local biologists are astounded, many claiming they’ve never seen so many whales in the bay. But the sudden bloom begs the question: why so many anchovies?

“It isn’t that they had a reproductive explosion and bloomed as algae would,” says marine biologist Baldo Marinovic of UC-Santa Cruz’s ecology and evolutionary biology department. “This is a redistribution issue. Something concentrated them coastward.”

Anchovy populations tend to fluctuate randomly, and sometimes mysteriously. The fish live hard and fast lives, laying oodles of eggs at a time and taking only months to sexually mature. Variation in water temperature, how much food is available, and other environmental factors can yield anchovy surpluses and deficits. They go where the water is mild and the plankton is plenty, sometimes forming swarms where conditions are especially favorable. For the past few years, we’ve experienced an anchovy deficit in Monterey Bay.

“They haven’t been particularly abundant in the Central California region for the past five or six years, and no one really knows why,” says Marinovic. “But now all that has been turned on its ear.”

The swarm of anchovies in Monterey Bay is massive and thick—a tempting lure for humpback whales, which rely on anchovies as one of their primary food sources, as well as seabirds like brown pelicans. Juvenile blue-footed boobies have also made a recent appearance in the bay amid speculation that they, too, are following the anchovies.

Read Blue-Footed Boobies Glimpsed in Santa Cruz
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To escape their mammalian attackers, anchovies employ a special strategy: they make a bait ball. A bait ball happens when anchovies pack themselves into a dense, spherical mob. Like a herd of zebras, the glimmering ball shines with the frantically quivering bodies of a thousand fish. Each one swims toward the sphere’s center, where they’re least likely to be attacked. As the sphere moves, the center shifts, and the fish must constantly fight for the safest positions.

But the whales have their own strategy. They blow bubbles.

Humpbacks patrol the ocean, migrating as far as 16,000 miles in search of food in the summer. They rely on stored fat when fasting for the rest of the year. Scientists speculate that, when a whale comes upon a group of anchovies like the one in Monterey Bay, they alert other whales through far-traveling songs.

From below a school of fish, multiple humpbacks will strategically and cooperatively blow bubbles in what is termed a bubble net, which frighten fish like anchovies and herd them into tightly packed formations (like a bait ball). When the anchovies are in just the right place, a humpback will propel itself upward, its mouth agape, and swallow as many anchovies as possible.

Marinovic likens the whales to fishermen.

“It’s no different than a bunch of fishing boats out when salmon are biting. It’s the same behavior.”

Until their anchovy supply runs out, the whales will continue fishing.

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