SANTA CRUZ — Marine life and seabirds of all sorts, from pelicans and gulls to migratory terns and shearwaters, have amassed off the coast in recent weeks — flapping, squawking, swirling and diving — feathers flying in a feeding frenzy as each creature fights for their share of the anchovies swimming below the ocean’s surface.
The spectacle is an expected one in the summer for those that study and admire the swarming seabirds such as the long-flying and deep-diving sooty shearwaters, shifting from east to west en masse along the Santa Cruz coast.
“It’s remarkably consistent year to year that we have large numbers of sooties in our bay,” said research wildlife biologist with the United States Geological Survey’s Western Ecological Research Center Josh Adams. “It may appear in certain years that there’s more of them or they are here for longer or they disappear sooner, but it’s hard to get a handle on the numbers because they travel in such large flocks.”
Adams mentioned that there can be tens of thousands of sooty shearwaters swirling and shifting off the Santa Cruz coast. Although the birds are based in New Zealand, they make their way to California and Monterey Bay during the summer months to molt and fatten themselves up in order to make the trip back home.
“When they come to this part of the Earth, their diet is known to shift from juvenile rockfishes and krill early on, to an anchovy-dominated diet in late summer,” said Adams. “The anchovies are the real fuel for their migration at the end. Their protein demand fluctuates through the summer and when they get here, they’re depleted from traveling from New Zealand and Chile and they need protein to replace their feathers, which are made from protein.”
Those out on the water who find themselves in the eye of a seabird storm may notice that the sooty shearwaters have a hard time taking off after diving underwater to fill up on anchovies.
“There’s a couple of things going on there,” said Adams. “They consume enough food in proportion to their body mass that it does require them much more effort to get up off the water. And they are also in a primary molt so they have less wing area. They’re cool because they’re designed to both fly underwater and huge distances across the ocean, and molting compromises their ability to get up and fly quickly, especially if they have a belly full of anchovies.”
For years, the actual dive depth and duration that the sooty shearwaters can stay underwater were murky statistics until professor of ecology and evolution at San Jose State Scott Shaffer and other researchers used archival tags to measure the birds’ habits in the 2000s when Shaffer was a postdoc researcher at UC Santa Cruz.
“We knew that they could dive close to 60 meters because some researchers had used these capillary depth gauges,” said Shaffer. “Those would just tell you the maximum dive depth. They don’t tell you anything about the duration or how frequently they made dives that deep. When we placed our time depth recorders on the animals, we were able to glean a lot more information about their diving patterns. They’re typical dive depths are somewhere between 30 and 45 feet.”
Shaffer and the researchers used the information they gathered to co-author a paper on the habits of the soot-colored seabird, which he studied in California and New Zealand. He mentioned that the sooty shearwater’s migratory habits between the northern and southern hemispheres allow them to experience an endless summer.
“It’s quite extraordinary to see the full migrations that these birds do and the notion of them living in a perpetual summer is really appropriate,” said Shaffer. “They are able to get the best of both worlds by crossing the equator and coming up here.”
Shaffer said that before the sooty shearwaters were tagged, they were thought to travel in a massive circular pattern as the birds tend to show up in California, Alaska and Japan before heading back to New Zealand, but the researchers found that the pattern they followed was more like a figure eight.
“The prevailing thought was that they did this large circuit around the Pacific, and we were able to show that in fact that’s not what happens,” said Shaffer. “The birds tend to target an area in the North Pacific and as far we know, they stay there.”
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In addition to the sooty shearwaters, Monterey Bay attracts other types of shearwaters in smaller numbers such as the short-tailed shearwaters from Australia, Buller’s shearwater also hailing from New Zealand and pink-footed shearwaters from Chile.
“It’s similar in size but has a little bit different morphology and also doesn’t dive as deep as the sooty,” said Adams. “Our summer is a sort of mixing time for a lot of species that travel to California from the Southern Hemisphere. We also have a handful of dominant resident breeding birds here that we see in big numbers off of Santa Cruz like western gulls and common murres and California brown pelicans and some other gull species. We have a really large diversity of seabirds here.”
Now that the summer is winding down, Adams said that most of the migratory sooty shearwaters will soon start making their return trip to New Zealand.
“We’ll start seeing them leave now and through the next three weeks or so and then they’ll all be gone,” said Adams. “The anchovies will be somewhat depleted by then because there are a lot of animals eating them, but the anchovies are not leaving our system.”
Although the clouds of the diving seabirds would seem like a hazard for boaters, Santa Cruz Harbormaster Blake Anderson said their attention is mostly fixated on the anchovies and they tend to avoid the boats. He said it’s the anchovies and not the birds that rouse a feeling of nervousness for boaters and those in the harbor especially.
“The birds themselves don’t come into the harbor but the anchovies do,” said Anderson. “Anytime we see the shearwaters out there, we have a pretty good idea that there’s large schools of anchovies close by, so we start monitoring where those anchovies are going, and if they’re getting close to the harbor, we’ll deploy our aeration system.”
Anderson said that some anchovies have entered the Santa Cruz Harbor and they have switched on the aeration system, but they remain hopeful that the fish will remain out at sea.
“We’re running our aeration system and monitoring the oxygen levels,” said Anderson. “It’s stable for the moment, but if we do get one of these schools coming in here, we could be in trouble.”
Anchovies swarmed the harbor a year ago, but the aeration system helped avoid a massive die-off. The last big baitfish die-off occurred in the late summer of 2014 and led to a massive cleanup effort at the harbor and a foul fishy smell to permeate along the affected coastline.
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