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Monday, 1 September 2014
by craig welch
Once-common bird population in decline – new studies show dwindling fish numbers as a significant cause.
THE bird-counters stood in the windy bow chattering into headsets and scanning the Strait of Juan de Fuca with binoculars. “Scoters,” Sherman Anderson said. “Three of them. At 11 o’clock. Look like surfs.”
“Marbled murrelets,” he added seconds later. “I see two.” Inside the boat’s cabin, another Washington Department of Fish and Wildlife worker listened through a headset of his own so he could record the tally on a computer.
Bird surveys like this and others done by plane are tracking a significant ecological shift in the US Pacific Northwest – a major decline in once-abundant marine birds. From white-winged scoters and surf scoters to long-tailed ducks, murres, loons and some seagulls, the number of everyday marine birds has plummeted dramatically in recent decades.
Scoters are down more than 75% from what they were in the late 1970s. Murres have dropped even more. Western grebes have mostly vanished, falling from several hundred thousand birds to about 20,000. The reasons often vary – from climate change and shoreline development to marine pollution and the rebound of predators such as bald eagles.
But several new studies now also link many dwindling marine bird populations to what they eat – especially herring, anchovies, sand lance and surf smelt, the tiny swimmers often dubbed forage fish. The relationship between marine birds and slick, fatty forage fish is complex. Some birds are here year-round while others pass through for just a few months. Some birds key in solely on silvery herring while others can just as easily eat flounder.
Some forage-fish species, such as herring, are a fraction of what they once were. But little information exists about the health of other species. But an analysis of bird diets and population trends found that marine birds relying exclusively on fish like herring were up to 16 times more likely to be in trouble than birds that ate non-schooling bottom-dwellers like sculpin.
“The result was remarkably strong,” said study author Ignacio Vilchis, formerly with the SeaDoc Society at the University of California, Davis.
“It showed that it’s the diving birds that go after forage fish which are much more likely to have a declining trend.”
There’s certainly no shortage of crashes to evaluate. Five years ago, one study showed the overall bird numbers in Puget Sound and British Columbia’s Georgia Strait were down 30% from the late 1970s. In Puget Sound alone, marine bird numbers have been cut in half. Biologists for years have tried to understand why the change hit so many species at once. But only recently have they really started to examine some of the systemic shifts that may cause or exacerbate declines.
“It’s one thing to have a rare species decline,” said Joe Gaydos, with the SeaDoc Society. “But we’re not talking about a few plovers. We’re talking about big, common species, and a lot of them.”
There is no easier way to view this decline than through the once-ubiquitous Western grebe. These black and white duck-like birds with their long necks and thin beaks settle in winter in colonies on lakes from south-west Canada to California, but gather their food from marine waters. While the bulk of that population once centred on Washington and lower British Columbia, sightings of larger flocks of grebes are unusual enough that when it happens “birders will talk about it on the Internet for days,” Gaydos said.
At first, it wasn’t clear whether this was a local or continental-scale problem, said Scott Wilson, a biologist with Environment Canada. It’s both: up and down the West Coast, the winter breeding population is half what it was in 1975. But something else was going on, too. While Puget Sound and lower British Columbia declines top 95%, grebe populations in parts of California have tripled. The centre of the bird’s range shifted 880km south.
“Food is one of the key resources species need to survive,” Wilson said. And grebes rely on forage fish.
In Puget Sound, the biggest stock of herring used to reside at Cherry Point, south of Bellingham. But since 1970, that herring stock has crashed, with more than 90% of the population all but gone. The loss of herring probably drove grebes away, Wilson hypothesised, but it also did so just as sardine stocks were recovering to the south. Down to a few thousand metric tonnes in 1985, sardine populations in California have since exploded to more than two million tonnes, providing an alternative food for hungry grebes.
Changes in other bird populations, too, could to some degree be related to changes in forage fish. Surf scoters, which primarily eat mussels and small crabs, also sometimes turn to herring eggs during spring migration. In 1978, bird surveyors counted 40,000 scoters near where herring were spawning. But in 2004 and 2005, surveyors counted fewer than 1,000 birds in the same location.
“I think the herring absolutely did play a role in the scoter decline, but exactly what that role is, we just don’t know,” said Joe Evenson, of the Washington Department of Fish and Wildlife.
“At that time of year, scoters are eating roe because they’re storing up a lot of fat and that can help determine whether they are successful at breeding. But I think there’s not just one thing that contributed to the scoter collapse.”
Meanwhile, the chief threat to marbled murrelets is still believed to be logging in their breeding grounds high in ancient Douglas fir forests. But some researchers have suggested that along the coast, murrelets are being forced to abandon their fatty fish diets and are eating less-nutritious fish lower in the food chain – especially just before the important period when they mate.
At the same time, some scientists believe the herring problem itself may be far worse than others acknowledge. Forage fish, particularly herring, are supposed to be so abundant they are eaten by almost everything, including hake, dogfish and sea lions and whales.
“They are the central node of the marine ecosystem, they are the thing through which everything else flows,” said Iain McKechnie, a coastal archaeologist with the University of British Columbia.
Modern humans presume populations of herring and other forage fish naturally swing wildly between booms and busts. But archaeological sites recently analysed by McKechnie suggest that may be a modern feature because herring populations today are a mere fraction of what they were historically.
“I count fish bones,” McKechnie said. He examined ancient shell middens from Alaska to Puget Sound and determined that Native Americans centuries ago caught and ate gobs of herring – so many that it clearly was among the most important fish they ate. Herring bones were present at almost every single one of 171 sites.
Most important, over 7,000 years of history, McKechnie saw no evidence of variability in herring populations. McKechnie suspects the most likely reason for the change is intensive industrial herring-fishing pressure in the late 19th and early 20th centuries. In a recent study, Wayne Landis of Western Washington University found that while Puget Sound herring used to live eight to 10 years, they now survive only to age three or four. That limits their ability to successfully replace themselves. “They don’t get old anymore,” he said.
That change could be the result of disease or toxic contamination or other changes, but the important thing is that it appears to be true across Puget Sound, he said. Usually after a bust, herring eventually recolonise, Landis said.
The question now: Is this bust different from those in the past? “If that part of the food web is in decline, there may be a food problem for some birds,” he said.
Even so, other scientists say these environmental systems remain far too complicated for researchers to know for sure. We often don’t know enough yet to say where things are headed. Gaydos put the bird decline picture this way. “Something’s happening on a big level,” he said. “But what is it?” – The Seattle Times/McClatchy Tribune Information Services
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