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Aerial surveys conducted each year to gauge abundance in the iconic kelp forests off the North Coast showed a slight improvement last fall, offering a glimmer of hope for the recovery of the coastal marine habitat, the California Department of Fish and Wildlife has reported.

But conditions for the “bull kelp,” an annual type of seaweed — especially off Sonoma County — have become so bleak in recent years that even a reported doubling of the forest canopy during last year’s growing season has done little to bring the underwater habitat back to full strength, scientists said.

Despite patches that resemble the historic ecosystem in some ways, large swaths of ocean floor off the North Coast remain devoid of bull kelp and other fleshy algal species, prompting continued starvation among common marine herbivores like red abalone and urchins, they said.

Even with growth in the overall canopy last year, data indicates the kelp off Sonoma and Mendocino counties covered at least 95 percent less surface area in 2016 than it did in the banner year of 2008, said Cynthia Catton, an environmental scientist with the state wildlife agency.

The apparent expansion of the kelp canopy “is deceptive,” said Sonke Mastrup, environmental program manager for the agency’s invertebrate program, “because 2016 is still way below anything we would consider normal.”

Bull kelp is a large, brown algae that anchors itself to substrates on the ocean floor and reaches for sunlight with a thick, flexible stem that grows up to 60 feet long. A gas-filled float with long, narrow blades forms the surface canopy.

The thick forests typical of the waters off the North Coast provide a source of nutrition and an important link in the food web. They also form nursery habitat for young finfish that need to remain concealed until maturity, including commercially caught salmon and rock fish. Abalone and red urchins, which both supply important North Coast fisheries, rely on it as well.

Scientists with state Fish and Wildlife use infrared photography to map and measure the surface area of the kelp canopy each September in what should be peak season.

But they sounded the alarm last spring after observing shifting dynamics over several years that by 2014 and 2015 had produced what’s known as an “urchin barren.”

An exploding population of voracious purple urchins had overgrazed the North Coast bull kelp so severely it was nearly decimated.

The agency cited aerial surveys from 2014 that showed a 93 percent reduction of the kelp canopy between San Francisco and the Oregon border when compared with 2008. The overall surface area of the kelp diminished by another 33 percent in 2015, Catton said.

Sonoma County was particularly hard-hit, and by 2015 had lost nearly 100 percent of its canopy coverage, she said.

The agency described what it called “a perfect storm” of large-scale environmental stressors. They included a harmful algal bloom off the Sonoma Coast in 2011 that proved catastrophic for red abalone in the area; widespread sea star wasting disease that killed off large numbers of the invertebrate predators along the North Coast, beginning in 2013, and an unchecked explosion of small purple urchins that resulted; and anomalous warm water conditions that spread down the West Coast from Alaska in 2013 and 2014, persisted in 2015 and weakened the kelp forest, which requires cold, circulating water to thrive.

The purple urchins, estimated at more than 60 times their historic densities in Northern California by last year, ate heavily of the kelp, out-competing abalone and razing huge areas of all growth, including other, smaller fleshy alga and, by last fall, even hard, crusty, calcified species that coat the rocks on the ocean floor, Catton said.

A key concern has been that bull kelp is an annual, unlike the giant kelp common to Southern California, and needs to be around to reseed itself each year, Mastrup said.

Any increase in forest density is an improvement, Rebecca Flores Miller, a state environmental scientist, said via email.

The most recent survey results reveal a number of unusual trends, including diminishing levels of kelp in Southern California, where the surface canopy was about half what it had been the previous year, Flores Miller said.

In areas north of Pigeon Point near Santa Cruz, the canopy had grown two to five times what it was a year earlier, though results were uneven, she said.

At the Timber Cove Boat Landing north of Fort Ross, where owner Brenda Verno grew up, the kelp in her little cove used to grow so thick in the summer it was necessary to cut a channel through the canopy so boaters could get out. These days, looking out from land, the water is undisturbed by the bull kelp fronds.

The winter storms that used to litter her beach with remnants for her garden compost haven’t produced enough for her to bother cleaning up.

Monte Rio diver and fisherman Matt Mattison, who runs an online forum and has fished off the Sonoma and Mendocino coasts for 3 ½ decades, said each location, each diver, tells a little something different about what’s happening under water.

There are places off the Mendocino Coast where sea stars are returning and appear to be curtailing the purple urchin growth, with the result that there’s more kelp and “big, fat, healthy abalone,” Mattison said.

But “you can go 50 feet down the reef, and it’s a totally different environment,” with abundant purple urchins, hungry abalones and no kelp, he said.

“I’m not going to panic. We’re seeing some improvement, which is good. We’re not seeing more loss, degradation. We’re slowly seeing some areas of improvement, which is a good sign.”

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