Daily Sitka Sentinel

Scientists Tackle Sitka’s Invasive Tunicate

By Anna Bisaro
Special to the Sentinel
    Scientists are searching for a method to eradicate the invasive tunicate species that has kept Whiting Harbor closed since 2010.

Didemnum vexillum is seen in Whiting Harbor. (Photo courtesy ADF&G, Jeff Meucci)

    This invasive sea squirt has been found all over the world and can have detrimental effects on marine ecosystems if not controlled. But killing the invasive is not so easy.
    “Sometimes people have this notion that you can just kill anything,” Ian Davidson, of the Smithsonian Environmental Research Center in Maryland, said in a recent interview. “There is not a standard template you can just follow and do.”
    Whiting Harbor is the cove between the Northwest end of the airport runway and the causeway linking the islands of the Fort Rousseau State Historical Park. If not for the tunicate contamination, Whiting Harbor would be the preferred access to the state park, which is accessible only by boat.
    This September, Davidson and other scientists from the Smithsonian will be testing a possible treatment method for the invasive tunicate to see if they might be able to remove the species from Whiting altogether.
    Didemnum vexillum, or D vex, is a fast-growing sea squirt sometimes called marine vomit.  It has been found all over the world and has greatly impacted ecosystems off the coasts of New Zealand and Wales and has been particularly harmful to scallop populations near Massachusetts.  Scientists believe D vex originated in Japan.
    “It establishes well over surfaces,” Tammy Davis, invasive species program director for the Alaska Department of Fish and Game, said.  “It’s a really fast grower.”
    Fortunately for Sitka and the rest of Southeast Alaska, despite the fast-growing characteristics of D vex surveyors have not found evidence of the tunicate spreading anywhere else in Alaska. 
    D vex often attaches to boats and fishing lines and is spread to other areas, so Davis said Whiting Harbor has been closed to all human activity since the discovery of the tunicate to limit the spread of the organism.  As for what brought it to Sitka, no one knows.
    “We can’t say what the vector was,” Davis said.
    Scientists can’t say just how long it’s been here either. 
    Marnie Chapman, a professor at the University of Alaska Southeast, was on the bioblitz expedition that discovered the tunicate in 2010. 
    “It’s hard to identify on first look,” Chapman said.  If the scientists hadn’t realized what they had found, “that would have been a nightmare scenario,” she said.
    Containing and ultimately eradicating the species is important because “invasive species compromise our sense of place,” she said.  “They take what is special and unique about a particular area and they make it less special.”
    But while the tunicate has remained contained in Whiting Harbor, scientists still don’t know how to get rid of it.  Davidson explained part of the research this fall will be testing the effects of increased salt content in the water of the harbor.  A higher salinity of the water may help kill the tunicate, he said, but the scientists need to figure out if they can control the salt content in the harbor long enough to be effective.
    Davidson’s team of scientists will return early next year or in the spring for full-on experiments in eradication, he said.  This first trip is just testing the methods. 
    “I want to emphasize that this is not an eradication attempt, but rather a trial to determine how one might go about an eradication effort,” Linda McCann of the Smithsonian Environmental Research Center said.  “We face several challenges with the work,” she said, including managing the delivery of the treatment and not harming the substrates the tunicate is attached to.
    Davidson said that mobile creatures in the harbor will disperse if the salt content gets too high for them during the testing.  He said the scientists were not worried about other invertebrates that may not be able to escape, because they were positive the harbor would repopulate because of Sitka’s healthy intertidal zones. 
    Getting rid of the D vex tunicate in Whiting Harbor is another important step in the management process.  Davidson said Alaska has less of an invasive problem than many other coastline states, particularly California. 
    “Alaska has a stronger reason to protect its territory,” Davidson said.  “You can get back to a pristine condition.”
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Anna Bisaro recently earned a master’s degree in journalism from Northwestern University’s Medill School of Journalism, and is a communications intern with the Sitka Conservation Society.