Research ProjectEradication of Introduced Species

Investigating Eradication Treatments for Didemnum vexillum in Sitka, Alaska

  • View leaving ADF&G Research Vessel Kestrel

    Alaska Department of Fish and Game Research Vessel Kestrel

    The Alaska Department of Fish and Game Research Vessel Kestrel, its crew, and divers were an integral part of this project. Photo by Ian Davidson (SERC).

  • Diver Michelle Marraffini with treatment enclosure dome

    Diver Michelle Marraffini and treatment enclosure dome

    Peering through the treatment dome window we could check on the progress of our treatments. Photo by Ian Davidson (SERC).

  • Close up view of Didemnum vexillum in a treatment plot.

    Close up view of Didemnum vexillum in a treatment plot.

    The colonial tunicate Didemnum vexillum inside a treatment plot, scientists measured its responses to multiple eradication techniques. Photo by Ian Davidson (SERC).

  • Underwater photo of Didemnum vexillum

    Underwater photo of Didemnum vexillum

    Underwater photo of Didemnum vexillum and small Metridium senile anemones in Whiting Harbor Sitka, Alaska. Photo by Kim Holzer (SERC)

  • Diver in the water testing out dome treatments

    Diver in the water testing out dome treatments

    Diver Jeff Meucci, Alaska Department of Fish and Game, testing treatment domes, which allowed treatments to be applied to small experimental areas. Photo by Tammy Davis (ADF&G)

  • Photo of D. vex in a kelp forest

    Didemnum vexillum in kelp forest

    Didemnum vexillum blankets the seafloor displacing or smothering other organisms in its path, like here in a kelp forest. Photo by. M. Marraffini

  • View inside a salt treatment dome.

    View inside a salt treatment dome.

    Salinity was increased inside a salt treatment dome. Photo by I. Davidson.

Affiliated Labs

Project Goal

The project goal is to investigate potential techniques for eradication of a colonial tunicate (Didemnum vexillum) on the seafloor. Previous work looked at the feasibility of different treatments and current work will scale up those experiments to cover larger areas of the seafloor.


Nonnative species management tries to prevent introductions from occurring through vector management and by reducing or removing introduced species that have already become established. Given the range of impacts caused by introduction, eradication or removal of a species can be a valuable tool for marine and coastal resource managers.  Eradication has been used successfully in many other systems, for example the removal of black rats in Barrow, Australia and the removal of feral pigs Santa Catalina Island, CA, but removal has been underutilized in marine systems.

Didemnum vexillum on rocks. Shows siphon openings and channels characteristic of this species.
D. vex growing on the rocky causeway that surrounds Whiting harbor. Photo by: I. Davidson.

One nonnative species that has become a concern in Alaska is Didemnum vexillum (D. vex or rock vomit). D. vex is a colonial tunicate made up of many individuals, called zooids, that are compiled into a mat or tunic. It is originally from Japan but has been introduced around globe.  Its ability to live in a variety of substrates and habitats, reproduce through fragmentation or “dripping”, and its fast growth rates make it a prolific invader. In many areas where it is invasive, D. vex can cover the seafloor smothering local species, for example, it covers over 200 km2 on Georges Bank off the coast of New England (Lengyel, Collie, and Valentine, 2009).  In 2010, D. vex was found in Whiting Harbor in Sitka, AK as part of a citizen scientist effort called BioBlitz (see related feature story).  Given its detrimental effects in other areas, there is concern about how it might affect the Sitka Sound ecosystem and steps were taken to limit its spread. We have worked with our partners at the Alaska Department of Fish and Game to remove infested debris and develop an eradication plan.



During summer 2015 we used custom-designed enclosures, like blue tents, to enclose areas of the seafloor and divers applied chlorine, lime, and salt treatments inside the enclosures.  Divers used photographs and examined the seafloor by hand to measure the amount of D. vex present before and after treatments. After three weeks, we revisited the treated areas to see how the D. vex responded. Salt and chlorine treatments tested in 2015 removed 100% of the D. vex on the flat seafloor and chlorine was most effective on the sloped rocky causeway surrounding the harbor. Based on efficacy and logistics, chlorine was chosen as the most feasible for scaled-up treatments.

In summer 2018 we scaled up our trials to test the feasibility of our method on a larger spatial scale. We deployed turbidity curtains, which created large enclosed cylinders of the water column in both open and shore-bound spaces. Inside the tightly contained curtains we treated D. vex with chlorine and monitored the response of the invasive tunicate and its background community.

Schematic eradication: left photo is of D.vex. Middle photo is of the treatment dome. Right hand photo is of  a treated plot where the Dvex has died.
Left: Close up of D.vex in a plot. Middle: Photo of the treatment dome attached to the seafloor which stayed in place for 4 hours. Right: The after measurements of plot treated with Chlorine where the D. vex has died (taken 3 weeks after treatment). Photos by I. Davidson. 
Diver photographing treatment plot and view inside a dome
Top: SERC diver Ian Davidson
photographs treatment plot. 
This will allow us to measure
the loss of D. vex cover.
Photo by M. Marraffini
Bottom: View inside a treatment 
dome. Photo by: I. Davidson


We are analyzing preliminary results from the scaled up trials. Although 100% removal of D.vex was not achieved, there were very significant reductions in cover which may provide feasibility for a harbor wide removal campaign. Chemical eradication involves some loss of non-target (native) species but local plants and animals can re-colonize these areas. Since there are nearby populations of these species, they can regrow and resettle.

Future plans

While D. vex remains in an early stage of invasion in Alaska, we have an opportunity to manage it because of the relatively small scale of its current distribution. If it expands outside of Whiting Harbor and into Sitka Sound, that opportunity will be diminished. Our data shows that chlorine treatment can be used to remove it from the seafloor and, if combined with other techniques (e.g. manual removal), the tunicate may be suitable for a harbor-wide eradication attempt. Funding from the North Pacific Research Board for this project helps to determine how management can reduce the risk of further D. vex spread and curtail its expansion on the seafloor in Sitka. Results from the scaled-up treatments will be reported in 2019.

For more updates on our progress and tales from the field follow us on our blog

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Alaska Department of Fish and Game

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Cohen, C.S., McCann, L., Davis, T., Shaw, L. and Ruiz, G., 2011. Discovery and significance of the colonial tunicate Didemnum vexillum in Alaska. Aquatic Invasions, 6(3), pp.263-271.

McCann, L.D., Holzer, K.K., Davidson, I.C., Ashton, G.V., Chapman, M.D. and Ruiz, G.M., 2013. Promoting invasive species control and eradication in the sea: Options for managing the tunicate invader Didemnum vexillum in Sitka, Alaska. Marine pollution bulletin, 77(1), pp.165-171.


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