Feature Story                                   November 2013

Killing Rock Vomit in Whiting Harbor, Alaska – A Thought Experiment

By Monaca Noble, Linda McCann, Ian Davidson, Kristen Larson, and Kimberly Holzer

Early detection worked. A new invader was discovered. Now what?

Didemnum vexillum by Ian Davidson
The beautiful leather star, Dermasterias imbricate, sits on a mass of yellow rock vomit in Whiting Harbor, AK. Photo by Ian Davidson. 

Back in 2010, the notorious “rock vomit” fouling organism Didemnum vexillum was discovered at an old aquaculture farm in Sitka, Alaska, during the first Marine Invasive Species Bioblitz (see the December 2010 Feature Story for details). Concern that the rapidly growing invader would impact local fisheries in the area by completely covering aquaculture nets, shellfish beds, and sensitive marine environments, led to a push by several agencies, including the Alaska Department of Fish and Game (ADF&G), Smithsonian Environmental Research Center (SERC), U.S. Fish and Wildlife Service (USFWS), University of Alaska Southeast (UAS), National Oceanic and Atmospheric Administration (NOAA), and San Francisco State University's Romberg Tiburon Center (SFSU-RTC), to explore ways to stop its spread. By August 2010, dive surveys had been completed mapping the extent of the infestation in Whiting Harbor, Sitka. The tunicate was found at the aquaculture farm surveyed during the bioblitz as well as on a variety of substrates on the seafloor beneath. A more detailed survey was carried out in January 2011 to look for changes in distribution. Learn more from ADF&G here.

Let’s consider now that you are the scientist tasked with eliminating this species. How would you do it? We’ll give you some background information from the study we did to get you started.

A net covered in rock vomit is cut into pieces. Photo by Kimberly Holzer.
Carrie Hisaoka places the net into the treatment bags. Photo by Kimberly Holzer.
Kristen Larson hangs the treatment bags off the test platform in Whiting Harbor. Photo by Kimberly Holzer
After the immersion treatments have been completed, the nets were removed from the Ziploc bags and hung in the water, where they were monitored for five weeks. Photo by Ian Davidson 

Testing treatment options

While Alaska Department of Fish and Game and others were determining the extent of rock vomit’s distribution in the harbor and evaluating response options, SERC researchers also went to work testing possible management options to rid the harbor of the tunicate and stop its spread. After researching previous eradication attempts on the tunicate and talking with state and local officials and stakeholders, SERC researchers began trials using five different immersion treatments: freshwater, high salinity brine (62ppt salinity), hypoxia (oxygen depletion, 0.5 mg L-1), acetic acid (10%) and chlorine bleach (1%).

Researchers conducted the experiments in Sitka at the site of the infestation between June and November 2011. Netting from the aquaculture farm covered with rock vomit was cut into pieces, placed in Ziploc bags containing either ambient water from the harbor (control) or one of the five treatment solutions mentioned above, and placed in the water. Each treatment was tested for different time periods ranging from minutes (chemical treatments) to hours (non-chemical treatments). After each treatment, rock vomit was removed from the bag and hung in the water so its mortality could be assessed over a five-week period. The results of the experiments are summarized in the following table (for details, see Marine Pollution Bulletin publication McCann et al. 2013).

Table 1: Summary results of the five submersion treatments from McCann et al. 2013.
Freshwater Complete mortality in all treatments by five weeks.
Saltwater brine 24-hour treatments resulted in complete mortality by five weeks, but the two-to-four-hour treatments were not 100% effective.
Hypoxia All of the rock vomit survived these treatments.
Acetic acid All of the rock vomit exposed to the acid died by 5 weeks.
Chlorine bleach Complete mortality was found in the ten-minute treatments but not the two- and five-minute treatments.

Scaling it up – a thought experiment
Now we know what treatments can kill rock vomit in a Ziploc bag, but how can any of these treatments be applied to the rock vomit growing on the floating docks, aquaculture equipment and all the different substrates of the sea floor below? Let’s assume for the moment that we have no monetary, technical or political constraints. We invite you to ponder this question and do your own thought experiments on how you would control rock vomit. Here are some things to consider:

Treatment area – what size patch to treat?
In 2011 the rock vomit in Sitka Harbor included the aquaculture farm and large and small patches of a 1.13 km area at the back of the bay. The aquaculture farm was a floating dock and was pulled out of the water. How would you treat the patches covering the seafloor and rock walls of the bay? The infected area is a complex, three-dimensional landscape covered with boulders, old aquaculture gear, World War II relics and other debris. What is the maximum size of a treatment patch, and how would you cover such an area in order to apply and contain the treatment for the required amount for time?

Containment - dilution both is and is not the solution
Each of these treatments requires a containment system that will hold the treatment solution at the correct concentration for the needed treatment duration. Leaky containment could dilute treatment solutions to a level that may not be lethal to rock vomit. Once you have found a containment system that can maintain a lethal concentration of the solution within the treatment area, how would that solution be collected or released?

Limiting mortality outside the treatment area
During the January 2011 survey, 63 species were recorded in the rock vomit areas. Containment and possible release, treatment type and duration are all factors that will affect these non-target species inside and outside the treatment zone. If the solution is released too quickly from the containment system it may result in damage to organisms in the surrounding area. Is this acceptable? After all, if eradication is successful, would not the habitat be recolonized by local animals and plants? Are some steps required to protect non-target organisms from the eradication treatments? What other factors need to be considered in order to reduce mortality outside of the treatment area?

Wind, waves, and tide
You’ve selected the size of your treatment area, selected and tested a containment system, and reduced non-target mortality. What other factors need to be considered in order to successfully apply the treatment? How could current, waves and weather impact the containment system? Will an incoming or outgoing tide help or hinder the successful application of the treatment?

Eradication of an established invasive species is complicated and potentially costly to accomplish. The research conducted by Linda McCann and her colleagues provide some potential solutions, and they conclude in their recent publication that targeted application of several treatment combinations appears feasible. However, as we can see from the above thought experiment, identifying the best approach for scaling up to treat large areas--needed for any effective eradication or control campaign--is not simple. They also conclude that successful eradication will depend on the efficacy of the treatment and the quality of post-treatment monitoring such that further "sweeps" can be done if needed.

If you have considered all of the factors listed above and believe that you have the perfect solution for removing rock vomit from Sitka, Alaska, we’d like to hear about it! Please email Monaca Noble at noblem@si.edu. Provided we receive enough responses, we will select the winning idea on December 16th and post it on our website.

We hope you have a new appreciation for how difficult it is to eradicate an invasive species once it is established and why prevention is a much more cost-effective solution to the invasive species problem.

McCann, LD, K Holzer, IC Davidson, GV Ashton, and GM Ruiz. 2013. Promoting invasive species control and eradication in the sea: Options for managing the tunicate invader Didemnum vexillum in Sitka, Alaska. Marine Pollution Bulletin. Available online October 25, 2013.