A variety of mitigation techniques are available to help control invasive ascidians. Preventative measures that keep surfaces free of ascidians (e.g., antifouling paints, air drying, mechanical scraping, etc) may be most useful because they limit the spread of ascidians. Once invasive ascidians become established in a new location they can be almost impossible to eradicate. Various physical and chemical treatments are used to remove ascidians from substrates, but most fouling control methods are inefficient, costly, time consuming or harmful to non-target species.
Keeping surfaces ascidian-free (especially boat hulls, docks, and aquaculture equipment) helps prevent ascidian invasions. One of the most commonly used preventive methods is the application of commercially available antifouling paints. Antifouling paints are very effective at stopping fouling and greatly reduce ascidian settlement (Bellas 2005, Bellas 2006, Darbyson et al. 2008a). A significant limitation of antifouling paints is that many are toxic or have negative environmental impacts (e.g., Claisse & Alzieu 1993, Fernandez & Pinherio 2007, Konstantinou 2006), thus they cannot be used in sensitive biological areas or for aquaculture. This may be changing, as a few promising antifouling paints have been developed that are non-toxic and can be applied directly onto living shellfish (De Nys et al. 2004).
Periodic exposure to air also keeps surfaces ascidian-free. Ascidians cannot tolerate desiccation stress (Valentine et al. 2007, Darbyson et al. 2008a), so air drying will kill them. As a result, floating docks and boats have been removed from the water to help control ascidian outbreaks (Coutts & Forrest 2007). Removing artificial substrates has the added benefit of reducing the amount of hard substrate available for ascidian growth (e.g., Glasby et al. 2007, Locke et al. 2007). Air drying can also eliminate ascidians from aquaculture gear and shellfish, but care must be taken to ensure that shellfish can withstand the desiccation stress. These stresses vary depending on air temperature, relative humidity and size of individual animals (Katayama and Ikeda 1987), so it is important to determine emersion tolerances of ascidians and bivalves prior to air drying (V alentine et al. 2007).
Site-specific mechanisms may be used to keep surfaces ascidian-free. For example, commercial shellfish growers have experimented with novel antifouling methods. First, they have temporarily placed aquaculture bags at the mouth of rivers in the hope that low salinity levels would reduce fouling (e.g., Thiyagarajan & Qian 2003, Bullard & Whitlatch 2008). Second, they have placed aquaculture bags in eelgrass (Zostera marina) beds to see if antifouling compounds produced by the plants (e.g., Zimmerman et al. 1995, Newby et al. 2006) would prevent fouling. Results of these efforts have been mixed, and more study is warranted to fully assess their effectiveness.
Public education provides an important key to prevention. Boaters, harbormasters, and private dock owners are often avid stewards of the marine environment and can help monitor and control invasive ascidian populations. Before the public can take action, they must first be made aware of the problems posed by invasive ascidians and taught to identify common invaders. To this end, numerous groups have prepared illustrated pamphlets that describe invasive ascidians (Biosecurity New Zealand, British Columbia Shellfish Growers Association, Environment Canada, Fisheries and Oceans Canada, NOAA Sea Grant, etc). Education efforts encourage interested users to combat invasive ascidians (by ensuring that their boat hulls are scraped regularly or by notifying governmental or scientific agencies when new invasive species are spotted) and increase public awareness of the problem. Increased awareness is an integral step to legislative support efforts. Current legislative regulations vary; for example, removal of invasive ascidians is voluntary for some user groups (recreational boat owners and dock owners), but compulsory for others (commercial shellfish growers).
There are many methods to remove ascidians from natural and artificial substrates. Physical removal treatments include mechanical scrapping, pressure washing, smothering, air drying (including UV light exposure; e.g., Bingham & Reyns 1999, Bingham & Reitzel 2000), etc. The most commonly used technique is the mechanical removal of fouling organisms by scraping or pressure washing (Coutts 2006, Minchin et al. 2006). Mechanical removal of hull fouling can prevent the spread of ascidians between areas (e.g., Minchin et al. 2006, Minchin & Sides 2006), though during heavy infestations the disposal of a large amounts waste ascidians can pose an environmental challenge (Locke personal communication; Figure 7). Ascidians can be physically removed from aquacultured bivalves to increase shellfish growth rates and to make the final shellfish product visually appealing. While manual brushing is highly effective, it is inefficient and costly because each bivalve must be individually handled. Pressure washing is an easier and quicker method for removing ascidians from shellfish (Clancey & Hinton 2003). For example, in PEI, Canada the tunic of solitary ascidians fouling mussel lines are ruptured with high pressure water jets that do not harm the bivalves. A significant concern associated with scraping and pressure washing is the production of viable ascidian fragments. Because fragments of some colonial ascidians can survive and reattach (Bullard et al. 2007b), cleaning activities should be conducted on land if possible and detached colonial ascidians should not be returned to the water.
Smothering also eliminates ascidians. Smothering is accomplished by covering infested surfaces with materials that prevent water and light exchange and result in anoxic conditions. Although highly effective, smothering is nonselective and non-target species are killed along with ascidians (Coutts 2006). Coutts (2006) details smothering techniques aimed at eliminating Didemnum vexillum from Shakespeare Bay, New Zealand (see also Coutts & Forrest 2007). In this situation, polyethylene plastic sheets were wrapped around wharf pilings and seafloor areas were covered with geotextile fabric and buried with dredge spoil. All of these smothering techniques proved effective, but had limitations. Most D. vexillum colonies were eliminated from the wharves, although some survived due to ruptures in the plastic wrap and the inability of the covering to form watertight seals over rough bottom features. Similarly, some colonies survived on the seafloor near the seams of the geotextile fabric. Dredge spoil (~10 cm thick) eliminated all D. vexillum from level seafloor areas (Coutts 2006, Coutts & Forrest 2007), but proved ineffective on sloped seabed areas (Coutts & Forrest 2007).
Ascidians can be removed using chemical sprays or immersions in chemical baths. Caustic chemicals (e.g., acetic acid, sodium hypochlorite etc.) and non-caustic chemicals that are harmful to marine organisms (e.g., freshwater, steam) have been used. Due to the potential risks to non-target species, especially in aquaculture settings, care is needed in selecting appropriate chemical treatments. Perhaps the most environmentally friendly "chemical" is freshwater. Unfortunately, some ascidians can tolerate freshwater unless they are immersed in it for relatively long periods (Katayama and Ikeda 1987, Denny 2008) and it often difficult to gain access to large quantities of freshwater in the field. Acetic acid (i.e., vinegar) effectively removes ascidians (Carver et al. 2003, Locke et al. 2008), but can harm bivalves and other organisms (Forrest et al. 2007, Locke et al. 2008) and may leave some ascidians alive after exposure (Denny 2008). Sodium hypochlorite (chlorine bleach) proved very effective at removing Didemnum vexillum from green mussel seed (Denny 2008); exposing D. vexillum to 0.5% chlorine bleach for two minutes killed 100% of ascidians but left seed mussels relatively unaffected. Other caustic chemicals have been used (formalin, detergents, calcium hydroxide, sodium hydroxide, etc.) with varying results.
A final method of ascidian control involves the biological control of ascidians by other organisms. Some predators feed on ascidians (Lambert 2005a) and may be useful in controlling ascidian populations (Osman & Whitlatch 1999, Osman & Whitlatch 2004). For example, sea urchins consumed solitary ascidians attached to scallop shells (Ross et al 2004). However, biological controls are highly species-specific (e.g., Simoncini & Miller 2007) and in some cases predators do not appear able to control ascidians (Carman et al. 2008b). Diseases may also be used to control invasive species (e.g., Mutze et al 2008) and could be effective at controlling ascidians (e.g., Moiseeva et al. 2004). To date, there has been no directed effort to culture any pathogen specifically for use in ascidian control.
The controls described above are primarily designed for use against small-scale infestations. They can be of great value to individual users who want to control ascidian growth in localized areas (e.g., individual shellfish growers or boat owners), but when used independently have limited impact on larger-scale ascidian outbreaks. Coastal management efforts involving large-scale coordinated approaches are needed to prevent the arrival of ascidians or to control them once they are established (e.g., Locke & Smith 2008).
One of the most vigorous and well documented attempts to control a newly established ascidian involve the efforts to eliminate Didemnum vexillum from Whangamata Harbor, Shakespeare Bay, New Zealand (Coutts 2002a, Coutts 2006, Coutts & Forrest 2007). The species was introduced to the area in December of 2001 when a heavily fouled barge was moored in Whangamata Harbor. The ascidian's place of origin is unclear because the barge had traveled extensively since its launch in Australia in 1969. D. vexillum had obviously arrived with the barge because the species had not been seen in the area before the barge's arrival and its distribution was initially confined to the barge's hull and the seafloor directly beneath it. Once introduced, D. vexillum began to proliferate rapidly. In August 2002, an initial containment effort using underwater vacuums removed ~80% of the ascidian biomass from areas immediately surrounding the barge, but by this time the species had already infested ~40% of the pilings at a nearby shipping wharf (Coutts 2002b, Coutts & Forrest 2007). Eleven months later, the species was firmly established in the bay and was located on pilings, boat hulls, moorings, and natural surfaces. It had also spread to an aquaculture facility 35 km away due to movement of an infested aquaculture pontoon. In September 2003 an intensive eradication campaign was launched in a bid to eliminate D. vexillum from Shakespeare Bay. Mitigation efforts included: 1) removing infested boats from the water to physically remove and desiccate the ascidian; 2) treating infested boat hulls with high doses of chlorine; 3) wrapping wharf pilings with polyethylene to smother the ascidian; 4) smothering infested seabed areas with dredge spoil; and 5) smothering infested seabed areas with small pore-size geotextile fabric (Coutts & Forrest 2007). Despite significantly reducing local D. vexillum population sizes, these herculean efforts failed to eliminate the species from the system. D. vexillum continues to persist and spread throughout the region (Coutts & Forrest 2007).
The results from Shakespeare Bay are significant because they demonstrate that vigorous control methods can temporarily reduce the biomass of introduced ascidians but may not be able to entirely remove them. In this case, initial control efforts began seven months postinvasion (Coutts & Forrest 2007). While it could be argued that this response time was too slow, it is probably well above average for government agencies lacking pre-existing, in place mitigation strategies (i.e., most governmental agencies worldwide). Since it was impossible to eradicate an ascidian invader under these favorable conditions, few ascidian invaders may be able to be contained after they become established. It is therefore essential that at-risk regions develop detection and control plans so that when faced with a new ascidian invasion they can mount rapid, coordinated responses supported by previously established funding sources (e.g., Locke & Smith 2008).
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