An Integrated Assessment of the Continued 

Spread and Potential Impacts of the Colonial 

Ascidian, Didemnum sp. A, in U.S. Waters 

NOAA Technical Memorandum NOS NCCOS 78 
Mention of trade names or commercial products does not constitute 

endorsement or recommendation for their use by the United States 

government. 

Cover images: Photo 1: Mussel cage covered with tunicates at Okeover Inlet, Malaspina 

Peninsula, BC, Photo credit Gordon King (Taylor Shellfish Farms, Inc). Photo 2: Close 

up of Didemnum colony surface at Sandwich Town beach, Mass,. Photo credit: Dann 

Blackwood (USGS) 

Citation for this report: 

.Daley, Bryon A and Don Scavia. 2008. An Integrated Assessment of the Continued 

Spread and Potential Impacts of the Colonial Ascidian, Didemnum sp. A, in U.S. Waters. 

NOAA Technical Memorandum NOS NCCOS 78, 61 pp. 
An Integrated Assessment of the Continued Spread 

and Potential Impacts of the Colonial Ascidian, 

Didemnum sp. A, in U.S. waters. 

Bryon A. Daley Don Scavia 

Great Lakes Science Center School of Natural Resources 

U.S. Geological Service & Environment 

Ann Arbor, MI 48105, USA University of Michigan 

Ann Arbor, MI 48109-1114, 

USA 

NOAA Technical Memorandum NOS NCCOS 78 

June 2008 

United States Department of National Oceanic and National Ocean Service 

Commerce Atmospheric Administration 

Carlos M. Gutierrez Conrad C. Lautenbacher, Jr. John H. Dunnigan 

Secretary Administrator Assistant Administrator
Summary 

Didemnum sp. A is a colonial ascidian or “sea squirt” of unknown geographic 

origin. Colonies of Didemnum sp. A were first documented in U.S. waters in 1993 at 

Damariscotta River, Maine and San Francisco Bay, California. An alarming number of 

colonies have since been found at several locations in New England and along the West 

Coast of the contiguous continental United States. Originally believed to be restricted to 

artificial structures in nearshore habitats, such as ports and marinas, colonies of 

Didemnum sp. A have also been discovered on a gravel-pavement habitat on Georges 

Bank at depths of 40-65m. The wide distribution of Didemnum sp. A, the presence of 

colonies on an important offshore fishing ground, and the negative economic impacts that 

other species of noninidigenous ascidians have had on aquaculture operations have raised 

concerns about the potential impacts of Didemnum sp. A. We reviewed the available 

information on the biology and ecology of Didemnum sp. A and potentially closely 

related species to examine the environmental and socioeconomic factors that may have 

influenced the introduction, establishment and spread of Didemnum sp. A in U.S. waters, 

the potential impacts of this colonial ascidian on other organisms, aquaculture, and 

marine fisheries, and the possibility that it will spread to other U.S. waters. In addition, 

we present and discuss potential management objectives for minimizing the impacts and 

spread of Didemnum sp. A. 

Concern over the potential for Didemnum sp. A to become invasive stems from 

ecological traits that it shares with other invasive species, including the ability to 

overgrow benthic organisms, high reproductive and population growth rates, ability to 

spread by colony fragmentation, tolerance to a wide range of environmental conditions, 

apparent scarcity of predators, and the ability to survive in human dominated habitats. 

At relatively small spatial scales, species of Didemnum and other nonindigenous 

ascidians have been shown to alter the abundance and composition of benthic 

assemblages. In addition, the Canadian aquaculture industry has reported that heavy 

infestations of nonindigenous ascidians result in increased handling and processing costs. 

Offshore fisheries may also suffer where high densities of Didemnum sp. A may alter the 

access of commercially important fish species to critical spawning grounds, prey items, 

and refugia. Because colonial ascidian larvae remain viable for only 12–24hrs, the 

i
introduction and spread of Didemnum sp. A across large distances is thought to be 

predominantly human mediated; hull fouling, aquaculture, and ballast water. Recent 

studies suggest that colony growth rates decline when temperatures exceed 21 oC for 7 

consecutive days. Similarly, water temperatures above 8 to 10 oC are necessary for 

colony growth; however, colonies can survive extended periods of time below this 

temperature threshold as an unidentified overwintering form. 

A qualitative analysis of monthly mean nearshore water temperatures suggest that 

new colonies of Didemnum will continue to be found in the Northeast U.S., California 

Current, and Gulf of Alaska LMEs. In contrast, water temperatures become less 

favorable for colony establishment in subarctic, subtropical, and tropical areas to the 

north and south of Didemnum’s current distribution in cool temperate habitats. We 

recommend that the Aquatic Nuisance Species Task Force serve as the central 

management authority to coordinate State and Federal management activities. Five 

objectives for a Didemnum sp. A management and control program focusing on 

preventing the spread of Didemnum sp. A to new areas and limiting the impacts of 

existing populations are discussed. Given the difficulty of eradicating large populations 

of Didemnum sp. A, developing strategies for limiting the access of Didemnum sp. A to 

transport vectors and locating newly established colonies are emphasized. 

 

KEYWORDS: Ascidian, Didemnum, integrated assessment, invasive species 

management, Large Marine Ecosystems, tunicate 

ii
Table of Contents 

Summary i 

Table of Contents iii 

List of Tables and Figures iv 

Glossary of Terms v 

Introduction 1 

Documentation of Status and Trends 5 

Didemnum distribution and population abundance 5 

 Environmental and socioeconomic impacts of Didemnum 10 

Description of Potential Causes and Consequences of Status and Trends 10 

 Potential causes of Didemnum distribution and abundance 10 

 General biology, ecology, and life history of Didemnum 10 

 Environmental tolerances of Didemnum 

 and interactions with other species 15 

 Potential methods of introduction and spread 18 

 Potential environmental and economic consequences of Didemnum 20 

Forecasts of Future Conditions with No Management Action 23 

Provision of Guidance for Potential Management Actions 33 

Acknowledgements 46 

Literature Cited 47 

Appendix I 61 

iii
List of Tables and Figures 

Figure 1. Didemnum colony growing on a rope in San Francisco Bay, CA. 2 

Figure 2. Large Marine Ecosystems of the United States. 4 

Figure 3. Locations where Didemnum colonies have been documented in the 

Northeast U.S. Large Marine Ecosystem. 6 

Figure 4. Locations where Didemnum colonies have been documented in the 

Gulf of Alaska and California Current Large Marine Ecosystems. 7 

Figure 5. Maps showing size and distribution of Didemnum invasion on 

Georges Bank from 2003 to 2005. 9 

Figure 6. Close up of Didemnum colony surface. 11 

Figure 7. Photographs showing colonies of Didemnum sp. A growing on natural 

and artificial substrates. 11 

Table 1. Locations and year of Didemnum sightings. 12

Table 2. Mean monthly and monthly maximum-minimum water temperatures 

from nearshore locations. 27 

Appendix I. United States National Ballast Water Program regulations. 61 

iv
v

Glossary of Terms 

Aquaculture - the cultivation of freshwater or marine plants and animals for food or 

 other purposes. 

Ascidian - invertebrate members of the Phylum Chordata (Subphylum Tunicata; Class 

 Ascidiacea) Also referred to as tunicates or ‘sea squirts’. 

Ballast water - water used as weight to improve vessel stability. 

Dispersal - the spread of a species, population, or individual’s progeny over time.

Invasive species - nonindigenous species whose introduction causes or may cause harm 

 to the economy, environment, or human health. 

Large Marine Ecosystem (LME) - large areas of the ocean (i.e., generally 200,000 km2 

 or greater) that are characterized by; bathymetry, hydrology, productivity, and 

 trophodynamics 

Nonindigenous species - a species that has been intentionally or accidentally transported 

 by human activities into a region where it does not naturally occur. Also 

 commonly referred to as non-native species, exotic species, or introduced 

 species. 

Propagule pressure – a composite measure of the number of individuals released into an 

 area to which it is not native. Includes the number of larvae or fragments 

 released during an introduction event and the number of times larvae of 

 fragments are introduced. 

Transport vector - The physical means or agent by which a species is transported to a 

 new habitat, such as ballast water and hull fouling. 
Introduction 

 Invasive species are nonindigenous species whose introduction causes or may 

cause harm to the economy, environment, or human health (Office of Technology 

Assessment 1993). In the U.S., the effects of invasive species are estimated to cost 

between $120 - 137 billion per year. This estimate includes the costs of eradication and 

control efforts, as well as damages to agriculture, fisheries, utilities, and other economic 

activities (Pimentel et al. 2000, 2005). The environmental impacts of invasive species 

include the disruption of ecosystem processes, such as natural fire regimes (Lippincott 

2000) and nutrient cycling (Vitousek and Walker 1989). They also pose a threat to native 

plants and animals through predation (Wiles et al. 2003), competition (Thomson 2004), 

habitat alterations (Botts et al. 1996, Mayer et al. 2001), and hybridization (Echelle and 

Echelle 1997) (see also, Wilcove et al. 1998, Mack et al. 2000, Simberloff 2005). 

Furthermore, many of the pressing health issues of the last three decades are the result of 

introduced pathogens, such as the Human Immunodeficiency Virus (HIV) and the West 

Nile Virus. Clearly, invasive species have significant socioeconomic and environmental 

impacts, and preventing or reducing these impacts is an important management priority. 

 Concern over the number, variety, and effects of nonindigenous species in marine 

systems has lagged behind that shown for terrestrial and freshwater ecosystems (Carlton 

1989, Office of Technology Assessment 1993, Ruiz et al.1997). However, increased 

monitoring efforts over the past 15 to 20 years have found that nonindigenous species are 

conspicuous components of marine communities throughout the world, especially in 

estuaries (Carlton 1989, Cohen and Carlton 1998, Ruiz et al. 1999, Ruiz et al. 2000). In 

particular, several nonindigenous species of colonial and solitary ascidians, invertebrate 

members of the Phylum Chordata (Subphylum Tunicata; Class Ascidiacea), have been 

found across widely separated geographical locations (Whitlatch and Osman 1999, 

Lambert 2002, Lambert and Lambert 2003, Pederson et al. 2005, Cohen et al. 2005, 

WGITMO 2005). Most recently, ecologists from Europe (Minchin and Sides 2006, 

Wijman and Smaal 2006), New Zealand (Coutts 2002), and North America (Bullard et al. 

2007a) have reported an alarming increase in the abundance and distribution of either a 

single species or an unknown number of morphologically similar species of the colonial 

ascidian genus, Didemnum (Family Didemnidae) (Fig. 1). 

1
Although the precise impacts of 

these newly discovered colonies of 

Didemnum are not known, their 

relatively sudden appearance and rapid 

expansion has raised concerns that they 

may become invasive. This concern 

stems in part from ecological attributes 

that this colonial ascidian shares with 

other invasive species, including 1) the 

ability to overgrow other species and to 

carpet bottom substrates, 2) high reproductive and population growth rates, 3) ability to 

spread by fragmentation, 4) tolerance to a wide range of environmental conditions, 5) 

apparent scarcity of predators, and 6) the ability to survive in human dominated habitats 

(Lodge 1993). In addition, the fouling of equipment and stock by other species of 

introduced ascidians has increased aquaculture maintenance and processing expenses 

(Cayer et al. 1999, Boothroyd et al. 2002). 

 Figure 1. Didemnum colony growing on a rope in 

San Francisco Bay, CA (Photograph is from Cohen 

2005 and credited to Gretchen Lambert, Univ. of 

Washington). 

It should be noted that although the recently discovered Didemnum colonies from 

Europe, New Zealand and North America appear morphologically similar, their 

taxonomic relationship and geographic origin remain unresolved issues. The earliest 

reported colonies were referred to as Didemnum lahillei or D. helgolandicum. However, 

these designations were subsequently determined to be incorrect for the recently 

appearing Didemnum populations (Bullard et al. 2007a). Two specimens of these 

recently discovered colonies have since been described as new species: Didemnum 

vestum Kott, 2004 from Portsmouth Harbor, New Hampshire, and Didemnum vexillum

Kott, 2002 from Whangamata Harbour, New Zealand. However, a full description of D. 

vestum was not possible because the specimen lacked larvae, a key component for 

describing new species of Didemnum. As a result, D. vestum is a questionable 

designation and remains limited to colonies from its original collection site at Portsmouth 

Harbor, New Hampshire (G. Lambert, personal communication). Colonies from Europe 

and North America have not yet been formally compared to D. vexillum, and until studies 

are completed are being referred to as Didemnum sp. A (G. Lambert, personal 

2
communication). The genus Didemnum is a difficult taxonomic group to identify to the 

species level. There are a large number of undescribed species in addition to the 

morphologically similar populations discussed here that are currently undergoing a rapid 

world-wide expansion. Until the taxonomy of these different populations can be 

clarified, the use of Didemnum sp. A is a necessary designation to distinguish these 

morphologically similar colonies that have recently appeared and are rapidly spreading 

throughout temperate marine systems from all other unidentified Didemnum spp. Genetic 

sequencing of the Didemnum sp. A populations is a research priority that will help 

determine their geographical origin, their taxonomic identity and the factors underlying 

their recent expansion. For the purposes of this integrated assessment, we refer to 

colonies from Portsmouth Harbor, New Hampshire as Didemnum vestum (Kott 2004), 

and provisionally refer to all other morphologically similar colonies on both U.S. coasts 

as Didemnum sp. A (hereafter referred to as Didemnum) (Cohen 2005, Bullard et al. 

2007a, and G. Lambert personal communication). 

 In the U.S., colonies of Didemnum have been documented in coastal New 

England from northern Maine to New York, Washington State, and at multiple locations 

along the California coast. Until recently, species of Didemnum were primarily observed 

attached to artificial structures in harbors and marinas, and were not believed to pose a 

serious threat to native species occurring on natural habitats (Connell 2001, Lambert 

2001). However, the discovery of Didemnum on Georges Bank (Valentine et al. 2007b) 

has raised concerns about the impacts that this ascidian could have on economically 

important off-shore habitats and the possibility that it will spread to other marine 

ecosystems. As a result, there is a need to determine the necessity and feasibility of a 

Didemnum management plan. There are several ongoing research activities including: 

monitoring changes in the distribution and abundance of Didemnum; assessing the 

physical and biological requirements of Didemnum; and examining the impact that 

Didemnum may have on native species. In addition, there are efforts to eradicate or 

control newly-established populations in Puget Sound, WA and Eastport, ME. While 

these research activities will provide valuable information on the biology and ecology of 

Didemnum, and the feasibility of colony eradication, developing an effective 

management policy is an interdisciplinary problem that depends as much on input from 

3
the social sciences and economics as it does the natural sciences. A formal approach to 

synthesizing and delivering relevant, independent input to decision making is an 

Integrated Assessment, a comprehensive analysis of existing economic, natural and social 

scientific information in the context of policy or management questions (Dowlatabadi et 

al. 2000). 

The present integrated assessment is the first step in a multistage Didemnum 

management process. One of the goals of this assessment is to identify management 

questions and objectives that will serve as a framework for future planning efforts. The 

assessment focuses on describing the potential spread and impacts of Didemnum in U.S. 

waters using qualitative analyses based on the environmental conditions within 

Didemnum’s current U.S. distribution and general information on the ecology of 

Didemnum and other ascidians. These analyses consider the potential spread and impacts 

of Didemnum across and within the 

10 Large Marine Ecosystems 

(LMEs) of the U.S. (Fig. 2) LMEs 

are large areas of the ocean (i.e., 

generally 200,000 km2 or greater) 

that are characterized by; 

bathymetry, hydrology, 

productivity, and trophodynamics 

(Sherman 2000). It is our hope that 

the results of this assessment will 

assist in describing the extent and 

magnitude of the environmental 

and socioeconomic problems posed 

by Didemnum. Furthermore, we hope to identify information gaps and research needs so 

that future management efforts can move towards the use of quantitative approaches, 

such as risk assessments and cost-benefit analyses, at smaller spatial scales. This 

assessment draws upon peer reviewed journal articles, technical reports, and edited 

volumes, as well as results from ongoing research activities to address two main 

questions: 

 Figure 2. Large Marine Ecosystems of the United

States including, clockwise from right to left: 1) 

Northeast U.S., 2) Southeast U.S., 3) Caribbean, 4) Gulf 

of Mexico, 5) California Current, 6) Gulf of Alaska, 7) 

Eastern Bering Sea, 8) Chukchi Sea, 9) Beaufort Sea, 

and 10) Insular Pacific-Hawaiian (Figure Source: 

USCOP 2004). 

4
5

1) To what degree will Didemnum continue to spread in U.S. waters within the 

next 10 years, and 

2) What would be the range of impacts of Didemnum on different marine 

ecosystems located within the U.S. and on society’s uses of those areas? 

Given these questions, the integrated assessment focuses on 1) documenting the 

status and trends of environmental and socioeconomic conditions in the affected regions, 

2) describing the potential environmental and socioeconomic causes and consequences

of those conditions, 3) providing forecasts of future conditions with no management 

action, and 4) providing guidance for potential management actions. 

Documentation of Status and Trends 

Didemnum distribution and population abundance 

Colonies of Didemnum have been documented in the Northeast US, the California 

Current, and the southern end of the Gulf of Alaska LME in British Columbia. Colonies 

were first documented in the U.S. in 1993 at Fort Island Narrows, Damariscotta River, 

ME and San Francisco Bay, CA. However, there are anecdotal accounts suggesting that 

colonies may have been present in Maine as early as the 1970’s. Additional colonies 

have since been documented at 54 sites in coastal New England from Eastport, ME to 

Shinnecock Bay, Long Island, NY (Fig. 2), as well as 57 sites along the west coast of 

North America from Puget Sound, British Columbia to San Diego, CA (Fig. 3). 

However, Didemnum is not restricted to coastal habitats as colonies have recently been 

discovered off-shore on Georges Bank and in eastern Long Island Sound. Furthermore, 

colonies of Didemnum, or closely related species, have been documented worldwide at 

several localities along the coasts of the Netherlands, France, and Ireland (U.S. 

Geological Survey 2006, Minchin and Sides 2006, Wijman and Smaal 2006). 

Within its North American range, Didemnum has a highly disjunct distribution. 

For example, Bullard et al. (2007a) found that colonies of Didemnum were present at 

approximately 50% of the 190 sites surveyed along the east and west coasts of the U.S. 

from 1998 to 2005. Interestingly, no colonies were observed at five sites 
Figure 3. Locations where Didemnum colonies have been documented in the Northeast U.S. Large Marine Eco

November 2006 (modified from U.S. Geological Survey 2006, Bullard et al. 2007a, Osman and Whitlatch 2007)numbers correspond to sites in Tables 1 and 2.
 Figure 4. Locations where Didemnum colonies have been documented in the Gulf of Alaska 

and California Current Large Marine Ecosystems as of November 2006 (modified from U.S. 

Geological Survey 2006 and Bullard et al. 2007a). Circled numbers correspond to sites in 

Tables 1 and 2. 

7
surveyed along the Oregon coast, eight sites south of Port San Louis, CA, or 14 sites 

surveyed south of Shinnecock, NY to Virginia Beach, VA. In a separate survey of west 

coast National Marine Sanctuaries and National Estuarine Research Reserves conducted 

in 2003 and 2004, colonies of Didemnum were found in Elkhorn Slough, Monterey Bay 

and the Gulf of Farallones, to the north of San Francisco’s Golden Gate. The survey did 

not find colonies at any of the 14 sites surveyed in the Olympic Coast National Marine 

Sanctuary, WA or the 9 sites surveyed in Kachemak Bay, AK (deRivera et al. 2005). 

However, updated surveys are needed as new colonies of Didemnum continue to be 

recorded, such as the recent colonies found in San Diego, CA (U.S. Geological Survey 

2006), suggesting that it has not reached its maximum distribution or range. 

 The presence of Didemnum on a section of gravel-pavement habitat on the 

northern edge of Georges Bank, approximately 160 mi. (257 km) east of Cape Cod is 

currently the larger of only two known occurrences of this colonial ascidian in an offshore seabed habitat, with the other being an approximately 1.5 square mile (4 km2) area 

in eastern Long Island Sound (Osman and Whitlatch 2007). However, Didemnum may 

be more common in coastal and off-shore open-water habitats than has been documented, 

as faunal surveys of these deep-water habitats are logistically difficult to perform and are 

conducted less frequently relative to the rapid assessment surveys conducted in shallow, 

near-shore habitats. 

While the distribution of Didemnum in U.S. waters has been relatively well 

documented, few studies have estimated the abundance and/or biomass of these 

populations. However, populations on Georges Bank have generally increased in size 

and abundance over the past few years, and have been resilient to habitat disturbance due 

to commercial fishing. Similarly, larval recruitment has increased steadily at two 

nearshore sites in the Northeast U.S. LME. In the fall of 2003, scientists from the 

National Oceanographic and Atmospheric Administration, U.S. Geological Survey, and 

University of Rhode Island discovered colonies of Didemnum distributed over an

estimated 6 square miles (16 km2

) of ocean bottom on Georges Bank. Subsequent 

surveys conducted in 2004 and 2005 assessed larger areas of the gravel-pavement habitat, 

and found colonies distributed over 40 (106 km2

) and 88 square miles (228 km2), 

respectively (Fig. 5) (U.S. Geological Survey 2006). In 2003 and 2004 many of the 

8
 Figure 5. Maps showing size and distribution of Didemnum sp. A populations (in 

yellow) on Georges Bank from 2003 to 2005. Colonies were restricted to semi-isolated 

outcrop of gravel habitat open to fishing in 2003 and 2004. In 2005, colonies were 

found on a larger area of gravel habitat including an area closed to fishing (labeled 

CAII on map) (modified from U.S. Geological Survey 2006). 

sampled sites had colonies of Didemnum covering 50 to 90 percent of the seabed. 

Interestingly, the percent cover of Didemnum observed in August 2005 was less than 

50% at most locations following a period of bottom fishing activity in May-July 2005 

(Robert Reid, personal communication). However, this reduction appears to have been 

short-lived as results from August 2006 showed that the abundance of Didemnum had 

doubled at 75% of the sites surveyed in 2005, and was similar to that observed in 2003 

with colonies covering 50 to 75 percent of the seabed. Despite the high percent cover 

and persistence of Didemnum in this 88 square mile section of gravel-pavement habitat, 

colonies did not appear to be spreading eastward. The 2006 Georges Bank survey also 

included gravel-pavement habitats located in Canadian waters, and found no evidence of 

the presence of Didemnum (U.S. Geological Survey 2006). 

Settlement plate studies in Connecticut and New Hampshire indicate that 

Didemnum recruitment at these sites has increased in recent years. Didemnum 

recruitment at Avery Point and the mouth of the Mystic River in Connecticut was 

approximately one order of magnitude greater in 2003 relative to 2001 (Osman and 

Whitlatch 2007). In Portsmouth Harbor, New Hampshire, Dijkstra et al. (2007a) found 

very little recruitment of Didemnum (possibly D. vestum) in 2003. However, Didemnum

was a major component of the fouling community in 2004, and by the spring of 2005 had 

9
replaced Botryllus schlosseri as the dominant colonial ascidian at the site. 

Environmental and socioeconomic impacts of Didemnum 

Despite the widespread distribution of Didemnum, no dramatic environmental 

impacts, such as species extinctions, have been reported. Similarly, there are currently no 

data to indicate that it is negatively affecting economic activities, including aquaculture 

production or fisheries yields. However, results from a small number of studies 

examining the impacts of Didemnum on the composition of benthic and subtidal 

communities in the Northeast U.S. LME have recently been published (see special feature 

in J. Exp. Mar. Biol. Ecol., 2007, v. 342 on the Proceedings of the 1st International 

Invasive Sea Squirt Conference, Robert B. Whitlatch and Stephan G. Bullard, editors). 

Because these studies span only 1 to 3 years and include only a small subset of the known 

Didemnum populations, we consider these findings to be preliminary. We defer 

discussion of these studies until the potential environmental and socioeconomic impacts 

of Didemnum are considered. 

Description of Potential Causes and Consequences of Status and Trends 

Potential causes of Didemnum distribution and abundance 

The specific factors underlying the relatively sudden appearance of Didemnum are 

not understood. Didemnum was first documented on the west and east coasts of the U.S 

in 1993, suggesting that it has been independently introduced at least twice. It is unclear 

how many of the subsequent recorded occurrences of Didemnum are the result of 

independent introductions, perhaps of different species, versus how many are due to the 

spread of Didemnum from these initial points of introduction. This section provides a 

general discussion of the potential factors underlying the introduction, establishment, and 

spread of Didemnum. In particular, we discuss biological and ecological attributes of 

Didemnum that may have contributed to the establishment of viable populations in the 

U.S. and potential transport vectors and dispersal methods. 

General biology, ecology and life history of Didemnum

Didemnum is a colonial ascidian with sessile, filter-feeding adults and motile, 

10
non-feeding larvae. Colonies consist of thousands 

of small individuals or zooids (~1 mm) embedded 

in a tough outer covering or tunic (Fig. 6), and 

vary in color from pale pink to yellow or orange 

(Kott 2004). Colonies range in morphology from 

long, ropey or beard-like in wave protected 

environments to low-lying undulating and 

encrusting mats in habitats with strong currents 

(Fig. 7). 

In the U.S., Didemnum colonies have been 

found in water temperatures ranging from -2° to 

24 °C (28° to 75 °F), from the low intertidal to depths of 65 m (213 ft) (Table 1), and 

salinities ranging from 26 to 32 ppt. Didemnum colonizes a variety of firm substrates 

such as rocks, gravel, docks, pilings, and aquaculture equipment, but is unable to 

establish colonies on mobile sand, mud, or other unstable substrates (Coutts 2002, 

Valentine et al. 2007b). Didemnum can also colonize and overgrow other benthic 

organisms including sea scallops, mussels, and seaweed (Table 1; Fig. 7), but has no 

known major predators (Dijkstra et al. 2007a, Valentine et al. 2007b). Furthermore, 

Didemnum appears to be able to tolerate substrate disturbances from fishing gear and ice 

scour (Valentine 2007a), and excess nitrogen inputs (Carmen et al. 2007). 

 Figure 6. Close up of Didemnum colony 

surface. Solid, upward arrows point to 

incurrent siphon openings of 3 zooids. 

Dashed, downward arrows point to 

common cloacal openings (Photograph is 

credited to Gretchen Lambert). 

11

 Figure 7. Photographs showing colonies of Didemnum sp. A growing on natural 

and artificial substrates. A) gravel seafloor, B) mussel shells, C) green algae, D) ship 

hull, E) mussel cage, and F) wood pilings (all photographs are from U.S. Geological 

Survey 2006, and photograph credits are shown at bottom left of each photograph). 
 Table 1. Locations and year of Didemnum sightings including depth, temperature 

 and substrate types from when specimens were first collected in the 

 Northeast U.S., California Current, and Gulf of Alaska LMEs. LI = lower intertidal; 

 ND = no data (sources: U.S. Geological Survey 2006, Bullard et al. 2007a, and Osman 

 and Whitlatch 2007). 

 

LMEs and Sites Depth (m) Temp (°C) Substrate Type Year

Northeast US 

Nearshore:

1 Cobscook Bay, ME 2003

 Eastport 5.0-6.0 ND Subtidal rocks, pilings

2 Schoodic, ME ND ND ND 2003

3 Damariscotta River, ME 1993

 Fort Island Narrows 12.0 ND Shell hash, Gravel

 Thrumcap Island 10.0-15.0 ND Rocks

 Boothbay ND ND Dock

 Clark’s Cove ND ND Dock

4 Portsmouth Harbor, NH 2001

 New Castle LI – 5.0 ND Ropes

 Fort Point 5.0 ND Coast Guard Pier

4 Piscataqua River, NH 2001

 Newington 12.0-15.0 ND Rock Surface

5 Isles of Shoals 2003

 Duck Island ND ND ND

6 Rockport, MA

 Sandy Bay < 5.0 14 Boulder and Cobble 2006

7 Winthrop, MA

 Boston Harbor 1.5-3.0 5 Solitary tunicate 2006

8 Plymouth, MA ND ND Docks 2004

9 Sandwich, MA 1998

 Sandwich town beach LI - 3.4 1 Tidepool rocks, floats

10 Provincetown, MA 2003

 MacMillan Wharf LI ND Metal pilings, Tires, Ropes

12 Wellfleet Harbor, MA ND

 Mayo Beach 3.3-3.6 ND Oyster bags

12 Town Cove, MA 2004

 Orleans LI – 0.1 ND Floating Dock

12 Chatham Harbor, MA 2003

 Chatham LI ND Wood Pilings, Mussels

12 Pleasant Bay, MA ND

 Strong Island 2.0-3.0 ND Rocks

13 Cotuit Bay, MA ND

 Osterville Grand Island LI ND Channel markers

14 Martha's Vineyard, MA 2004

 Vineyard Haven LI – 0.5 0 Floats

 Oak Bluffs LI – 2.0 0 Rope, Steel bulkhead

15 Woods Hole, MA 2000

 Eel Pond LI-10 2.2, -2.0 Floating Docks, Ropes

 Iselin Dock 18.0-20.0 ND Steel piling

12
 Table 1. Locations and year of Didemnum sightings continued. 

LMEs and Sites Depth (m) Temp (°C) Substrate Type Year

17 Westport, MA ND

 Tripps Marina ND ND

18 Narragansett Bay, RI 2003

 Coasters Harbor Island ND ND

 Newport Shipyard ND ND

 Schl of Ocean, URI LI ND Pilings, Rocks, Mussels

 Beavertail Point LI ND Rock surface

19 Groton , CT 2000

 Mystic River ND ND Plexiglass plates

 Avery Point ND ND Plexiglass plates

 Pine Island 40.0 2.0-21.0 Plexiglass plates

 Niantic Bay ND ND Plexiglass plates

19 Groton , CT 2000

 Groton Long Point ND ND Plexiglass plates

 Bushy Point ND ND Plexiglass plates

20 Shinnecock Bay, LI, NY LI – 0.1 ND Floating Dock 2004

Offshore & Open water:

5 Isles of Shoals, NH ND ND Suspended fish cages 2003

21 Georges Bank, NW 

Atlantic 41.0-47.0 ND Seafloor gravel, scallops 1998

22 Tillies Bank, MA ND ND Gravel 1996

Gulf of Alaska (Canada)

Vancouver Island

1 Meares Island 2005

 Tofino - Lemmens Inlet ND ND Oyster Farm

2 Baynes Sound 2005

 Nanoose - Deep Bay ND ND Algae, Oyster Farm

British Columbia

3 Pendrell Sound, B.C.

 East Redonda Island 6.0-15.0 ND Rock wall

4 Strait of Georgia, B.C. 2005

 Jedediah Island LI -1 ND Rock surfaces 

5 Okeover Inlet, B.C. 2003

 Malaspina Penninsula 12.0-15.0 12.2 Mussel cages

 Lions Rock ~10 12.8 Rock surfaces 

5 Jervis Inlet, B.C. ~6 ND Rocky seabed

5 Agamemnon Channel, B.C. 2004

 Nelson Island 6.0-12.0 ND Rock wall

California Current

6 Larrabee State Park, WA ND ND ND 2007

7 Dabob Bay, WA ND ND ND 2007

8 Poulsbo, WA 2007

13
14

 Table 1. Locations and year of Didemnum sightings continued. 

LMEs and Sites Depth (m) Temp (°C) Substrate Type Year

13 Dockton Park, WA ND ND ND 2007

14 Des Moines, WA 5.0 ND crab cage 2004

15 LongBranch Marina, WA ND ND ND 2007

16 Steamboat Isl., WA ND ND ND 2007

17 Totten Inlet, WA 0.5-1.0 ND mussels, rope 2004

18 Humboldt Bay, CA 2001

 Woodley Island 1.0 ND PVC fouling panels

19 Bodega Bay, CA ND ND Docks, Rocks 2003

20 Tomales Bay, CA ND ND Docks 2001

21 San Francisco Bay, CA 1993

 Sausalito ND ND Rope, Mussels, Bryozoa

 Horseshoe Bay ND ND ND

 Fisherman's Wharf ND ND Rope on floating dock

22 Monterey Bay, CA 1998

 Elkhorn Slough ND ND Docks

23 Morrow Bay, CA ND ND Docks 2000

24 Port San Luis, CA ND ND Docks 2002

25 San Diego, CA 2007

 Mission Bay 0.3 ND Foam Dock Floats, 

Colonial ascidians, such as Didemnum, brood their young before releasing mature 

larvae that live 12-24 hours. As with most ascidians, newly recruited Didemnum larvae 

probably reach sexual maturity within a few weeks following settlement and 

metamorphosis (Lambert 2002). At Groton, Connecticut and Bodega Bay, California, 

recruitment occurs between July and November with a peak in late summer or early fall 

(Whitlatch and Osman 2007, Bullard et al. 2007a). Furthermore, colonies containing 

mature larvae have been collected in May at Sausalito, California, in November at Puget 

Sound and Georges Bank, and as late as December in British Columbia (Valentine et al. 

2007b, Bullard et al. 2007a). Didemnum can also produce new colonies through asexual 

budding, and colony fragmentation (Valentine et al 2007b, Bullard et al 2007b). 

Prior to settlement, many ascidian larvae become negatively phototactic (Bates 

2005, Bingham 1997) and preferentially settle in shaded microhabitats, such as that found 

s, docks, pontoons, and boats. In addition, pontoons and other 

floa way and Connell 2002), 

on the undersides of rock

ting structures experience consistent, mild wave action (Hollo
and for reasons that are not fully understood, the presence of this ‘swash zone’ appears to 

favor ascidian establishment (Connell 2000, Glasby 2001). 

Environmental tolerances of Didemnum and interactions with other species 

Tolerance to temperature. Recent studies suggest that colony growth and 

reproduction occurs at water temperatures between 10 and 25 °C. For example, colonies 

grown in laboratory aquaria under constant temperature regimes for 1 week grew 

approximately 1.5 to 2X more rapidly in water temperatures between 15 and 20 °C 

relative to colonies grown in water temperatures maintained above 21 °C (McCarthy et 

al. 2007). Similarly, under natural mid- to late summer temperature regimes in eastern 

Long Island Sound, Osman and Whitlach (2007) found that Didemnum colonies were 4 

times larger and 3.5 times more abundant (estimated as percent cover) in open-water sites 

with high water temperatures of 21 °C compared to colonies in the marina sites where 

summer temperatures exceeded 25 °C. Observations of the seasonal growth cycle of

Didemnum 

this cycle of

e to salinity.

colonies in a low intertidal pool at Sandwich, MA showed that colony size 

and morphology varied substantially across seasons (Valentine et al. 2007b). Over a 28 

month period, average daily water temperatures at four sites at the Sandwich tide pool 

fluctuated from -0.6 to 7.3 °C in the winter and from 13.3 to 21 °C in the summer with a 

minimum temperature of -1.8 °C and a summer maximum of 25.6 °C. As is typical of a 

number of colonial ascidians, Didemnum colonies were reduced to small basal patches 

during cold winter months (Satoh 1994). Colonies began growing by asexual budding 

when water temperature reached 8 to 12 °C in the spring, and continued accumulating 

biomass through the summer. Colonies began to degenerate again when water 

temperatures declined relatively rapidly to 15 °C in late fall or early winter. In contrast, 

 growth and degeneration was not been observed on Georges Bank, which 

experiences less pronounced daily temperature variations and has more equitable summer 

and winter water temperatures. Average winter and summer water temperatures on 

Georges range from 4 to 15 °C, and colonies collected in November 2004 did not exhibit 

signs of colony degeneration as observed at the Sandwich tide pool (Valentine et al. 

2007a). 

Toleranc In general, ascidians are not able to tolerate salinities below 

15
25‰ (Lambert 2005), and colonies of Didemnum in San Francisco Bay have only been 

found in areas with salinities greater than 26‰ (Cohen 2005). In addition, short-term 

changes in salinity can have dramatic effects on ascidian populations. For example, 

Didemnum populations in the Damariscotta River, ME were visibly less abundant 

following severe flooding (L. Harris, personal communication). However, mortality

resulting from seasonal changes in salinity is probably rarely, if ever, a major factor 

controlling the distribution of Didemnum over long temporal scales. In a number of 

Southern California harbors, Lambert and Lambert (2003) found that ascidians in the 

uppermost 0.5 m of the water column suffered complete mortality following heavy winter 

rains. However, the ascidians removed by the decrease in salinity were quickly replaced

by recruits from adults living below the affected area after the rains had subsided. 

Tolerance to environmental stress and disturbance. The ability of Didemnum to

establish a self-sustaining population may also depend on the frequency and intensity of 

environmental disturbances. Communities exposed to novel disturbances (e.g., human 

activities) are predicted to be more vulnerable to species invasions (Mack et al. 2000). 

Specifically, native species unable to acclimate or adapt to the novel disturbance may be 

replaced by pre-adapted, nonindigenous species. Ports, harbors, and marinas are 

obviously heavily impacted by human activities. In particular, water quality in most of 

the world’s estuaries is being degraded due to increasing nitrogen inputs, which can 

stimulate algal blooms and bacterial production (Galloway et al. 2003, Howarth

2000). Didemnum appears to be able to tolerate moderate levels of nitrogen pollution. A 

recent survey of ascidian diversity in 17 southern Massachusetts coastal habitats only 

found Didemnum colonies at 7 of the 10 sites where water quality was ranked as ‘fair’ 

based on nitrogen concentration, water clarity, and the frequency of algal blooms. In

contrast, colonies were not found at sites where water quality was considered ‘good’ or 

‘poor’ (Carman et al. 2007). 

In

 et al. 

 general, ascidians are efficient filter feeders that can take advantage of the 

higher bacterial levels associated with high nitrogen inputs. For example, the colonial 

ascidian, Trididemnum solidum, was historically a rare component of coral reef 

communities in the Caribbean. However, as concentrations of suspended bacteria 

16
increased with coastal development, colonies of T. solidum tripled in biomass, 

overgrowing and killing corals (Bak et al. 1996, 1998). 

In addition to water quality degradation associated with high nutrient 

concentrations, many of the larger U.S. estuaries face pollution and sediment 

contamination from heavy metals. While heavy metals are lethal to most benthic 

organisms, many adult ascidians are able to tolerate and even accumulate heavy metals, 

including arsenic, cadmium, chromium, cobalt, copper, iron, lead, mercury, selenium, tin

(as tributyltin), and zinc (Papadopoulou and Kanias 1977, Philp et al. 2003). 

n 

curing primary space. It is currently unclear if the 

temper cess 

ip can 

Physical disturbances rather than water quality degradation may have played an 

important role in the establishment of Didemnum on off-shore habitats, such as Georges 

Bank. The destruction of benthic habitats by dredging and trawling is a major concern i

many off-shore fishing grounds (Collie et al. 2000, Fogarty and Murawski 1998, 

Hermsen et al. 2003). Physically disturbing benthic habitats reduces the abundance of 

native species and increases the availability of bare space that could potentially be

colonized by Didemnum. Furthermore, physical disturbances may increase colony 

fragmentation resulting in greater local dispersal and colony growth (Stoner 1989). 

Changing temperature regimes due to global warming may have provided 

Didemnum with an advantage over native species for securing settlement sites. For 

example, Stachowicz et al. (2002) suggested that the successful invasions of three 

nonindigenous species of ascidians in New England may be due to the increase in mean

winter temperatures observed over the past 25 years. Specifically, they found that 

nonindigenous ascidians recruited earlier than native species following warm winters, 

giving them an advantage in se

ature changes documented by Stachowicz et al. (2002) have influenced the suc

of Didemnum, as additional information concerning the effects of water temperature on

the reproduction and growth rates of Didemnum are needed before this relationsh

be established. 

Effects of competition. Because Didemnum is a sessile organism, its ab

limited in part by the availability of hard substrates. How

undance is 

ever, Didemnum exhibits a 

number hic 

th due to 

 of characteristics that allow it to successfully compete with other bent

organisms for limited space. In addition to early maturation, rapid colony grow

17
asexual budding, and the ability to overgrow other benthic organisms, Didemnum ma

prevent other benthic organisms from settling and growing on colony surfaces by 

sequestering acidic and organic allelopathic compounds in their tunics (Bryan et al. 2003, 

Joullie et al. 2003). For example, Dijkstra et al. (2007b) found that significantl

y 

y fewer 

individuals and species settled on the surface of Didemnum (possibly D. vestum) 

compared to benthic species that do not synthesize allelopathic chemical compounds, 

such as solitary ascidians, bryozoans, hydroids, and bivalves. 

Effects of predators. Although a variety of macro- and micro-predators co

recruits of the nonindigenous ascidians Botrylloides and Diplosoma (Osman and 

Whitlatch 1995), there have been no experimental studies examining the effect of 

predators on the abundance or distribution of Didemnum. However, Valentine et al. 

(2007a) observed the periwinkle, Littorina littorea, grazing on senescing colonies at

nsume 

Sandwi eding 

, 

 little 

ds of introduction and spread 

lly, 

, 

ch, MA. Similarly, chitons, sea urchins and seastars have been observed fe

on colonies of D. vexillum in New Zealand. Predators may actively avoid Didemnum

because it is a low quality resource or unpalatable. For example, researchers at the

National Marine Fisheries Service have found that colonies of Didemnum consist largely

of water and considerable cellulose (indigestible by most marine species), and offer

nutritional value to potential consumers (Valentine et al. 2007b). In addition, Pisut and

Pawlik (2002) found that Didemnum candidum and Didemnum vanderhosti tunics had a

surface pH less than 3, which served as an effective deterrent to bluehead wrasse 

predation. 

Potential metho

 The introduction and spread of Didemnum is thought to be predominantly the 

result of three transport vectors; hull fouling, aquaculture, and ballast water. Historica

hull fouling has been the primary vector for the transport and introduction of 

nonindigenous species in marine habitats (Carlton 1989). Although the use of modern 

anti-fouling paints has greatly reduced the transfer of nonindigenous species via hull 

fouling, recent studies from Hawaii (Godwin 2003), California (Takata et al. 2006)

Germany (Gollasch 2002) and New Zealand (Coutts and Taylor 2004) indicate that 

fouling remains an important and overlooked transport vector for sessile marine 

18
organisms (see also, Godwin et al. 2004). For example, a total of 84 species (30 n

and 44 nonindigenous) were found attached to the hulls of 7 overseas cargo barges and

floating dry dock in Oahu, Hawaii (Godwin 2003). More specif

ative 

 a 

ically, Didemnum 

vexilliu

bridge 

outts 

s in 

s 

um on 

 result, 

. (2007a) suggested that Didemnum may have been introduced to New 

ngland as a fouling organism on seed stock of the European flat oyster, Ostrea edulis

 

 transport 

ge 

 a 

 in 

 to 

 of 

m (Kott 2002) was introduced to the South Island of New Zealand on the hull of a 

barge. The barge is believed to have been infested while moored at the Tauranga 

marina located on the North Island before traveling to Picton on the South Island (C

2002). Species of Didemnum can be introduced as larvae spawned from adult colonie

the fouling community, or through colony fragmentation. Indeed, colony fragments a

small as 5 cm2 collected from the Sandwich, MA tidepool were 15 times larger after 30 

days (Valentine et al. 2007a). Furthermore, fragments of Didemnum colonies from 

fishing boats or gear may have resulted in the introduction and/or spread of Didemn

Georges Bank (Robert Reid and Page Valentine, personal communications). 

The transport of aquaculture equipment and shellfish stock between facilities can 

inadvertently transfer nonindigenous species, and may have played a role in the 

introduction of Didemnum to the Northeastern U.S. The species first appeared at Fort 

Island Narrows, ME 11 years after oyster culture was introduced to the area. As a

Dijkstra et al

E

and/or the Japanese oyster, Crassostrea gigas. 

The threat of species introductions due to ballast water dumping has received a 

great deal of attention (Carlton and Geller 1993, Cangelosi 1999, Lavoie et al. 1999). 

However, in the case of Didemnum, it is likely the least important of the three

mechanisms. Colonial ascidians produce lecithotropic larvae that have a planktonic sta

lasting from 12 to 24 hours (Lambert 1968; Olson 1985; Svane and Young 1989). As

result the likelihood of Didemnum larvae surviving long distance transport in ballast 

water is low. Indeed, Carlton and Geller (1993) found that ascidian larvae were rare

ballast water samples collected from 159 cargo ships that had traveled from Japan

Oregon. However, the likelihood that larvae supplied from colonies growing in ballast 

tanks or that colony fragments could be transported in ballast water has not been 

examined and may pose transport risks (Drake et al. 2005). Similarly, the transport

larvae in ballast water during intercoastal voyages lasting less than 24 hours may have 

19
played a role in the spread of Didemnum following its initial introduction and 

establishment. 

As with its introduction, the spread of Didemnum is most likely human mediated. 

Because of the short lifespan of Didemnum larvae and their tendency to settle near adult

the spread of Didemnum due to larval dispersal is likely to be primarily local. However

ports, harbors, and marinas have probably served as important corridors for the regio

spread of Didemnum due to ballast water and hull fouling. For example, Didemnum 

vexillum was spread to different bays on the South Island of New Zealand as a 

s, 

, 

nal 

fouling 

organis

tland 

g 

ation due to fishing gear may have been 

importa , 

p 

l 

er 

tention. 

Potent

m on barges ad recreational vessels (Coutts and Sinner 2004). Similarly, Wasson 

et al. (2001) documented colonies of Didemnum in Elkhorn Slough, a coastal we

with recreational boat traffic and aquaculture facilities, but no international shipping. 

The role of recreational boats and aquaculture operations in spreading nonindigenous 

organisms beyond large ports to smaller bays, including marine reserves, is a growin

concern (Wyatt 2005, Floerl and Inglis 2005). 

As previously discussed, colony fragment

nt in generating the high abundance of Didemnum on Georges Bank. In addition

colony fragments caught in fishing gear may be spread to other areas when gear used in

locations where Didemnum is present is pulled up and reset in areas outside of the areas 

infested by Didemnum. Similarly, scallop fisherman may promote the spread of 

Didemnum due to the practice of shucking scallops at sea. In particular, discarded scallo

shells colonized by Didemnum could generate new colonies (Page Valentine, persona

communication). 

Although the spread of Didemnum has primarily been attributed to human 

mediated processes, other dispersal mechanisms need to be investigated. For example, 

larvae and colony fragments could be passively transported to new areas by water

currents, or released by colonies that have colonized the carapaces of crustaceans or oth

mobile, hard shelled organisms. The importance of these dispersal mechanisms and the

spatial scales over which they may operate are unknown, but deserve further at

ial environmental and economic consequences of Didemnum 

20
A potential consequence of the establishment of Didemnum could be a decrease in 

the abundance of native species and an increasing risk of local extinctions. There are 

numerous anecdotal accounts of Didemnum and Didemnum vexillum overgrowing and

smothering benthic organisms, including scallops and mussels on Georges Bank and in 

New Zealand, respectively (Coutts 2002, U.S. Geological Survey 2006, Valentine et al. 

2007b). However, it is unclear if overgrowth and smothering by these species of 

Didemnum is a significant source of mortality in benthic communities. At relatively 

small spatial scales, species of Didemnum and other nonindigenous ascidians have bee

shown to alter the abundance and composition of benthic assemblages (Oren and 

Benayahu 1997, Osman and Whitlatch 1995). Fo

n 

r example, Oren and Benayahu (1997) 

found t

t 

s Bank, 

gh 

in community structure were present. Specifically, two polychaete 

species s 

f 

 by 

can provide a mechanistic understanding of the direct and indirect interactions (e.g., 

ganisms, 

underst

es. 

 

hat Didemnum granulatum and an unidentified species of Didemnum overgrew 

and excluded benthic organisms on 50 x 50 cm experimental reefs in the Red Sea. In 

contrast, surveys of benthic assemblages on Georges Bank have not shown a strong effec

of Didemnum on the diversity and abundance of benthic organisms. 

Despite the high percent cover of Didemnum colonies observed on George

the diversity and abundance of benthic organisms prior to the discovery of Didemnum did 

not differ significantly from surveys conducted after Didemnum was discovered, althou

subtle differences 

 were more abundant in areas with dense Didemnum than in areas where it i

scarcer (Valentine et al. 2007b). Although the impact of Didemnum on mortality rates o

benthic organisms is equivocal, Didemnum may still adversely impact native fauna

limiting larval recruitment. In particular, Didemnum may restrict larval recruitment 

directly and indirectly by dominating hard substrates and chemically preventing larvae 

from settling on colony surfaces (Dijkstra et al. 2007b). While small-scale experiments

overgrowth or recruitment limitation) between Didemnum and benthic or

anding the effects of Didemnum on the abundance and diversity of benthic 

organisms will require additional studies conducted at larger spatial and temporal scal

However, even without changing the abundance of benthic and infaunal organisms,

Didemnum could negatively impact consumers at higher trophic levels in several ways. 

In areas where colonies of Didemnum cover significant portions of the benthos, 

21
consumers may be unable to reach prey items. Specialized predators or those with

limited mobility would be particularly vulnerable to food limitation as their access to 

prey items decreases. Alternatively, the abundance of generalist predators capable of 

switching to Didemnum could increase, putting additional predation pressure on na

prey species. Similarly, mobile predators could move to other locations, increasing 

predation pressure in these areas. 

In addition to the potential negative impacts of Didemnum on species diversity,

there are also concerns that Didemnum will have negative economic impacts on 

commercially important aquaculture operations and fisheries. Heavy i

tive 

nfestations of 

nonind

ving 

nd 

crease 

r size 

 al. 

(1992) n 

osseri

les 

r 

um

igenous ascidians in Canadian aquaculture operations have increased handling and 

processing costs (Cayer et al. 1999, Boothroyd et al. 2002). Lines and cages weighed 

down by ascidians require cleaning before they can be retrieved, and ascidians need to be 

removed from shells before they are marketable (Neil MacNair, personal 

communication). Didemnum will likely have similar impacts as it is capable of achie

high population densities and overgrows shellfish. 

Studies examining the impacts of ascidians on shellfish mortality (Dalby and 

Young 1993, Mazouni et al. 2001) and meat yields (Petersen et al. 1997) have not fou

a consistent relationship between ascidian infestations and shellfish survivorship or 

production. For example, Dalby and Young (1993) found that overgrowth by ascidians 

had variable effects on the survivorship and size of the oyster, Ostrea equistris. 

Specifically, they found that allowing ascidians to colonize oyster cultures could de

or enhance oyster survivorship, and either had no effect or actually increased oyste

relative to ascidian exclusion treatments. Other studies suggest that bivalves and 

ascidians are capable of coexisting because they partition food resources (Petersen et

1997, Lesser et al. 1992, Mazouni et al. 2001). Feeding trials conducted by Lesser et al. 

showed that blue mussels, Mytilus edulis, had significantly higher feeding rates o

particles ranging in size from 3 to >16 μm compared to the ascidians, Botryllus schl

and Ciona intestinalis. In contrast, colonial ascidians have been shown to retain partic

smaller than 2 μm; whereas, bivalves are less efficient at filtering these smaller items 

(Petersen et al. 1997). While differences in resource utilization may limit competition fo

food, interference competition may still be an important factor. Colonies of Didemn

22
have been observed overgrowing mussels, scallops, and oysters to the point where the 

ability 

l 

ant food supply 

nd holes for avoiding predators 

(Gotcei

ably 

r some 

gs of 

f Didemnum’s range, the factors controlling its abundance, and the 

lative In 

nomic 

of these bivalves to open their shells and feed is compromised (U.S. Geological 

Survey 2006). 

 Finfish fisheries on Georges Bank may also be negatively impacted by 

Didemnum. The gravel habitat where Didemnum is currently found serves as an 

important nursery for juvenile cod and haddock (Collie et al. 2000), as well as a critica

spawning ground for Atlantic herring (Smith and Mores 1993). The high water column 

and benthic productivity of this habitat provides young fish with an abund

(Townsend and Thomas 2002). In addition, the structural complexity of the gravel 

matrix likely provides juvenile fishes with crevices a

tas and Brown 1993). High densities of Didemnum may dramatically alter the

availability of food and refugia for these commercially important fish species. Before 

there was intensive bottom trawling and dredging in this area, the gravel bottom prob

supported more emergent epifauna (e. g., hydrozoans, bryozoans) than it presently does. 

This epifauna may increase the forage and shelter value of the gravel pavement fo

species and life stages. Recovery of the emergent epifauna could be more difficult in 

areas with dense colonies of Didemnum. In addition, the allelopathic chemicals that 

prevent benthic organisms from overgrowing Didemnum may be harmful to the eg

fish that rely on the gravel pavement for spawning sites, such as the Atlantic herring 

(Dijkstra et al. 2007b). Atlantic cod, haddock, and Atlantic herring had a dockside value 

of over 55 million dollars in 2004, and accounted for approximately 24% of the total 

dockside value of fish landings in New England (National Marine Fisheries Service 

2005). Didemnum has the potential therefore to negatively impact the abundance or 

distribution of these economically important fish species. 

Forecasts of Future Conditions with No Management Action 

 At present, there is insufficient information to accurately predict the future 

distribution and impacts of Didemnum in U.S. waters. In particular, the northern and 

southern limits o

re importance of potential transport vectors remain to be fully examined. 

addition, studies documenting the effects of Didemnum on native species and eco

23
activities have not been completed. However, new populations of Didemnum, or closely

related species, continue to be found in the U.S. and worldwide (U.S. Geological Survey

2006, Minchin and Sides 2006), suggesting that the spread of Didemnum has not 

subsided. Furthermore, the studies and examples discussed in the proceeding section

indicate that it is conceivable that Didemnum will negatively impact native species, 

aquaculture, and fisheries. Although the lack of information prevents the use of 

quantitative analyses to predict the spread and impacts of Didemnum, general predictio

about the potential spread of Didemnum can be made using existing information on the 

tolerance of Didemnum to water temperature and temperature regimes within its current 

range. 

 1. New colonies of Didemnum will continue to be found in the U.S. Northeas

 California Current, and Gulf of Alaska LME’s. 

 2. The ability of Didemnum to establish colonies in the Southeast U.S., 

Caribbean, Gulf of Mexico, and Insular-Pacific Hawaiian LMEs is limited due

high summer water temperatures. 

3. The ability of Didemnum to establish colonies in

s 

ns 

t, 

 to 

 the East Bering Sea, 

Chuckchi Sea, and Beaufort Sea LMEs is limited due to the short duration of the 

ion 

eld 

en 

 this 

growing season. 

 Most studies that have examined the ecological factors controlling the distribut

and abundance of Didemnum have focused on water temperature. Results of these fi

observations and experimental studies suggest that colony growth rates decline wh

temperatures exceed 21 oC for 7 consecutive days (McCarthy et al. 2007). However, 

colonies can survive in habitats where daily temperatures fluctuate between ~15 and 26 

oC (Valentine et al. 2007b). Similarly, water temperatures above 8 to 10 oC are necessary 

for colony growth; however, colonies can survive extended periods of time below

temperature threshold as an unidentified overwintering form (Valentine 2007b). We 

investigated the potential spread of Didemnum by examining mean monthly water 

24
temperatures and monthly maximum and minimum temperatures at multiple loca

within LMEs of the U.S. Areas where mean monthly temperatures were above 25 oC and 

minimum temperatures were greater than 21 oC for consecutive months were considered

potentially unsuitable habitats for supporting large populations of Didemnum. 

Temper

tions 

ature data for locations where Didemnum colonies currently exist were included 

 provide insights into the number of months colonies are capable of overwintering. The 

dcurrents.noaa.gov). These 

ations are located onshore in ports or other nearshore habitats, and record subtidal water 

mper

Northeast U.S., California Current, and Gulf of Alaska LMEs, the spread of Didemnum 

eyond its current range in the Northeast U.S. and Gulf of Alaska may be limited. 

Colonie

souther

monthly temperatures above 25 oC and temperatures ranging from 21 oC up to the low 

0’s begin just south of Long Island at Sandy Hook, NJ (Table 2). Similarly, the ability 

f Dide

 is 

h 

ton 

 western 

to

data used for this analysis were collected from NOAA’s National Oceanographic Service

tides and currents meteorological stations (http://tidesan

st

te atures every hour. 

Although we expect new colonies of Didemnum to continue to be found in the 

b

s are currently restricted to the northern half of the Northeast U.S. and the

nmost region of the Gulf of Alaska (Figs. 3 and 4). In the Northeast U.S., average

3

o mnum to extend its range northward in the Gulf of Alaska may decrease as the 

length of the growing season shortens. Seasonal temperatures in Ketchikan, AK are 

similar to those in Eastport, ME, the coldest location where Didemnum colonies have 

been documented. However, the 6 months with temperatures above 9 oC in Ketchikan

reduced to 4 months in Anchorage and areas to the west, which may prevent the 

establishment of Didemnum or limit it to small colonies (Table 2). In contrast to the 

Northeast U.S. and Gulf of Alaska, nearshore water temperatures suitable for the growt

of Didemnum occur throughout the U.S. portion of the California Current LME. As a 

result, areas currently free of Didemnum, such as the coasts of Oregon and Washing

may be susceptible to the establishment of Didemnum. 

 Water temperatures become less suitable for Didemnum establishment as you 

move away from the temperate systems where Didemnum is currently found towards 

subarctic LMEs in the north and subtropical and tropical LMEs in the south and

pacific. The East Bering Sea, Chukchi Sea, and Beaufort Sea LMEs are remote, high 

25
26

ove 

establish large, viable populations at nearshore sites in these 

LMEs.

r 

latitude systems with extended periods of sea ice cover and water temperatures below 9 

oC for 7 to 12 months of the year (Table 2). Mean monthly water temperatures in the 

Southeast US, Insular Pacific-Hawaiian, Gulf of Mexico, and Caribbean LMEs are ab

25 oC for 4 to 12 months of the year (Table 2). These temperature regimes should hinder 

the ability of Didemnum to 

 

 We emphasize that these forecasts are tentative and subject to a number of data 

limitations and other uncertainties. For example, our assessment is not applicable fo

predicting the spread of Didemnum in off-shore habitats in the Northeast U.S, the 

northern part of the Southeast U.S., and the Gulf of Alaska LMEs. Because water 

temperatures in deeper habitats are colder and less seasonally variable, Didemnum’s 

distribution limit in off-shore habitats in the Northeast U.S. may extend further south than

suggested by our analysis of nearshore water temperatures. For example, mean water 
Table 2. 2006 monthly mean and maximum-minimum water temperatures from multiple nearshore locations within six Larg

2006. Mean monthly averages 25 oC or above and minimum temperatures above 21 oC are highlighted in ye are highlighted in blue. Numbered locations correspond to Figures 3 and 4. 

llow. Mean mont

LME and Buoy 

Location Month 

JAN FEB MAR APR MAY JUN JUL AUG SEP OC

Northeast U.S. 

5.4 4.1 3.4 4.9 6.9 8.7 10.6 12.3 12.8 12.

1 Eastport ME (4.1, 6.7) (2.4, 5.2) (2.2, 4.7) (3.9, 6.3) (5.5, 8.6) (7.2, 10.0) (8.5, 12.9) (11.1, 13.4) (12.0, 13.7) (11.2, 1

3.8 2.1 2.0 5.2 9.0 12.2 14.5 14.1 13.7 12.

2 Bar Harbor ME 

(0.5, 4.7) (-0.9, 3.9) (0.3, 4.0) (3.1, 7.9) (6.5, 11.6) (9.9, 15.7) (6.2, 18.7) (11.4, 17.8) (12.5, 15.7) (9.9, 1

4.3 3.4 3.2 6.6 9.1 12.9 14.2 15.1 15.2 12.

Portland ME 

(2.9, 5.4) (1.5, 4.3) (1.8, 5.4) (4.7, 8.4) (7.5, 12.7) (11.0, 15.2) (11.2, 18.2) (13.3, 19.8) (13.2, 17.2) (-0.1, 1

4.3 3.6 3.2 8.1 11.0 14.8 15.8 17.9 18.0 13.

7 Boston Harbor MA 

(3.3, 5.9) (2.0, 5.2) (1.5, 6.1) (5.2, 11) (9.3, 14.7) (11.3, 18.5) (12.6, 19.4) (15.3, 23.4) (15.1, 19.9) (10.4, 

3.9 3.0 2.6 7.8 11.9 17.2 21.6 22.3 19.4 15.

15 Woods Hole MA 

(3.1, 5.5) (0.5, 5.1) (0.1, 6.1) (5.0, 10.6) (9.6, 15.2) (14.7, 20.6) (19.2, 24.0) (19.9, 25.5) (17.7, 20.8) (11.0, 

5.4 4.5 4.7 8.1 11.1 15.8 20.4 20.7 18.7 15.

18 Newport RI (2.4, 7.4) (1.0, 6.7) (2.1, 7.4) (5.6, 11.0) (8.9,16.7) (12.0, 21.0) (16.0, 26.5) (17.8, 27.2) (17.2, 20.5) (10.0, 

5.0 4.6 5.1 8.8 12.0 16.4 20.2 21.2 19.8 16.

19 New London CT 

(2.4, 7.2) (1.9, 6.5) (2.0, 8.4) (5.8, 12.0) (8.8, 16.2) (12.4, 22.0) (17.1, 24.7) (19.0, 24.1) (18.6, 20.9) (10.8, 

4.1 3.4 4.3 9.6 14.7 18.5 20.6 22.6 20.2 17.

Kings Point NY (0.9, 6.1) (-1.3, 6.1) (-0.7, 9.0) (6.8, 19.3) (12.1, 24.4) (16.1, 24.5) (17.6, 25.5) (16.3, 26.0) (7.5, 25.9) (11.3, 

5.2 4.0 6.1 11.8 15.9 20.2 23.7 25.0 20.8 15.

Sandy Hook NJ (2.9, 8.5) (2.0, 6.7) (1.4, 12.8) (8.6, 16.7) (13.0, 20.0) (17.6, 23.3) (18.0, 30.0) (21.6, 31.5) (16.7, 23.6) (9.6, 1

6.6 7.3 8.1 14.6 18.1 21.9 22.9 25.0 22.1 17.

Cape May NJ (4.3, 12.5) (3.7, 15.6) (3.6, 12.0) (11.2, 18.7) (15.2, 21.7) (18.8, 34.0) (18.5, 27.3) (21.8, 27.2) (20.3, 24.2) (10.0, 

6.0 5.4 6.5 11.6 16.3 20.2 21.5 23.4 21.9 17.

Lewes DE 

(3.2, 7.6) (2.7, 7.2) (3.5, 9.3) (8.5, 15.3) (12.0, 20.6) (18.0, 24.2) (16.8, 26.1) (20.7, 27.8) (19.7, 23.8) (12. 

2, 
Table 2. 2006 monthly mean and maximum-minimum water temperatures continued. 

LME and Buoy 

Location Month 

JAN FEB MAR APR MAY JUN JUL AUG SEP OC

Northeast U.S. 

5.0 4.8 6.6 13.1 17.6 23.3 27.5 27.3 23.2 18.

Baltimore MD 

(3.4, 6.8) (3.3, 6.4) (4.3, 11.4) (9.7, 19.3) (1 ) (1 2) 4.6, 24.2 8.9, 26. (2 2) 5.0, 29. (2 ) (2 2) (1 5.4, 29.4 0.6, 26. 2.4, 

6.5 5.0 7.0 12.8 18.2 23.3 27.5 28.5 24.0 19.

Solomons Island MD 

(5. 0) 6, 12. (4 ) (3 5) (8 .2, 6.4 .9, 10. .8, 17.1) (15. .9) (20. .2) 1, 22 5, 29 (25. 8) 0, 30. (26. 5) 4, 31. (22. 7) (13. 

3, 26. 3, 

Southeast U.S. 

8.5 7.7 8.1 11.8 15.3 18.1 19.6 23.6 23.4 19.

Cape Hatteras NC (6.0, 11.4) (5.8, 9.4) (5.8, 12.2) (7.9, 16.9) (11.6, 21.6) (11.8, 24.6) (12.7, 26.9) (17.9, 28.9) (21.6, 31.6) (15.0, 

11.5 10.4 12.7 19.1 21.4 25.8 28.3 28.6 25.2 21.

Beaufort NC 

(9.0, 13.4) (8.5, 17.3) (10.1, 17.1) ( 16.0, 22.1) (15.8, 27.7) (20.1, 28.7) (25.1, 30.6) (25.4, 30.5) (22.7, 28.7) (15.8, 

14.2 13.8 16.1 21.3 24.0 27.9 30.6 31.1 28.9 24.

Charleston SC 

(13, 15.4) (12, 15.2) (13, 18.9) ( 17.1, 24.4) (21.5, 28.4) (27.0, 30.9) (28.4, 32.2) (29.2, 32.9) (27.1, 31.2) (19.1, 

14.4 13.8 16.6 20.6 23.8 27.5 29.3 30.0 27.8 23.

Fernandina Beach FL 

( 12.5, 16.8) (10.3, 16.6) (13.4, 20.1) (17.5, 24.2) (19.8, 27.6) (25.7, 29.8) (27.0, 31.0) (27.1, 31.0) (26.2, 29.9) (19.6, 

18.5 19.3 21.9 25.2 27.3 30.2 30.9 27.9 31.1 28.

Trident Pier FL 

( 15.8, 21.8) (16.4, 21.9) ( 19.7, 24.2) (22.8, 28.6) (24.2, 30.0) (28.0, 31.9) (26.7, 32.8) (21.4, 30.7) (28.5, 31.6) (25.9, 

21.2 21.5 23.2 25.6 27.1 29.4 29.7 30.2 25.6 

Virginia Key FL (18.8, 23.5) (17.3, 25.2) (21.2, 26.0) (22.4, 28.9) (23.8, 29.6) (27.6, 31.6) (28.2, 31.4) (27.9, 32.3) (22.0, 31.8) ND Caribbean 

21.4 21.3 23.5 26.3 28.2 30.0 30.1 30.7 30.5 28.1 

Key West FL 

(18.7, 24.1) (17.4, 24.7) (20.8, 27.4) (22.7, 29.5) (25.9, 30.7) (27.7, 31.3) (28.9, 31.3) (29.0, 32.3) (28.7, 31.8) (25.0, 30.0) 

Gulf of Mexico 

19.0 18.5 22.1 25.2 28.2 30.0 30.2 31.2 30.7 27.

Naples FL (16. .8) (14. 3, 22 8, 22.7) (19. .4) 0, 26 (21. 5) 1, 29. (25. 3) 3, 28. (27. 0) 6 ,32. (28. 4) 3, 32. (28. 8) 9, 32. (29. 1) 2, 33. (23. 

0, 

17.6 17.2 20.8 23.4 26.0 29.0 30.5 31.1 29.7 26.

St. Petersburg FL (16.2, 20.3) (14.4, 20.7) (18.6, 23.5) (21.1, 26.6) (23.1, 28.9) (26.5, 31.2) (28.6, 32.5) (29.4, 33.4) (28.6, 31) (22.1, 
Table 2. 2006 monthly mean and maximum-minimum water temperatures continued. 

LME and Buoy 

Location Month 

JAN FEB MAR APR MAY JUN JUL AUG SEP OC

Gulf of Mexico 

17.5 17.9 22.3 26.9 28.0 29.9 31.9 32.1 28.7 25.

Pensacola FL 

( 14.4, 19.4) (14.3, 19.9) (19.4, 26.1) (23.1, 30.6) (25.3, 35.8) (27.0, 32.4) (30.2, 33.3) (30.8, 34.2) (26.7, 31.7) (20.7, 

Pilot Station East, SW 12.3 10.8 14.2 18.6 29.7 30.5 28.6 27.Pass LA (10.4, 15.0) (9.9, 12.2) (10.0, 18.5) (13.4, 22.7) ND ND (28.6, 31.1) (29.5, 32.1) (23.7, 35.7) (20.0, 

14.9 14.2 18.3 23.2 25.7 28.4 29.7 30.9 28.7 24.

Galveston Bay TX (12.4, 17.3) (10.1, 17.4) (13.5, 21.5) (19.6, 26.4) (23.5, 28.0) (25.2, 30.7) (27.6, 32.6) (29.2, 32.9) (24.9, 31.7) (18.8, 

17.8 16.8 20.2 24.5 26.8 28.8 30.1 30.5 28.7 26.

Corpus Cristi TX (15.6, 20.2) (13.4, 19.2) (16. 4) 1, 23. (21.7, 27.7) (21.8, 29.0) (26.5, 31.0) (28.2, 32.4) (28.7, 32.3) (26.4, 31.9) (21.7, 

18.1 17.5 8 24.7 20. 26.8 29.7 30.2 30.0 29.6 27.

Port Isabel TX 

(15.5, 20.4) (13.6, 20.2) (16. 5) 5, 25. (22.2, 27.0) (25.1, 28.6) (26.3, 33.3) (27.7, 46.7) (27.3, 38.5) (27.1, 37.5) (23.1, 

Beuforet Sea 

-1.7 -1.7 -1.7 -1.8 -1.7 -0.7 7.3 8.6 8.9 4.4 

Red Dog Dock AK (-1.8, -1.7) (-1.8, -1.5) (-1.8, -1.7) (-1.8, -1.7) (-1.8, -0.4) (-1.7, 7.5) (-1. 0) (6. 4) (6. 7) 0, 15. 8, 10. 6, 12. (0 

.0, 

East Bering Sea 

-5.3 -3.1 -1.4 -2.4 0.5 6.7 11.3 11.4 9.0 4.6 

Nome AK 

(-10.6, -1.1) (-11.6, 0.0) (-8.4, 0.0) (-8.4, 0.0) (-1.8, 5.9) (0.9, 12.4) (5.7, 15.0) (6.0, 14.3) (5.3, 12.3) (-1.5, 

3.3 2.9 3.7 4.1 6.0 8.9 8.9 9.8 8.7 7.0 

Unalaska AK 

(1.2, 4.5) (0.6, 4.3) (3.1, 4.4) (3.1, 5.1) (3.4, 11.6) (7.5, 11.3) (6.8, 11.1) (9, 11.1) (7.9, 10.1) (5.2, 

7.3 6.9 7.2 7.4 8.1 8.6 9.3 10.5 11.0 10.

Adak Island AK 

(6 ) .4, 8.0 ( 3) 6.0, 7. (6.6, 7.6) (7.0, 8.6) , 9.5) , 9.8) , 10.7) .4, 12.8) .5, 13.4) .4, 1

(7.3 (7.7 (8.5 (9 (9 (9 

Gulf of Alaska 

3.9 2.9 3.0 3.2 4.3 6.9 9.8 10.7 10.6 9.8 

Sand Point AK 

(2 ) .0, 6.3 (1 ) .6, 3.7 (2 ) .1, 3.9 (0 ) .5, 3.9 (3 ) .0, 7.9 (4.3, 8.6) (6. 2) (9. 4) (9. 7) (8. 

7, 11. 7, 11. 8, 12. 5, 1

4.2 1.9 1.9 4.4 6.7 9.9 10.8 11.9 11.8 8.9 

Kodiak AK 

(0 ) .4, 5.4 (0 ) .4, 3.0 (0 ) .5, 3.6 (3 ) (4 ) (8 2) (9 .2, 5.7 .9, 10.3 .5, 11. .6, 11.8) (10. 1) (10. 8, 14. 3, 12.9) (6. 

0, 1
Table 2. 2006 monthly mean and maximum-minimum water temperatures continued. 

LME and Buoy 

Location Month 

JAN FEB MAR APR MAY JUN JUL AUG SEP OC

Gulf of Alaska 

-1.0 -1.1 -1.1 0.5 6.4 11.9 13.9 13.0 11.4 7.4 

Anchorage AK (-1.1, 2.4) (-0.2, 2.9) (-1.2, -0.2) (-0.9, 2.8) (2.3, 11.8) (10.6,13.5) (12.4,15.5) (9.8, 14.6) (9.4, 12.5) (3.5, 1

5.6 3.9 4.6 4.6 6.2 9.7 12.1 13.4 12.2 9.8 

Seward AK 

(3.5, 6.9) (2.8, 5.3) (3.7, 5.7) (3.9, 6.0) (4. 4) (5. 5) 7, 11. 3, 13. (10.4, 14.8) (10.0, 14.6) (8.6, 13.2) (5.1, 1

5.2 4.3 5.4 5.6 7.4 9.4 10.6 10.8 11.2 10.

Valdez AK 

(3.4, 6.6) (2.1, 6.1) (3.9, 6.2) (4.2, 6.6) (5.4, 11.4) (6.9, 12.8) (7.2, 12.1) (4.4, 11.9) (8.8, 11.8) (4.5, 1

6.3 5.2 4.2 5.3 6.9 11.7 12.4 12.6 11.8 10.

Yakutat AK (5.3, 7.0) (4.3, 5.4) (2.3, 4.9) (4.2, 6.7) (5.6, 11.1) (9.0, 13.6) (8.6, 15.3) (8.1, 14.0) (11.2, 12.9) (7.7, 1

3.8 3.2 3.8 5.1 7.4 Juneau AK (2004) (1 3) .0, 5. (1 ) .2, 3.9 (3 3) .3, 4. (3 8) .7, 7. (4 5) .8, 12. ND ND ND ND ND 6.4 5.4 6.0 6.9 10.0 11.6 14.0 15.0 Sitka AK (2005) (5.6, 7.4) (4.5, 6.0) (5.6, 6.4) (5.8, 9.9) (7.1, 12.7) (9.2, 14.4) (12.5, 15.8) (12.5, 18.2) ND ND 7.7 7.2 7.4 8.1 9.3 13.1 14.6 14.0 12.7 10.

Ketchikan AK 

(6.7, 8.4) (6.3, 8.0) (7.1, 7.9) (7.6, 8.6) (7.9, 11.5) (10.9, 14.9) (11.8, 17.0) (12.0, 15.8) (11.4, 19.1) (9.6, 1

California Current

8.4 8.0 8.0 9.0 10.5 11.5 13.6 10.

Cherry Point WA (7.9, 8.7) (7.2, 8.5) (7.3, 8.8) (8.3, 10.8) (9.0, 12.7) (9.7, 15.9) (10.8, 16.5) ND ND (9.8, 1

9.2 8.6 8.6 9.0 10.2 11.8 12.8 13.4 13.3 12.

6 Seattle, WA (8.4, 9.8) (8.3, 9.2) (8.2, 9.0) (8.6, 10.2) (9.3, 11.6) (10.2, 14.0) (11.9, 14.8) ( 12.7, 14.9) (12.9, 14.4) (11.8, 

9.4 8.1 8.3 9.9 10.9 12.6 11.6 11.5 10.8 9.8 

Neah Bay WA (8.7, 10.1) (6.1, 9.5) (7.3, 9.5) (8.8, 11.6) (9.2, 12.8) (10.3, 15.4) (10.0, 14.4) (9.5, 14.6) (9.1, 12.6) (8.5, 1

7.0 5.8 6.7 10.4 14.3 17.0 20.2 20.0 18.2 15.

Astoria OR (6.3, 7.8) (3.9, 7.8) (4.8, 9.0) (8.5, 15.4) (11.6, 16.8) (14.9, 20.0) (15.4, 23.0) (15.2, 22.7) (14.9, 20.8) (12.0, 

11.0 10.1 9.8 11.3 11.6 13.0 11.9 11.0 11.1 10.

Charleston OR 

(9.4, 12.2) (7.7, 11.8) (8.2, 12.0) (10.0, 13.8) (8.5, 15.5) (8.8, 16.6 (9.3, 15.5) (8.9, 19.7) (9.3, 14.1) (9.2, 1
Table 2. 2006 monthly mean and maximum-minimum water temperatures continued. 

LME and Buoy 

Location Month 

JAN FEB MAR APR MAY JUN JUL AUG SEP OC

California Current 

11.4 10.4 10.2 11.3 11.3 12.6 12.4 12.8 12.2 12.

7 North Spit CA (9.9, 12.7) (9.3, 12.2 (8.8, 11.5) (9.3, 13.3) (8.5, 15.4) (9.6, 16.0) (10.0, 16.2) (10.6, 16.5) (10.3, 15.9) (10.7, 

12.0 10.8 10.8 11.2 9.9 10.2 10.4 11.5 11.3 13.

Arena Cove CA ( ) 11. 9 0, 12. (8.4 4) , 12. (8.9 1) , 12. (9.1, 13.7) (8.1 4) , 13. (8.3 6) , 14. (8. 6) 3, 16. (9. 7) 6, 13. (10. 9) 0, 12. (11. 

3, 

11.9 12.0 11.6 12.5 13.6 13.6 14.5 16.4 15.2 15.

10 San Franscico CA (11 7) (11 1) (10 .1, 13. .3, 14. .8, 13.2) (11 2) (12. 6) .7, 14. 6, 15. (12. 3) (13. 8) (15. 5, 16. 0, 17. 8, 17.8) (14. 4) (14. 2, 17. 3, 

12.5 11.9 12.3 13.5 14.0 14.0 14.3 16.3 14.7 15.

11 Monterey CA (11 5) (11 8) (11 .1, 13. .0, 12. .6, 13.0) (11 1) (11. 2) .6, 15. 0, 16. (10. 3) (11. 3) (13. 2, 16. 8, 17. 1, 17.8) (13. 7) (13. 2, 16. 6, 

12.4 11.9 11.5 12.4 12.7 13.3 14.8 15.0 14.6 

15. 13 Port San Luis CA (11 9) (11 1) (10 .3, 13. .2, 13. .1, 12.8) (9. 4) (10. 8) 3, 14. 4, 14. (11. .1) (11. 2) (13. 2, 16 8, 18. 1, 18.8) (12. .4) (14. 

6, 16 6, 

14.8 14.8 13.2 13.5 16.5 17.6 18.6 17.6 18.5 17.

Los Angeles CA (13 9) (13 8) (11 .5, 15. .8, 15. .9, 15.1) (12 8) (14. 5) .2, 14. 3, 18. (13. 1) (13. 7) (15. 4, 20. 8, 21. 3, 20.6) (15. .2) (15. 9, 20 8, 

15.2 15.6 15.5 17.2 18.8 21.2 23.1 22.2 21.2 19.

14 San Diego CA (14 0) (14 6) (14 .6, 16. .7, 16. .6, 16.5) (15 7) (16. 6) .9, 20. 7, 21. (18. 1) (17. 9) (19. 1, 23. 5, 24. 2, 25.5) (18. 2) (17. 9, 23. 7, Insular Pacific-Hawaiian 

25.1 24.6 24.4 25.0 25.6 26.5 27.3 28.0 28.2 27.

Nawilliwil HI 

(22 8) .8, 29. (22 2) .1, 26. (22 4) .0, 26. (23. 9) 1, 26. (24. 4) 5, 27. (25. 1) 7, 28. (26. 3) 0, 29. (26. .5) 3, 29 (26. .8) 5, 29 (26. 

2, 

26.4 26.3 26.5 26.4 27.2 27.5 29.2 28.9 28.

Honolulu HI 

( ) 22. 6 1, 32. ( ) 25. 4 3, 27. ( 25. 3) 2, 27. (23. 8) 9, 27. (26. 0) 5, 28. (27. 8) 5, 28.

ND 

(28. 2) 4, 30. (28. 4) 4, 29. (27. 

9, 

24.2 24.0 24.2 23.8 25.1 25.8 26.2 26.8 26.7 27.

Kahului HI (23. 0) 2, 26. ( ) 22. 1 1, 25. ( 23. 3) 4, 25. (22. 1) 4, 25. (24. 4) 0, 26. (24. 0) 8, 27. (25. 6) 1, 27. (25. 0) 7, 28. (25. 9) 5, 31. (25. 

5, 

24.3 23.5 23.3 23.7 24.0 25.2 25.7 25.8 26.5 26.

Hilo HI 

( 21.4, 25.9) (20.8, 24.2) (20.1, 24.1) (22.3, 24.7) (20.6, 25.3) (23.3, 26.4) (23.8, 26.7) (24.2, 27.0) (24.7, 27.4) (24.2, 
temperatures from September through Novem r 0 wer pp im ly oC at four 

40 m deep off-shore sites located between Delaware Bay and Chesapeake Bay 

(Rasmussen al. re t DE and 

Solomons Island, MD ranged from 21.9 to 12.5 oC and 24.9 to 12.5 oC, respectively 

(Table 2). Simila th constrain 

the northward expansion of Didemnum more than predicted based on nearshore 

temperature ta. ing tween 

water temperature an h o o i n does not allow us to distinguish between 

areas where w b st e s ll onies itt o rn versus areas 

where Didemnum stra lly create 

serious problems. Finally, the predicted range expansion of is based on 

current tem tu e u warming 

could result in a northward extension of temperatures suitable for the establishment of 

idemnum. sp re

ubtropical, and trop l E e fi nt n orable th stablish t of large 

olonies of D emnu th fu e c in fo should focus on the predicting the 

pread of D n ge ent 

istribution. 

Our lity num ic area will 

enefit from analyses that incorporate local environmental factors in addition to water 

mperature ch ail uch of the 

horeline in the sout n ti of N theast U.S. is located with he Chesapeake 

ay where surface s it c e lo as ‰ Simila th bi o demnum to 

olonize off ore U.S o the 

redominance of mobile substrates unsuitable for Didemnum ent. Future 

nalyses sh al env are 

avorable to Didemnum establishment versus sites where Didemnum is likely to be 

trod d. Th ssibility t a noni genous spe will be introduced into a new 

abita nd n b f iv a el d ring an introduction event and 

e nu of sp e in duced, which taken together are referred to as 

be 200 e a rox ate 12

 et 2004). In contrast, autumn nearsho emperatures at Lewes,

rly, colder off-shore temperatures in e Gulf of Alaska may 

 da In ad

d t

ditio

e gr

n, ou

wth

r cur

f D

rent 

dem

unde

um

rstand of the relationship be

Didemnum ill 

s w

e re

l d

rict

in

d to

e b

ma

th

col

ub

of l

 an

le c

cou

nce

colonie il om at en ic s tes d ld potentia

Didemnum 

pera re regimes. An increase in water temperatur s d e to global 

D

s

c

s

d

b

te

s

B

c

p

a

f

in

h

th

 De ite th

ica

ese u

LM

ncert

s ar

ainti

 suf

es, th

cie

e tem

ly u

peratu regim

 to

es in the subarctic, 

fav e e men

id

idem

m, 

at 

at 

ma

tur

r s

fore

tia

ast

cal

g ef

 at

rts

um s lle pa l s es the ed s of and within its curr

 abi to predict the likelihood of Didem spreading to a specif

, su as sa

her

linity

sec

, dep

on 

th, h

 the

ard s

or

ubstrate av ability. For exam

in t

ple, m

alin

bi

ies

s i

an b

he

 as

ut

w 

rn

 18

rt

. 

st 

rly

ay

e a

e li

lity

ted

f Di

-sh ha tat n t so he No hea . m b mi due t

 estab

nm

lishm

ould so distinguish between sites where iro ental conditions

uce

t depe

mber 

e po

s on

time

hat

er o

s is

ndi

idu

cies

 the du

s a 

um

eci

 ind

tro

ls r ease

32
propagule pressure (Carlton 1996). The number of individuals released during a 

urrently 

ort 

aculture 

re 

ry 

s. In the U.S., nonindigenous aquatic species management and 

 be 

s 

ugh 

Didemnum introduction should vary with the size and condition of the colony, the 

presence of larvae, and the possibility of colony fragmentation. However, it is c

not possible to estimate the number of individuals or fragments that may be released 

during an introduction event, or how this variable may differ between types of transp

vectors. Although the number of times Didemnum may be introduced into a habitat may 

be approximated by examining the vessel traffic statistics and the number of aqu

facilities, details on vessel traffic patterns and aquaculture shipments are needed befo

we can confidently estimate propagule pressure for specific locations. 

Provision of Guidance for Potential Management Actions 

Devising a management strategy for Didemnum is clearly a national issue. 

However, the methods and resources required to control Didemnum will differ between

industries, states, and other private and public stakeholders. Coordinating these 

independent needs requires a central management authority. Coordinating policy 

development will prevent confusion over variable but overlapping industry and state 

management policies, minimize gaps in management activities, and clarify regulato

responsibilitie

coordination is conducted by the Aquatic Nuisance Species Task Force (ANSTF). The 

ANSTF is an intergovernmental committee established by Congress under the 

Nonindigenous Aquatic Nuisance Prevention and Control Act of 1990 (Act, 6 USC. 

4701-4741), and later amended by the National Invasive Species Act of 1996. The 

ANSTF is charged with developing and implementing a national strategic plan for 

preventing the introduction and dispersal of nonindigenous aquatic species, and 

coordinating the development of management plans for specific species considered to

of national significance. 

 The ANSTF also addresses regional and state nonindigenous aquatic specie

issues and has created six regional panels made up of federal, state, industry, public, and

environmental stakeholders. Furthermore, states that submit State Management Plans 

are eligible for funding to implement these plans following ANSTF approval. Altho

33
the ANSTF does not have any regulatory authority, it can serve as an advisory bod

the coordination of state and federal regulations. The ANSTF has the institutional 

framework, access to financial resources, and personnel network to address the tasks and 

activities needed to develop a cohesive Didemnum management plan. Below, we out

a Didemnum management and control program that focuses on preventing the spread of 

Didemnum to new areas and limiting the impacts of existing populations. Specifically, 

we outline a management plan that addresses the following objectives: 

1. Continue conducting research on the environmental and socioe

y in 

line 

conomic impacts 

of Didemnum sp. A. 

ds of Didemnum 

to better understand and manage the risks and threats associated with this 

ting 

s can 

ill 

ponsibilities. 

2. Continue conducting research on the ecology and control metho

nonindigenous species. 

3. Prevent the spread of Didemnum to new areas. 

4. Develop monitoring programs to detect changes in the abundance of exis

populations of Didemnum as well as the establishment of new colonies 

outside of its current distribution. 

5. Eradicate pioneering colonies, and determine the feasibility of eradicating or 

reducing the abundance of established colonies. 

6. Educate the public, private industry, and policy makers about potential 

environmental and economic costs of Didemnum, and how their action

reduce the risks and impacts of this nonindigenous species. 

For each objective, we describe potential management actions and provide 

technical guidance where feasible. It is our hope that this management outline w

facilitate the development of formal management plans that identify funding sources and 

state and federal agency res

Objective 1. Continue conducting research on the environmental and 

socioeconomic impacts of Didemnum sp. A. 

34
Not all nonindigenous species become invasive and eradicating or controlling 

invasive species is costly. For example, the Didemnum vexillum eradication and co

program attempted in the South Island of New Zealand cost approximately $120,000 

(Pannell and Coutts 2007). Although there are a number of viable reasons to eradicate 

and control nonindigenous species, including aesthetics and biotic homogenization, 

management decisions are often based on cost-benefit analyses. The effort and expense

of eradication or control efforts are compared to the likelihood of succes

ntrol 

s and the 

environmental or economic conditions that will be maintained. Complete eradication of 

all U.

Rather, l t and control efforts and funding will be needed. Although 

Didem

spread ra uses 

for conce ave negative environmental and economic impacts, 

there of these potential impacts. 

Unde

will help 

S. populations of Didemnum is unlikely, particularly colonies in offshore habitats. 

ong-term managemen

num’s wide distribution, ability to overgrow other benthic organisms, ability to 

pidly, and occurrence in aquaculture facilities and on Georges Bank are ca

rn that Didemnum will h

is currently little quantitative data on the magnitude

rstanding the impacts of Didemnum on native and economically important species 

justify the need for any management actions and long-term funding. 

Objective 2: Continue conducting research on the ecology and control methods of 

Didemnu hreats associated with this 

nonin

A

managem s. However, 

reliminary speculation about the effects of Didemnum on native species, aquaculture, 

and ma at the 

limitations are identified and addressed. 

Some preliminary research priorities and information needs highlighted throughout this 

integrat

1. Determine the taxonomic identity and geographic origin of Didemnum. 

m to better understand and manage the risks and t

digenous species. 

s with any effort to control a newly discovered nonindigenous species, the 

ent strategies outlined here contain a number of uncertaintie

p

rine fisheries, as well as the continued discovery of new colonies, indicate th

problem may worsen without management intervention. As a result, an adaptive 

management approach is needed, in which control efforts are viewed as hypotheses to be 

tested, monitored, and modified as information 

ed assessment are described below: 

35
y 

um

ces 

n 

um

ould determine if open ocean ballast exchange is an effective vector control method. 

tion of ballast water, we have very little information on the role 

of hull um

l 

bjective 3:

The forecasts and management recommendations presented in this integrated 

assessment are based on the assumption that colonies on the East and West coasts of the 

U.S. and in nearshore and offshore habitats are the same species. It is possible that the

are different species with unique environmental tolerances that would require unique 

management plans. Similarly, we have focused on minimizing the spread of Didemn

from populations from sources in U.S. waters. However, there is a need to determine 

where and how these populations arrived so that additional introductions from sour

outside of U.S. waters can be prevented. 

2. Quantify the environmental tolerances and preferences of Didemnum. 

 Our ability to identify locations that have the greatest risk of being colonized

by Didemnum is currently limited by our incomplete understanding of the environmental

tolerances and preferences of this nonindigenous ascidian. In addition to continuing 

research on the effects of water temperature on colony growth and survival, informatio

on other environmental variables known to be important to ascidians would be useful. 

For example, a better understanding of the range of salinities tolerable to Didemn

w

3. Create and maintain databases on transport vectors in addition to ballast water. 

 With the excep

fouling, aquaculture and other potential transport vectors in spreading Didemn

as well as other nonindigenous species. Information housed in the National Ballast 

Information Clearinghouse provided a valuable resource for identifying ballast water 

management practices and vessel traffic patterns, and ultimately in developing a BWM 

plan. A similar program is needed for hull fouling, aquaculture, and other potentia

transport vectors. This will help identify methods for minimizing the risk of spreading 

Didemnum, and allow managers to quickly identify transport vectors for other 

nonindigenous species. 

O Limit the spread of Didemnum to new areas. 

36
 Limiting the spread of Didemnum will require controlling the transport vectors

that contribute to its dispersal: vessel fouling, aquaculture, commercial fishing, and 

ballast water. Programs developed to control these transport vectors need to include 

voluntary or mandatory practices that minimize the risk of spreading Didemnum as we

as mechanisms for regulatory agencies to monitor compliance and identify individual, 

high-risk vectors that require inspection or other management actions. Current U.S. 

vector management policies and regulations focus predominantly on preventing the 

unintentional introduction and dispersal of

ll 

nonindigenous aquatic species in ballast 

ater. Specifically, in 2004 the U.S. Coast Guard began requiring mandatory ballast 

iance penalties 

e 

iscuss the potential effectiveness of existing and pending management policies and 

w

water management practices, record keeping requirements, and noncompl

for all vessels equipped with ballast tanks entering and operating in U.S. waters 

(Appendix I). However, some states (e.g., Hawaii and California), and other countries 

(e.g., New Zealand) are considering programs to control the introduction of 

nonindigenous species due to hull fouling, aquaculture, and fisheries (Coutts 2005, Floerl

et al. 2005, Godwin 2005, Takata et al. 2006). Similarly, the aquaculture industry has 

been actively developing methods for controlling the abundance of fouling organisms on 

equipment and stock, including a number of nonindigenous tunicate species. Below, w

d

industry practices to minimize the spread of Didemnum, and where possible make 

suggestions for improvements. 

Ballast water: Vessels equipped with ballast tanks can potentially spread 

Didemnum by transporting colony fragments or larvae in their ballast water. An 

additional concern is the ability of Didemnum to colonize ballast tanks, as these vessel

could serve as continual sources of colony fragments and larvae. The most effective wa

of minimizing the risk of spreading Didemnum through ballast water is to prevent co

fragments and larvae from entering ballast tanks. Currently, this means that ballast wate

uptake needs to be conducted in areas free of Didemnum. However, methods fo

out or separating organis

s 

y 

lony 

r 

r filtering 

ms from ballast water are being developed and may be available 

 the future. 

should avoid taking ballast 

water in areas with known infestations of harmful nonindigenous aquatic species and in 

in

 Federal regulations do stipulate that vessel operators

37
shallow water where propellers can disturb the bottom. Although these guidelines will 

reduce the risk of colony fragments and larvae from entering ballast tanks, prohibitin

vessels from taking ballast water in areas with large populations of Didemnum, or wh

colonies are reproducing would be an improvement over existing guidelines Vessel 

operators need to be strongly encouraged to not take ballast water in areas with known 

populations of Didemnum until methods for removing or destroying colonies and larva

from ballast water can be verified. 

The most widely used method for removing or destroying organisms taken up

ballast water is to conduct an open ocean ballast water exchange. Exchanging ballast

water in the open ocean reduces the density of coastal organisms in ballast tanks and 

replaces them with oceanic organisms that should have a lower probability of sur

in nearshore waters with lower salinities. Although some states (i.e., Oregon, 

Washington and California) require vessels involved in intercoastal shipping

ballast water exchanges, the BWM program only requires vessels arriving from outside 

of the U.S. Exclusive Economic Zone to conduct ballast water exchanges. As a result, 

current ballast water exchange requirements will have little impact on reducing the spread

of Didemnum in U.S. waters. Furthermore, ballast water exchange may not be an 

effective treatment as colonies attached to tank walls would not be flushed out. How

ballast water treatment is the subject of extensive research, and several technologie

methodologies for removing or 

g 

en 

e 

 in 

viving 

, to conduct 

ever, 

s and 

destroying organisms taken up in ballast water are being 

tested (

um in 

re, they could be used by the coast guard or port authority to identify 

e.g., sterilization with ultraviolet light, ozone, heat, or biocides). 

Because there are no proven methods for preventing the spread of Didemn

ballast water, there needs to be a mechanism for identifying high risk vessels. The 

national BWM program requires all vessels to submit ballast operation reports, which 

include information on the port of departure, location of ballast water operations, as well 

the use of any ballast water treatment methods. Currently these reports are required to be 

sent to the National Ballast Information Clearinghouse (National Ballast Information 

Clearinghouse 2004). If these reports were required to be sent to the vessels’ arrival port 

prior to departu

vessels that have taken ballast water in high risk areas and are potentially transporting 

Didemnum in their ballast water. 

38
Vessel fouling: As with ballast water, controlling the spread of Didemnum due t

hull fouling will largely depend on developing a process for identifying high risk vessel

At present, there are no programs or regulations that focus specifically on preventing the 

introduction of nonindigenous species due to vessel fouling. The BWM program requ

that anchors and chains be rinsed at their place of origin and that fouling organisms be 

removed from hulls, piping, and tanks on a regular basis (Appendix I). However, what 

constitutes a “regular” basis is not de

o 

s. 

ires 

fined. In addition, the focus on vessels with ballast 

tanks m

” 

rs 

 For 

 deal 

ouling coatings 

do not 

gson 

e 

001). Overall, there is a need to collect information 

isses other types of potential fouling vectors, such as recreational vessels. 

Hawaii is developing a hull fouling management strategy, which includes a “risk matrix

for identifying high risk vessels and possible actions to be taken before the vessel ente

state waters. Although this management strategy provides a good framework for 

managing hull fouling, a number of important details remain to be worked out.

example the “’risk matrix” has not yet been formally adopted as the criteria for 

determining a vessels “risk” have not been finalized, such as what hull maintenance 

standards should be adhered to, or how long a vessel can remain in port before it is 

considered a fouling risk (Godwin 2005). 

 Although there are a variety of antifouling coatings that can prevent a great

of vessel fouling when properly maintained, maintenance schedules are dictated by 

economic concerns (e.g., fuel efficiency) rather than preventing the introduction and 

spread of nonindigenous species. Furthermore, recent studies have found that even with 

antifouling coatings, organisms can still attach to vessel areas where antif

adhere well or are difficult to apply, such as along weld seams, sea chest intakes, 

rudder posts, and where wood blocks are placed while the vessel is in dry dock (Dod

and Coutts 2002, Minchin and Gollasch 2003, Godwin 2003). 

 Despite the emphasis on preventing the introduction of nonindigenous species in 

ballast water, a growing number of studies indicate that vessel fouling remains an 

important mechanism by which nonindigenous species are introduced into new areas 

(e.g., Eldredge and Carlton 2002, Godwin 2005, Takata et al. 2006). In addition, the 

importance of hull fouling as a transport vector is likely to increase in the future, as th

use of antifouling paints containing tributyltin (TBT) become restricted due to their 

toxicity (EPA 2003, see also IMO 2

39
on how hull maintenance patterns and fouling levels vary so that a fouling management 

plan can be developed. 

 Until specific guidelines can be established, management should focus on 

identifying high risk vessels. One option is to expand the ballast water reporting forms 

required by the U.S. Coast Guard to include information relevant to hull fouling, such a

estimated speed during voyage, time since last antifouling coating application or 

cleaning, length of stay at port of departure, and hull was visually inspected for fouling 

organisms prior to departure. For example, researchers in Hawaii found that slow 

moving vessels or those that have had a long inactive period were more likely to harbor 

fouling organisms relative to faster commercial vessels with short residency times 

(Godwin et al. 2004). 

s 

Aquaculture: Aqua-culturists employ a variety of techniques to control fouli

organisms on equipment and shellfish stock, including low concentration metalantifouling paints, manual cleaning, chemical treatments (e.g., vinegar, chlorine), hot

water baths, exposure to air, and exposure to benthic predators (Goldburg et al. 2001

Studies need to be conducted to determine w

ng 

based 

). 

hich of these techniques is most effective for 

controlling Didemnum. There is also a need to provide preshipment and arrival 

inspectors with the training necessary to identify Didemnum. 

Commercial Fishing: Controlling the spread of Didemnum through commerci

fishing activities could be done by closing areas with known populations or requiring 

vessels to clean all equipment after fishing in an area known to support Didemnum. 

Similarly, it will be necessary to restrict the disposal of potentially infected materials 

such as scallop shells. 

al 

Objective 4: Develop monitoring programs to detect changes in the abundance of 

existing populations of Didemnum as well as the establishment of new colonies out

its current distribution. 

 A monitoring program would greatly compliment efforts to control the spread 

Didemnum. Tracking changes in the size of existing colonies of Didemnum as well as th

abundance of larvae would help identify locations and seasons where vector control 

efforts should be closely observed. Monitoring in areas where colonies of Didemnum

side of 

of 

e 

40
have not yet appeared but are at risk of an introduction would provide a way of 

measuring the success of vector control programs. Furthermore, early detection greatly 

increas 3). 

ition, 

ould span a range of geographical locations and environmental 

el. 

at are 

d the general vulnerability 

ency researchers that contributed to 

is integrated assessment, have research programs involving tunicates, and should be 

oods Hole Oceanographic Institution (WHOI), Woods Hole, 

es the likelihood of eradicating newly established populations (see objective 

Developing a monitoring program requires identifying monitoring sites and developing a

network of university and agency researchers who can assist in establishing standardized 

monitoring protocols and data collection. 

Although any of the locations with existing populations of Didemnum could be 

used to track changes in colony and larval abundance (Table 1, Figs. 3 and 4), we 

recommend monitoring existing populations near areas where there are substantial 

amounts of vector activity, such as aquaculture facilities, ports, and marinas. In add

we suggest that sites sh

conditions. Coordinating with researchers with ongoing research programs such as the 

surveys being conducted on Georges Bank by scientists from NOAA, USGS and the 

University of Rhode Island could reduce the time and expense needed to train personn

Such collaboration would also take advantage of existing long-term data sets that may 

provide insights into the effects of Didemnum on native species. 

Efforts to detect new populations of Didemnum should be done in areas th

vulnerable to colonization. Although we have already discusse

of different LMEs, specific locations could be identified using port statistics, vessel 

traffic patterns, aquaculture equipment sharing practices or stock shipping patterns. 

Areas along the edges of the current distribution of Didemnum, particularly at the 

boundary of the Northeast and Southeast U.S. LMEs, should also be monitored. 

Contact information for University and Ag

th

contacted about developing a monitoring program are provided below: 

Mary Carman W

 Massachusetts. 

 mcarman@whoi.edu

Andrew Cohen San Francisco Estuary Institute (SFEI), San Francisco, California. 

 acohen@sfei.org

41
Jeremy Collie Graduate School of Oceanography, University of Rhode I

(URI), Narragansett, Rhode Island. 

jcollie@uri.gso.uri.edu

Jennifer Dijkstra University of New Hampshire (UNH), Durham, New Hampshire. 

dijkstra@cisunix.unh.edu 

Larry Harris University of New H m

sland 

a pshire (UNH), Durham, New Hampshire. 

s, 

lifornia. 

 bullard@hartford.edu 

 lharris@hypatia.unh.edu

Gretchen Lambert University of Washington (UW), Friday Harbor Laboratorie

Seattle, Washington. 

glambert@fullerton.edu

Richard Osman Smithsonian Environmental Research Center (SERC), St. 

Leonard, Maryland. 

osmanr@si.edu 

Bob Reid NOAA Fisheries (NMFS), Highlands, New Jersey. 

robert.reid@noaa.gov

John Stachowicz University of California, (UCD), Davis, Ca

 jjstachowicz@ucdavis.edu

Page Valentine, U.S. Geological Survey (USGS), Woods Hole, Massachusetts. 

pvalentine@usgs.gov

Robert Whitlatch University of Connecticut (UCONN), Groton , Connecticut. 

robert.whitlatch@uconn.edu

Gregory Ruiz Smithsonian Environmental Research Center (SERC), St. 

Leonard, Maryland 

ruizg@si.edu 

Stephan Bullard University of Harford, West Hartford, Connecticut 

Objective 5: Eradicate pioneering colonies, and where feasible, eradicate or reduce the 

emnum rapid response plan, which includes 

abundance of established colonies. 

Although a monitoring program can help locate newly established colonies, 

resource managers need to develop a Did

42
identifying proven and cost-effective eradication methods. It may also be desirable to 

reduce the abundance nge as part of a larger 

prevention program. There have been few systematic attempts to eradicate or control 

num

recently appeared in te orld-wide. The results of these 

vexillum from benthic substrates, dock pilings, and boat hulls in Shakespeare Bay, New 

Zealand (Sinner and C t eradication methods employed include 

al and 

treating boat hulls with n (Sinner and Coutts 2003). Follow-up 

surveys conducted in 2004 showed that smothering with dredge material killed 100% of 

eneous seabed 

substrate. In contrast, o smother colonies in spatially complex 

lonies; however, the surface of the 

un 

vexillum quickly reocc researchers manually removed 

ctive 

in killing colonies on b by securing the edges of a tarp the 

water 

between the plastic an ng material, including D. vexillum, was killed 

within 48 hours. 

om 

Shakes all scale attempts to eradicate 

colonies of Didemnum in the U.S. In 2005, volunteer divers also used bleach to 

 a tarp over the 4 m2

 colony and added 4 chlorine 

tablets 

 

of Didemnum along the edge of its ra

colonies of Didem , or colonies of potentially closely related species that have also 

mperate, marine systems w

attempts have been variable, but demonstrate that small colonies can be eradicated by 

smothering or bleach. 

 In 2003, the Cawthron Institute attempted to eradicate colonies of Didemnum 

outts 2003). The differen

smothering with dredge material, filter fabric, and plastic, as well as manual remov

 a dilute bleach solutio

the colonies occupying an approximately 3200 m2 area of relatively homog

 the use of filter fabric t

habitats with large boulders was ineffective. Wrapping pilings with 50 μm 

polypropylene plastic smothered the underlying co

plastic wrap was colonized by recruits from other areas. Similarly, colonies of Didemn

upied a boat hull after 

approximately 80-90% of the colonies. However, the use of bleach proved to be effe

oat hulls. Bleach was applied

barge and then poring two 20 liter buckets of granulated chlorine mixed with fresh

d the barge. All fouli

In addition to the various efforts to eradicate Didemnum vexillum fr

peare Bay, New Zealand, there has been two sm

eradicate a small colony from the hull of a submerged boat in the Edmonds Underwater 

Park in Puget Sound. Divers secured

to the approximately 32 gallons of entrapped water. The chlorine treatment 

appears to have been effective, as new colonies were not found in follow-up surveys. 

43
In spring 2007, Dr. Larry Harris from the University of New Hampshire began a pilot 

attempt to eradicate or control Didemnum by manually removing colonies from a 

commercial wharf in Eastport, ME. Follow up surveys have shown that while manual 

removal succeeded in reducing size of the colonies, difficult to remove fragments in 

crevices were beginning to regenerate. 

These examples emphasize the need for a rapid response program. Eradication

large established coloni

 of 

es is probably not feasible, or desirable given current eradication 

methods. For example bleach and smothering are effective, but have negative effects on 

other native species. While complete mortality of all benthic organisms may be 

acceptable at small spatial scales, the environmental cost will eventually outweigh the 

benefit of eradicating Didemnum. 

Objective 6: Educate the public, private industry and policy makers about potential 

environmental and economic costs of Didemnum, and how their actions can reduce the 

risks and impacts of this nonindigenous species. 

Minimizing the spread and impact of Didemnum will require public a

stakeholders participation in the management process. Human activities play an 

important role in spreading Didemnum, and preventing further introductions will require 

changing individual behaviors and industry practices. However, identifying 

behaviors pose the greatest risk of spreading Didemnum, and how these behaviors need to 

be modified to limit the risk of further spread, depends in part on stakeholder provided 

information. For example, the BWM program is based on ballast water reports submitted

by the shipping industry as part of the National Ballast Information Clearinghouse 

database jointly o

nd private 

which 

verseen by the Smithsonian Environmental Research Center and the 

U.S. Co databases 

g and 

y 

s. 

ast Guard (National Ballast Information Clearinghouse 2004). Similar 

are needed to determine how different hull and equipment cleaning practices and 

schedules or vessel travel patterns influence the relative risks posed by hull foulin

aquaculture, respectively (on aquaculture, this would need to be done on state by state 

basis, unless in federal waters). Devising a management strategy also requires that polic

makers and government agencies adequately fund and support management activitie

The willingness of resource users and policy makers to contribute to management efforts

44
will be greatly improved provided that they understand the scope of the threat posed by

Didemnum. 

 A number of invasive species websites already contain information on 

Didemnum, including sites maintained by the U.S. Geological Survey 

(woodshole.er.usgs.gov/project-pages/stellwagen/didemnum/), the San Francisco Estuary 

Institut

s for 

ian. There are also a number of national and state 

program tion in general that need to incorporate 

 i rmation available at industry and 

trade sh

eradicating well-established colonies indicates that an interim plan is necessary. 

e (www.exoticsguide.org/), and the Invasive Species Specialist Group 

(http://www.issg.org/database/welcome/). Similarly, a number of web-based articles and 

press releases are available, which describe local concerns regarding Didemnum (e.g., 

www.pnwscuba.com/critterwatchers/invasive.htm). These are excellent resource

stakeholders that are already aware of Didemnum, and interested in actively learning 

more about this nonindigenous ascid

s dedicated to invasive species educa

information on Didemnum. For example, the ANSTF’s Stop Aquatic Hitchhikers! 

campaign’s focus on freshwater systems could be expanded to include marine systems. 

Other activities that may be useful include making nfo

ow and with state boating license registration packages. 

Conclusions 

Although a large number of Didemnum colonies have only recently been 

discovered in U.S. waters, this nonindigenous colonial ascidian may conceivably have 

negative impacts on native species, aquaculture, and marine fisheries. As a result, 

management agencies, policy makers, industry members, and the general public need to 

recognize Didemnum as an invasive species. Raising the awareness of these different 

stakeholder groups will require developing and supporting education and outreach 

programs, and should provide stakeholders with sufficient information to work 

collaboratively to identify actions and resources needed to limit the spread and impacts of

Didemnum. While providing all stakeholders with the opportunity to contribute to the 

development of Didemnum management plan will greatly improve the long-term 

management of this nonindigenous species, such a process will take time. However, the 

rate at which new colonies are being discovered and the lack of proven methods for 

45
Current management actions should focus on limiting the spread of Didemnum, 

via hull fouling and the movement of aquaculture stock and equipment. In addition, 

olonies in 

ogy and ecology 

. Levels 

a 

ze the many individuals who graciously 

 unpublished data and results for use during the development of this 

integra

increasing monitoring efforts are needed to rapidly locate and eradicate newly c

areas likely to be invaded. Given our current understanding of the biol

of Didemnum, areas most susceptible to invasion are those with growing season of 6 

months or longer and where mean monthly water temperatures are below 25 oC

of vector activity could be determined through vessel traffic patterns, aquaculture 

shipment patterns, and fishing activity. Areas at the edges of and within Didemnum’s 

current distribution should be monitoring priorities, followed by the northern end of the

Southeast U.S. LME. As an important precaution, we recommend monitoring the 

southern end of the Southeast U.S. LME. Recent evidence suggests that hull fouling is 

larger problem than currently believed. 

Acknowledgements 

The authors wish to thank and recogni

provided us with

ted assessment including (in alphabetical order); Stephan Bullard, Jennifer

Dijkstra, Richard Osman, Bob Reid, Robert Whitlatch, and Page Valentine. Comments

by Drs. Page Valentine, Gretchen Lambert, and 4 anonymous reviewers greatly improved 

the clarity and quality of this manuscript. Funding was provided by the Alaska 

Department of Fish and Game, the Pacific States Marine Fisheries Commission and 

NOAA’s National Centers for Coastal Ocean Science (NCCOS). 

46
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Appe

. Mandatory practices for all vessels with ballast tanks operating in all U.S. waters. 

2.

Near dredging operations. 

d. Where tidal flushing is poor or when a tidal stream is known to be more turbid. 

e. In darkness when organisms may rise up in the water column. 

3.

4. ater in coastal and internal waters. 

5. val to remove organisms and sediments at 

their place of origin. 

6. Remove fouling organisms from hull, piping, and tanks on a regular basis and dispose of 

any removed substances in accordance with local, state and federal regulations. 

7. Maintain a vessel specific ballast water management plan. 

8. Train vessel personnel in ballast water and sediment management and treatment 

procedures. 

9. Submit Ballast Water Reporting Forms. 

10. Ballast water records must be kept on board the vessel for a minimum of two years. 

. Additional Mandatory Practices for all vessels transiting to U.S. waters with ballast 

ater that was taken on within 200 nautical miles of any coast after operating beyond the 

.S. Exclusive Economic Zone. 

1. Conduct one of the following: 

a. conduct mid-ocean ballast water exchange prior to entering U.S. waters; 

b. retain the ballast water on board while in U.S. water; or 

c. use a Coast Guard approved alternative environmentally sound method to treat the 

ballast water . 

C. Penalties for failing to comply with the Mandatory BWM Requirements. 

1. Maximum fine of $27,500 per day 

2. Willful violations = Class C felony 

D. Exemptions 

1. Crude oil tankers engaged in coastwise trade; 

2. Vessels of the Department of defense, Coast Guard, or any of the Armed Services as 

3. Vessels that operate exclusively within one Captain of the Port Zone (COTPZ) 

ndix I. United States National Ballast Water Program regulations. 

A

1. Avoid ballast operations in or near marine sanctuaries, reserves, parks, or coral reefs. 

Avoid ballast water uptake: 

a. Where infestation, harmful organisms and pathogens are located. 

b. Near sewage outfalls. 

c.

f. In shallow water or where propellers may stir up the sediment. 

g. Areas with pods of whales, convergence zones and boundaries of major currents 

Regularly clean ballast tanks to remove sediment. 

Only discharge minimal amounts of ballast w

Rinse anchors and anchor chains during retrie

B

w

U

defined within 33 USC 1322 (a) and (n); 
e 

Secretary 

Vice Admiral Conrad C. Lautenbacher, Jr. USN (Ret.) 

National Ocean Service 

John H. Dunnigan 

Assistant Administrator for Ocean Services and Coastal Zone Management 

United States Department of Commerc

Carlos M. Gutierrez 

National Oceanic and Atmospheric Administration 

Undersecretary of Commerce for Oceans and Atmosphere