Principal Investigators: Donald Scavia (CILER)

Ecologists from Europe, New Zealand, and North America have recently reported an alarming increase in the abundance and distribution of either a single species or an unknown number of species of the colonial ascidian genus, Didemnum (Family Didemnidae) (U.S. Geological Survey 2005). Although the impacts of Didemnum in these different systems are not known, previous ascidian introductions demonstrate that nonindigenous ascidians can quickly dominate fouling communities, alter species diversity, and have become significant economic problems by fouling aquaculture equipment and stock (Cayer et al. 1999, Boothroyd et al. 2002).

In the U.S., colonies of Didemnum have been documented in coastal New England from northern Maine to New York, and at multiple locations along the California coast. Until recently, Didemnum colonies 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 (Lambert 2001). However, the discovery of Didemnum sp. on Georges Bank (Valentine et al., in press) has raised concerns about the impacts that this ascidian could have on economically important off-shore habitats, the possibility that it will spread to other marine ecosystems, and what management actions should be taken to prevent or limit any adverse impacts. We conducted an integrated assessment for Didemnum to address two main questions:

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 the 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) provide guidance for potential management actions.