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Mitigating Invasive Sea Squirt Impacts to Shellfish in the Gulf of Maine

September 7th, 2012

By: Felipe Paredes, UMaine School of Marine Sciences

In the marine world, invasive species are very hard to detect and often impossible to eradicate. The key is always in preventing introductions, but if that fails, we need to mitigate impacts where possible. In order to accomplish this we must understand their basic biology and environmental preferences.

Ascidians on mussels

Invasive ascidians compete with shellfish for food and space (Felipe Paredes)

Maine’s estuaries are unique ecosystems with a long history of introductions of non-native species. These systems have dramatically changed due to introduced mobile species such as the periwinkle snail, L. littorea and the green crab, Carcinus maenas, both introduced centuries ago. Recently, non-native colonial and solitary invertebrates have been increasing in abundance along the coast of Maine! Some of the most common species are ascidians, or “sea squirts,” which are filter feeders attached to hard substrates. These recent invaders not only threaten native marine communities but also foul suspended and bottom aquaculture gear (nets, floats, cages) and often successfully compete with cultured shellfish for food and space. Shellfish farmers try to mitigate losses from invasive ascidians by exposing gear to air or by replacing the gear. Both options are time consuming and expensive in materials and labor.

Despite the economic impact of these introduced fouling species to aquaculture, we know remarkably little about what drives the “bloom and bust” in their populations. There are no studies of seasonal timing, distribution in estuaries and rate of colonization by this group. The project “Mitigating invasive ascidian impacts on Maine’s shellfish aquaculture industry” has the goal of determining the seasonal patterns in settlement and growth of the dominant non-native ascidians in the Damariscotta River, the heart of Maine’s multimillion dollar oyster industry. The basic idea was to collect information to help minimize interactions of invasive ascidians with the shellfish aquaculture industry.

Fouling invasive ascidians

Colonies of invasive Golden star ascidians (Botryllus schlosseri) at the center and the orange or red sheath ascidians (Botrylloides violaceus) (Felipe Paredes)

We deployed hundreds of settlement plates at 1 meter depth at different sites along the Damariscotta River (approx. 20 km long) and measured environmental parameters such as salinity and temperature. We found extremely high abundances of invasive ascidians in all areas but with different patterns of dominance. In the upper section of the estuary, with temperatures that can reach up to 25ºC, there is an early settlement (July) and dominance of colonial ascidians such as Botrylloides and Botryllus. The mid sections of the estuary are dominated by solitary ascidians, such as Ciona, Ascidiella and Styela, which settle later (August-September) and add extra weight to aquaculture gear. The lower section of the estuary, with “coastal” salinities and temperatures, is dominated by the short lived Diplosoma, and other colonial species.

Common to all stations was Didemnum, a well-known and aggressive colonial ascidian which adapts to many different conditions. Compared to other estuaries, such as the Kennebec and the Sheepscot – where invasive species are rare or absent , Damariscotta’s relatively low freshwater discharges and higher water temperatures might provide benign conditions to invasive ascidians, which nurtured the distribution, abundance and diversity of non-native species in fouling communities. Project funded by Maine SeaGrant.

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