Invasive Tunicate Spreading in Point Reyes Shellfish Farm

Didemnum vexillum ("marine vomit") smothering oysters

in Point Reyes shellfish farm

[click here for the Marin Indepent Journal article]

 

The non-native invasive tunicate, Didemnum vexillum (Dv) is spreading within the commercial oyster farm located in the Point Reyes National Seashore, part of the Drakes Estero area. Dv is considered the most invasive of the tunicate family and is found in Totten Inlet within Taylor Shellfish's existing mussel farm in Gallagher Cove. It spreads when colonies are broken off, in this case during oyster harvesting or, in the case of Totten Inlet, when mussels are harvested.

Invasive Tunicate Dv from

Taylor's Gallagher Cove Mussel Farm



(photo courtesy of Woods Hole Science Center)

[cick here for additional photos from Taylor's Gallagher Cove mussel farm]

 

Dv is a "colonial" tunicate which can spread when portions are dislodged. Those parts float free in the current and are able to re-attach for up to 4 weeks, more than ample time to leave Totten Inlet and enter the waters of south Puget Sound. 

 

A recent peer-reviewed study published in "Bilogical Invasions" has shown that anthropogenic activities, including commercial shellfish farming, can be a threat to increasing the spread of this invasive tunicate, noting: " D. vexillum can inflict ecological and economic damage through biofouling and habitat modification. Natural and anthropogenic processes that fragment colonies of D. vexillum may be accelerating the spread of this invader. Reattachment success and fragment viability were confirmed in the laboratory after four weeks of suspension in experimental aquaria." [click here for abstract]

 

The spread of the invasive Dv tunicate in the Point Reyes National Seashore is most likely tied directly to the operation of the shellfish farm. According to Ted Grosholz, a University of California at Davis marine ecologist who conducted the estero survey, the oyster farm structures provide a base for "fouling species" like D. vexillum. As seen in the picture above, the harvested oysters also provide a suitable substrate.

 

Dislodging colonies during harvesting allows them to drift within the National Seashore area and reattach when suitable substrate is found, including " establishing new colonies on the various structures used in shellfish farming or the planted shellfish.

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Taylor Shellfish's proposed 58 raft facility will have over 40,000 lines of mussels on which Dv will be able to establish itself, as it did in Gallagher Cove. Each time these mussels are harvested, established colonies will have portions break loose which will be allowed to float free within the waters of Totten Inlet, flowing into south Puget Sound on the outgoing tides.

 

Cumulative impacts are real, in part why the permit for Taylor's proposed mussel farm was denied. Thurston County Commissioners will hear Taylor's appeal to this permit denial November 14.