Shellfish aquaculture in the U.S. typically has three phases of production: hatchery (holding and spawning mature broodstock to produce larvae), nursery (nurturing larvae to plantable seed), and growout (culturing seed in a protected environment to yield marketable shellfish).
While shellfish aquaculture is similar throughout the U.S., each region has its own environmental conditions, industry practices, and regulatory oversight.
Prevention:
There may be a number of unwanted species in plantable size shellfish seed in a nursery, including crustaceans (e.g., crabs), gastropods (e.g., whelks and moon snails), and echinoderms (e.g., starfish).
The tiny larvae or juveniles of these species might be concealed in seed from a hatchery; however, they will grow larger in the nursery system and can be detected and removed when finally visible to the eye.
Growers should carefully inspect all seed shipments and remove undesirable species, paying particular attention during grading or washing activities. The examination of market-ready shellfish may reveal hitchhiking indigenous or nonindigenous species that could make the shipment unacceptable for receipt in other states.
For example, several species of nonindigenous tunicates and crabs have been increasing in range, including European green (Carcinus maenas) and Chinese mitten (Eriocheir sinensis) crabs.
A non-native tunicate (Didemnum vexillum) has fouled mussel gear in the Atlantic waters of Canada. Some species have been found on large swaths of the substrate on Georges Bank off Massachusetts and New England tidal lagoons and estuaries (Morris et al., 2009).
A second non-native tunicate, Styela clava, is affecting oyster culture in Rhode Island (Karney and Rhee, 2009). Native tunicates are common on shellfish gear in the mid-Atlantic, with Molgula manhattensis being predominant.
The risk of their spreading to other regions via shellfish can be reduced if they are removed by hand before shipping. Fortunately, these soft-bodied invertebrates are easily seen once they grow to about ΒΌ-inch, and they can be picked out of a shipment.
Flushing shellfish with freshwater produces osmotic pressure changes in these soft-bodied animals, causing them to die. If tunicates are present in the area, it is a good idea to be alert for them and wash the seed in freshwater.
State regulations may allow only native or naturalized species to be cultured. Check with the state regulatory agency and be alert for regulations that prohibit the movement of live shellfish from one water body to another.
Such requirements have been implemented to reduce the risk of spreading pests and they have been effective.
Authors:
Paul W. Zajicek, Jeffrey E. Hill, Nathan Stone, Hugh Thomforde,
Cortney Ohs, Diane Cooper, Gef Flimlin, Brad McLane and William D. Anderson
