Aquatic invaders are like uninvited house guests who refuse to leave. Recent invaders in the southern Gulf of St. Lawrence have become serious pests. This problem affects everyone using our waters - recreational boaters, cottage owners, commercial harvesters, aquaculturists and others. A population explosion of clubbed tunicates limited harvesting of mussels in southeastern P.E.I. in October 2001. Clubbed tunicates and oyster thief weigh down floating docks, making them difficult to remove from the water. Green crabs affect the eel fishery. Eels won't enter a trap filled with green crabs. Green crabs, clubbed tunicates and oyster thief can compete with, or prey on, native fishes, invertebrates and plants. Invasive species can be introduced in several ways: by moving ships and boats, by accidental release, and intentional introductions. Getting rid of these visitors is next to impossible. Keeping them out in the first place is the best approach.
Mussel lines covered in clubbed tunicates, P.E.I.
This green alga is a native of Japan and was detected in 1996 at Caribou, N.S. Oyster thief smothers mussels and oysters, preventing them from opening their shells to filter feed. Starved and weakened shellfish are easy targets for predators. Gas bubbles trapped under dense mats of oyster thief can lift shellfish off their beds, and float away with them. Fouling caused by the alga results in increased labour costs in shellfish harvesting. Oyster thief displaces native kelp, a preferred habitat used by many species, including sea urchins and lobster.
The clubbed tunicate, a native from the western Pacific, was reported in the Brudenell River, P.E.I., in January 1998. Clubbed tunicates grow in dense clumps of up to 1000 individuals per square metre. Adult tunicates can reach a length of 16 to 18 cm. Infestation of docks, buoys, and other hard surfaces has occurred from the low tide mark to depths of 4 to 5 metres. Clubbed tunicates interfere with the settlement of oyster and mussel larvae and compete for space and food with young oysters and mussels. They are serious pests of aquaculture.
The European green crab was observed in St. Georges Bay, N.S., in 1995. Green crabs are a dominant predator eating a variety of species, especially shellfish such as clams and mussels. Green crabs are smaller and more agressive than the native rock crab. Green crab populations have spread more than 100 km in some years. Green crabs probably contributed to the collapse of the softshell clam industry in Maine during the 1950s.
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