A study to quantify the decreasing numbers of rock crab in Prince Edward Island waters has resulted in a discovery that is being looked at closely by provincial and federal scientists.

Rock crab, the study suggests, can be utilized to control and even diminish the numbers of tunicate, an invasive species that is wreaking havoc on mussel socks and putting undue pressure on the $25 million dollar a year mussel industry. The objective of the three-year study (2005-2008) is to examine the effects of the rock crab on mussel socks.

“Mussel socks tend to be cleaner and tighter with the presence of rock crabs,” says Marc Ouellette, a scientist with DFO in Moncton, NB., explaining that rock crab help control the fouling organisms that colonize the mussel sock over time. He says rock crab presence also stimulates a better attachment of the mussels to the socks. “What we’ve found is the added weight of the tunicate on the socks can cause the mussel to fall off,” says Ouellette. Study sites were set up in Malpeque Bay, New London Bay, Rustico Bay and Georgetown Harbor. The PEI Shrimp Fund has provided financial support for Ouellette’s research.

Ouellette says at this point DFO does not have the data to specifically quantify the rock crab’s direct effects on tunicate, but notes, “These results certainly indicate that the rock crabs could play a role in controlling tunicates — it’s another piece of the puzzle.” He adds that the level of infestation depends on which species of tunicate mussel farmers are dealing with.

To utilize the rock crab, a farmer lowers the lines on the seabed to the bottom of the ocean, where the rock crab take over. The drawback with this method is that starfish can also climb on the mussel socks, and will eat larger mussels.

It is thought that these aquatic invasive species have been transplanted into Canadian waters from ballast water taken on by merchant ships in Asian waters. Various species of the tunicate have been in PEI waters for a number of years, particularly in eastern areas where fisherman see their mussel socks covered with hundreds of tunicate, adding to harvesting time and labor costs.

With PEI supplying about 80 per cent of the US market for mussels, scientists south of the border have an interest in what is happening in PEI waters as well as their own. Andrea Locke, another research scientist with DFO in Moncton, worked with four US scientists on an invasive species in the Gorges Bank area.

Locke spent 10 days in July on a boat in the Georges Bank area where the team studied the effects of that tunicate on waters, the bottom living animals and plants. She was working with Dr. Larry Harris of the University of New Hampshire, Jennifer Dykstra and Mary Carman from Woods Hole Oceanographic Institution and Dr. Page Valentine of the United States Geological Surveys office in Woods Hole, Mass.

Locke, who draws on her experience and involvement with the problem in New Brunswick, where minimal sightings have occurred, as well as in PEI and U.S. waters, says it’s the warm waters and soft, muddy bottom that keeps this sea creature a part of daily life for PEI’s mussel industry.

The Atlantic Canada Opportunities Agency (ACOA) Innovation Fund in cooperation with the Prince Edward Island Aquaculture Alliance, Department of Fisheries and Aquaculture, DFO, the Shrimp Fund and the Atlantic Veterinarian College in Charlottetown have, together funded a $3.8 million project that is looking at methods for early detection and prevention through developing of anti-fouling compounds and control techniques. q