Rock scallop: Viable candidate for aquaculture?
By Tom WalkerFeatures Shellfish Joth Davis purple-hinge rock scallop (Crassadoma gigantea)
The purple-hinge rock scallop (Crassadoma gigantea) is indigenous to the west coast of North America. It’s tasty, hardy and has a developed culture technology. One of Joth Davis’ current jobs is to turn it into a commercial species, and he is excited.
“The bottom line is that the rock scallop adductor muscle is quintessentially one of the finest scallops in the world, it’s amazing,” says Davis from the Puget Sound Restoration Fund (PSRF) office on Bainbridge Island. “It should be developed purely for the sushi market. It has an outstanding flavor that wins all kinds of taste tests.”
Rock scallops are one of only a few scallops native to the North American west coast that might be amenable to aquaculture, Davis explains. “This is different from the hybrid that crosses the Japanese and the weather vane scallops that has been produced on Vancouver Island for the last 20 years,” he says. “The hybrid is simply unavailable to the aquaculture industry south of British Columbia.”
Davis, an affiliate professor at the University of Washington School of Aquatic and Fisheries Sciences and a senior scientist with PSRF, calls David Leighton from California the “grandfather” of rock scallop research. “David is the first guy to realize that this is a species potentially very amenable to aquaculture,” says Davis. “I initiated work on rock scallops in the early 2000s that followed work done by Ken Chou who was my mentor and advisor at the University of Washington during my PhD.”
“The most recent work has focused on a Western Regional Aquaculture Center Grant to myself and folks in California to really look at rock scallop aquaculture again from A to Z and try to iron out the wrinkles in production.”
Davis says they are about half way through the project and are focusing on three things.
First, they are looking at practical, commercial grow-out methods. “We don’t have any bio-economic information to support the notion that this is a viable candidate for aquaculture,” he says.
Second, the work will focus on finding viable means of raising enough seed to be able to support an industry. “Growing enough seed is a bottleneck,” says Davis. The third piece of the research is triploiding, as for most scallops raised for commercial production.
Trials are underway at seven sites in Puget Sound in cooperation with Taylor Shellfish. “They are growing really well so that’s fabulous,” Davis says. “We are about a year away from a commercial sized scallop; that would be about 100 to 110 mm in shell length.”
“We have scallops that are pushing 70-80 mm now,” he says. “So the leading edge is certainly going to be in that size class a year from now. Three years grow-out from larvae is about what it will take under the right conditions.”
Those conditions include salinity from 26 or 28 to 31 ppm and temperatures below 18C. “They like high current areas and they thrive on rich phytoplankton stocks,” says Davis. “They do very well in productive environments.”
“This is a sub tidal species,” he adds. “All of our sites are suspended culture.”
“We are working on a suite of grow-out strategies,” says Davis. They are looking at having the larvae cement on a PVC panel, so they can be easily popped off as a single scallop. Some work has investigated using cement columns as a substrate.
Rock scallops hardy animals. “They are very tough to kill, you ignore them and they just chug along,” says Davis. “There are no known diseases so that is very helpful.”
Rock scallop brood stock maintenance and hatch is a fairly straightforward says Davis. They condition quite well and spawning can be induced. “Larval care is about 23-25 days, very similar to care and diet for raising of larval geoduck, which may not be all that coincidental because they are both native species,” Davis adds.
“The post-settlement piece is a focus we need more information on,” he says. “The culture conditions make a big difference. The diet is important. We know from previous work that scallop larvae love the sterols that they get from Pavlova. These sterols seem really important for settlement.”
“The other thing is that flow characteristics and temperature tolerances in the nursery system are really important,” Davis adds. “Based on some preliminary work from last year, we think we have a pretty good set of settlement system options to test for the research this coming year.”
“My firm goal in the next few years is to have a good approach to rearing that is going to produce enough rock scallop seed to support a beginning industry.”
Problems induced by ocean acidification (OA) are not showing up yet. Davis says this is “fairly consistent” with other native species to the Pacific Northwest. “That could obviously change, but we are not seeing it,” he adds.
“We have not looked at OA challenges at the larval level where it hits oysters and other bi valves but we are seeing recruitment in the wild regularly in areas that have OA issues,” explains Davis. “We find small scallops setting in our nets that come from the wild. There is more to do in that area for sure, but I think they handle it better than some species.”
Domestic adults that do not reproduce in the wild (for bio security) and develop a larger adductor muscle, are the goals of triploiding. “We have triploids already but we want to develop the next phase which are tetraploid lines of rock scallops,” says Davis. “Which is a standard approach in the oyster industry.”
— Tom Walker
The purple-hinge rock scallop is a cementing species, which means it glues itself to structures, explains Joth Davis from the University of Washington’s School of Aquatic and Fisheries Sciences. “As a result it grows into something that doesn’t look like a single scallop very much. They can cement together, grow into the nets or grow into the cages depending how you do it.” This could lead to higher harvest costs and damage to gear or shells, when the scallops are removed.
“Part of our research is to find ways to grow them so they don’t cement into the gear,” says Davis. “That is more of a handling approach than anything.”
Rock scallops are susceptible to bio toxins like other bivalves, and parts of the tissues, such as the hepatopancreas, can both retain and store bio toxins for unknown amounts of time.
“The adductor muscle doesn’t tend to take up very much toxin compared to the rest of the body tissues,” Davis points out. “Most scallop fisheries are based on the adductor muscle only, so that is in our favor.”
“We have just submitted a proposal to the National Sea Grant program to look at bio toxin up-take, retention and depuration,” says Davis. “We don’t know if that will be funded”.
“Bio toxin up-take and retention is problematic,” adds Davis. “The depuration piece we don’t know, that is for the research. We do know they don’t retain it for ever, but we don’t have any idea of the time frame.”
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