Quahog Hot Spots in Narragansett Bay

Some areas of Narragansett Bay, such as Barrington Beach, are well-known quahog hot spots. But how did they get that way? And is a “spawning sanctuary” in the bay, where no shellfishing is allowed, effective? Should it be moved elsewhere, or should an additional site be established?

Quahog fishermen working in Narragansett Bay

Researchers at the University of Rhode Island Graduate School of Oceanography (GSO), Roger Williams University (RWU), and the Rhode Island Department of Environmental Management, with funding from Rhode Island Sea Grant, set out to answer these questions to aid in shellfish management and discussed their findings as part of the Coastal State Discussion Series on March 28.

It might seem obvious that microscopic larval quahogs, spawned up into the water column by adult quahogs, would be at the mercy of tides and currents, which would entirely determine where they would land and bury themselves in the sand.

But, while newly spawned quahogs—which will spend 8 to 12 days in the larval stage before settling on the bottom and becoming juvenile quahogs—are very much dependent on water circulation to determine where they will end up, they are able to swim, and their behavior, to a degree, factors into where they settle.

Researchers on this project combined computer modeling with a hands-on approach creating “drifters” from hardware-store materials, such as PVC pipe and canvas, to follow the path of the quahog larvae. Scott Rutherford, a professor of marine and natural sciences at RWU, discussed that while larvae have little ability to swim against a current or determine their direction horizontally, they are able to determine their place vertically within the water column, and “where they are vertically (affects) where they go laterally.”

While they grow and their shells get heavier, their swim muscle does not grow, so they do slow down as the days go on, moving from a position close to the surface in the first few days of existence to lower and lower in the water column before eventually landing on the bottom.

Canvas-sail drifters help researchers examine how quahog larvae may be affected by circulation in Narragansett Bay.

Researchers equipped 59 drifters with GPS transponders and released them over the course of the summer from six sites in the bay known to have large populations of quahogs. The drifters had underwater canvas “sails” that were set to 1, 3, or 6 meters in depth. Those sails were intended to examine circulation at different depths to see where the circulation of the water might take the larvae. This was important because water at different depths may travel in different, even opposite, directions.

The drifters were observed for several days until they ran aground or left the bay, and later retrieved. Where the drifters ended up were incorporated into computer models known as ROMS models (for Regional Ocean Modeling Systems), that take into account river inflow, tides, winds, and meteorology. But “that’s just the circulation,” said Rutherford, explaining how this data was added that to a larval transport model that takes into account where the larvae might be and their behavior—that ability to swim up or down. Since little is known about quahog larval behavior, the researchers used oyster larval behavior to supplement the model.

What the researchers found was that larvae from anywhere in the bay can end up anywhere, but there are patterns that most larvae follow depending on where and when they are spawned. And larval swimming behavior matters. According to the models, when larval behavior is NOT accounted for, 61 percent of the larva end up leaving Narragansett Bay, and are considered “lost” from the system and the fishery. However, when the models did account for larva behavior, that number was reduced to 51 percent.

The research found that quahogs that spawn in Conditional Area A, an area that is periodically closed due to pollution, is an important source of larvae for the Providence River and a lot of the West Passage, with only 20 percent being lost from the system. Conditional Area B is a source for the West Passage, as is the spawner sanctuary, which researchers—as well as fishermen in the audience—agreed is an effective spot for the sanctuary in terms of the amount of larvae it contributes to the system. Greenwich Bay seeds itself as well as a spot by Jamestown.

What was not considered by the project were other factors beyond circulation and behavior that would affect the number of larvae that reach the bottom and mature—factors such as predation by ctenophores (comb jellies) or menhaden, or whether crowding at spawning areas, such as the sanctuary, reduces the spawning capacity.

Some of those questions are going to be examined in follow-up work, Christopher Kincaid, professor of oceanography at GSO, said.

“This research provides us at Fish and Wildlife our first estimate for sources and sinks for larvae throughout the bay … This information will be highly valuable as we continue to monitor the quahog population,” said Conor McManus of DEM.

The data from this project will be used in future assessments that describe quahog population status and to inform management strategies. The spatial data can also be used to assess how future operations, such as aquaculture and dredging, or other coastal development may impact quahog larval dispersal and settlement.

Michael McGiveney, the president of the Rhode Island Shellfisherman’s Association, was in the audience and commented that the distribution of quahogs throughout the bay helped disperse the fishing effort, avoiding too much concentrated pressure in certain areas. He said that if, as improvements to water quality continue in the bay thanks to sewage treatment upgrades in Providence, the upper bay is opened to shellfishing, “We have to have a good management plan; we have to worry about the golden goose.”

One audience member commented that while these model results showed larvae being lost from the system once they exit the bay, in fact, some will re-enter the bay from Rhode Island Sound. Kincaid said that they have new models that couple the bay with the sound, so it is possible now to look at what really happens to quahog larvae that exit the bay. Their work to better understand quahog dispersal and settlement will continue, said Rutherford: “One of our next steps is to try to figure out habitat preference for settling larvae.”

 

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– Monica Allard-Cox | Rhode Island Sea Grant Communications

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