Taking Stock of Currents and Quahogs

RESEARCHERS SEEK BETTER UNDERSTANDING OF CLAM DISPERSAL IN NARRAGANSETT BAY

by  ZoeGentes

Over 39 million clams were harvested from Narragansett Bay in 2012, supporting a $5.15million commercial fishing industry, according to figures by Jeff Mercer, principal biologist in marine fisheries for the R.I. Department of Environmental Management (DEM).

The estimates of clams in the Bay are used to set fishing limits. In Rhode Island, commercial shellfishermen use a bull rake for harvesting clams. The DEM, however, uses a hydraulic dredge to collect clams for population estimates. Fishermen say that the dredging method of harvesting is inefficient and inaccurate, and is likely to result in unnecessary limits on the commercial fishing operations.

Dale Leavitt, an associate professor of marine sciences at Roger Williams University, is conducting a study that compares the efficiency of dredge gear to that of a bull rake. He is going out on commercial shellfishing boats that are using bull rakes alongside DEM dredges to compare the results for clam population numbers.

Being able to accurately take stock of clam populations in Narragansett Bay will be important for making more effective management decisions concerning commercial shellfishing. Another aspect of population dynamics researchers are addressing is determining where the clams are coming from within the Bay and how they are dispersed.

Understanding dispersal of quahogs, specifically, is of particular interest to researchers because quahogs do not move much once they settle as larvae. They may move only a couple meters in their whole lifetime. To better target their harvesting efforts, “knowing where the quahog larvae move to is incredibly important for fishermen,” says Azure Cygler, an extension specialist from Rhode Island Sea Grant who is leading the R.I. Shellfish Management Plan.

One management strategy that has been used to a small degree in Rhode Island is to create “spawning sanctuaries” by closing off areas and prohibiting fishing where large numbers of quahogs are located. “The idea is that they maintain a population of reproductively active quahogs where they will spawn and broadcast larvae out for distribution about the Bay,” Leavitt explains. However, if it isn’t known where the larvae will go, it is difficult to judge how effective the sanctuary may be in replenishing the bay with quahog seed. Also, potential overcrowding at the spawning site may lead to poor conditions and even to low reproduction rates, defeating the purpose of the supposed sanctuary.

Leavitt and collaborators are using a hydrodynamic computer model called the Regional Ocean Modeling System (ROMS) to make an educated guess as to where the quahog larvae may be distributed when originating from a specific area.They are testing a number of locations to see which ones contribute the most seed to the upper Bay.

ROMS has been developed and calibrated against years of detailed hydrographic information with Sea Grant funding by Christopher Kincaid, a professor and researcher at URI’s Graduate School  of Oceanography (GSO) and David Ullman, a marine research scientist at URI GSO, to create a picture of how currents, circulation, and nutrients affect water quality in the Bay.

Kincaid, Ullman, and a number of URI students now use ROMS simulations to predict circulation and transport within the Bay under different conditions. For instance, if they want to see how a certain area of the Bay will circulate and flush without wind, they can select for those parameters. Then they can add the wind back in, and see how the system behaves differently. “Using ROMS in this way you can hone in on which environmental and manmade factors lead to which response in the estuary,” Kincaid  says.

By simulating quahog larvae in the Narragansett Bay ROMS program, including adding a larval behavior component, researchers can predict how the larvae will be dispersed by currents within the estuary under certain conditions. These predictions can help shellfishermen better understand where the quahogs are ending up, and what their populations might be.

These efforts are being under taken as part of the R.I. Shellfish Management Plan, which is being developed to provide comprehensive policy guidance for management and protection measures for shellfish located in state marine waters.

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