During November, young scallops, which have drifted in the sea and along ocean currents since their parents’ late-summer spawning, are settling down on patches of sand and gravel seafloor along the Maine coast. No one knows where they have come from, or where exactly they will go, but some will land in a sheltered area and grow to market size. Maine’s scallop harvesters, who have been fishing discrete scallop beds in coastal waters for decades, already know that some places are better for Placopecten magellanicus than others.

Once one of the state’s most valuable fisheries, scallop landings declined to an all-time low six years ago, and now provide only part-time jobs for a few fishermen in most coastal communities. However, the fishery remains important in some areas, particularly in the Cobscook Bay region, where nearly 70 percent of Maine’s harvest is landed.

In response to declining numbers of scallops, the Maine Department of Marine Resources (DMR), which regulates the fishery in state waters within three miles of shore, closed a number of scallop fishing grounds in 2009. The closures were inspired by the success of rotational closures established in offshore scallop fishing grounds, including on Georges Bank, since the 1990s. The offshore scallop fishery is one of the most valuable in the United States and the rebuilding scallop population is one of the Northwest Atlantic’s few fishery success stories.

On a map, the closed areas don’t look that big. But to a fisherman, they are huge. One scalloper might spend half the season in one area the size of those that are closed.

The nearshore closures are scheduled to sunset in May 2012. The question on everyone’s mind is, did the closures work? And should they be re-opened?

The answer appears to be yes, at least in some places.

Surveys of closed areas have shown increased numbers of harvestable scallops (those larger than four inches across), for example, from 8,000 to 45,000 pounds in Whiting and Dennys bays. The DMR is surveying the remaining closed areas this month, with assistance from fishermen. Its findings will influence whether and how the areas are reopened to fishing.

Closures work in part because of the nature of the beast: scallops grow fast, tend to live a long time, and don’t move around much once they’ve settled down and grown up.

Intuitively, this reasoning makes sense: close areas to fishing, fish come back. But more important, for both the managers and the fishermen, and the question that is not so easy to answer, is why. Where are the new scallops coming from? Are new scallops the result of spawning, fertilization, and settlement within the closed areas, or are the new colonizers drifting in to the protected habitat from elsewhere?

No one knows whether the protected scallop grounds are serving as sources of new scallops or sinks that attract settling larvae. For example, fishermen report that scallops have returned to the area adjacent to the inner Blue Hill Bay closure, suggesting that scallops in the closed area could be sources of new juveniles. But this effect is not consistent: the area outside of the Gouldsboro closure is still depleted. Heavy fishing in deeper water offshore may have wiped out the spawning stock that supplies the closed area. The question is, how are different scallop grounds related to each other?

Dr. Paul Rawson of the University of Maine and Dr. Erin Owen, now at Husson University, looked for genetic evidence of such connectivity. They sampled scallops from Georges Bank, and Casco, Penobscot, Gouldsboro and Cobscook bays to determine the number of separate gene pools (genetic populations) represented. Their results suggest that scallop larvae don’t all travel the same route. “Gouldsboro is very unique genetically,” Dr. Owen explained, “which means its source of young scallops is different than Cobscook and western Maine, although we still don’t know the source.”

Cobscook Bay scallops were also unique, but had more similarities with western Maine scallops. In Cobscook, the researchers sampled over multiple years, and each year showed a different signature, suggesting that scallops came from a different source each year, and that local oceanography such as currents and tidal mixing might play a role.

Dr. Rick Wahle, a marine ecologist at the University of Maine, recently began a study of scallop reproduction based on his previous research with sea urchins. Scallops, like urchins, are broadcast spawners: males and females release sperm and eggs into the water where, under the right conditions, they mix and eggs become fertilized and turn into drifting larvae. Wahle is evaluating whether scallop density on the sea floor affects spawning effectiveness.

Laboratory and field experiments with urchins found that urchins present in low densities never seemed to spawn effectively. In denser aggregations where spawning was successful, said Wahle, some signaling between the animals leads to coordinated pulses of spawning that increase the overall fertilization. The new research on scallops, which includes establishing experimental plots in the Damariscotta River, will evaluate whether the density of scallops in closed areas is important for repopulating surrounding areas.

Identifying where scallops in rebuilding beds are coming from is vital information for refining the design of closures. If done in the right area, a small closure could support a large resource. “If we could find a way to open these areas and still protect spawning stock, that will be ideal,” said Owen.

This article is made possible in part by funds from Maine Sea Grant.

Heather Deese holds a doctorate in oceanography and is the Island Institute’s vice-president of programs. Catherine Schmitt is communications coordinator for Maine Sea Grant.