Sixth Year of Die-Offs Hit Bay Scallops; Genetic Diversity Could Be Factor

The biologists doing surveys of local bay bottoms ahead of the annual bay scallop harvest on the East End next month have once again found widespread die-offs of the adult bay scallops throughout the Peconic Estuary and its tributaries, and few signs of the young-of-the-year scallops that would give hope the species might next year break its now six-year losing streak.

But the researchers say there are signs that Mother Nature is showing some resiliency to the biological blows that have driven the die-offs, giving them hope that their own research and scallop seeding work may be able to again restore the once-robust stocks and valuable annual harvest.

“I’ve found two live adult scallops,” Harrison Tobi, the head of the scallop research team at the Cornell Cooperative Extension Marine Program, a Suffolk County-funded research and education center dedicated to restoring eelgrass beds and shellfish stocks throughout the East End’s waters, said last week. “So, it’s not good.”

Up to that point, Tobi had gone through about three-quarters of the 21 survey sites scattered throughout the Peconics, Gardiners Bay and Shinnecock Bay that the program is using to track the mortality and survivability rates of both the wild scallops and the various broods of bay scallops raised in their hatchery, in hopes of finding the right genetic combinations to rear scallops that are more resilient for the future. Ultimately, a total of 10 live scallops combined were found.

Over the last few years, researchers from Cornell and Stony Brook University, working in cooperation, have been looking for genetic avenues to survival — selecting brood stock for the seeding program from among scallops that seemed to survive the onslaught of stresses better.

They’ve tried releasing their hatchery stock later in the year, so that they spend less time exposed to a parasite that infects nearly all scallops in the Northeast before going through the stress of their first spawn and the heating of the water around them in summer. They’ve brought in scallops from Martha’s Vineyard, where the wild population hasn’t seen the same die-offs as the Peconics, and they’ve used the adult scallops that do survive when more than 95 percent of their brethren die in the mud next to them.

But the latest thinking may be that at least part of the problem may be the scallop seeding program’s own past success.

The Peconic Bay stock of bay scallops, always the region’s largest, may be suffering from a lack of genetic diversity that makes it more vulnerable to deleterious impacts of the parasite and warming waters in the bay — in part because nearly the entire stock is descended from the same small brood used to start the seeding program decades ago.

“That loss of genetic diversity over time is the result of a small founder population,” Tobi said, while piloting a small skiff with two CCE employees and an observer aboard through the narrow channel from Cedar Beach Creek in Southold and into the neck of Little Peconic Bay. “Basically, we started a massive population from a small population. And we kept taking scallops from the wild for brood stock. But, in reality, it was still related to that founder population, so you had too small of a genetic population.”

After the local bay scallops stock was nearly wiped out by the infamous “brown tide” algae blooms in the 1980s and 1990s, the biologists at the Southold hatchery set out to rebuild the stock. They used surviving wild scallops spawned in captivity to create millions of “spat,” as the larval scallops are known, raising them in captivity and then seeding them into the bays — and they repeated that process over and over, year after year after year, ultimately releasing more than 10 million scallops into the wild.

After some 20 years of some successes and many setbacks, their efforts finally seemed to be paying off with a large population that had regenerated itself several years in a row — bay scallops only live for about 18 months — and harvest numbers that had steadily ticked upward through the end of the 2018 season.

Then, in the summer 2019, the huge number of adults that had littered the bay bottoms and boosted hopes for a banner harvest that would finally be within site of the volumes before the brown tide years all suddenly died. Literally, almost all of them.

More than 95 percent of the adult scallops in the bay died that summer, and each summer since. The population has persevered because the mass mortalities have only set in after the scallops spawned in the spring — the stress of the spawning itself seen as a potential contributing factor to the die-offs.

Scientists from around the region have settled on there not being one single factor but several — topped by the presence of the parasite and local water temperatures in summer now far above what they used to be. While brown tides have not been seen locally in many years, new algae blooms have also contributed to the stresses on the scallops, the scientists have said, and the suite of impacts appears to have finally just gotten to be too much for the scallops.

The hypothesis that perhaps the homogeneity of the Peconic population could be the reason for the massive die-offs has been bolstered by the sudden emergence of a newly resurgent population of bay scallops in eastern Moriches Bay, an area that had not seen a significant “set” of scallops in more than 20 years.

When analyzed in the lab, the Moriches Bay scallops were found to be substantially genetically different from the scallops in the Peconic, the biologists say, and weathered the summer die-offs that decimated the scallops in the Peconics despite being exposed to many of the same stresses.

Last week, Tobi donned a wetsuit and scuba tank and flung himself into the waters of Flanders Bay, above one of his survey sites. He carried mesh bags and a tape measure.

He soon reemerged, and handed a rope to Brooke Del Prete and Michael Bunn, the CCE staffers assisting him. The pair hefted a dozen satchels made of black plastic mesh into the skiff and began sorting them by the letter displayed on a tag. Each held a small collection of scallops with different lineage or birthday.

While they did this, Tobi returned to the bottom, with a tape measure, and began swimming survey laps — out 50 meters and back, in three directions, picking up whatever scallops and other crustaceans and marine creatures he came across as he went. As had been the case at most of his survey sites this fall, he found few living adult bay scallops.

When he was back in the boat, the team meticulously recorded the findings from each of the bags retrieved from the bottom — how many scallops from inside each bag were alive and how many were dead. The work is the precursor to the serious test of the “genetic bottle neck” hypothesis will get next summer — all part of the effort to speed the natural selection amid the scallop population toward a more resilient stock.

“Next year, we’ll have more comparative field trials with the Moriches, Peconic, Martha’s Vineyard and Maine scallops,” Tobi said. “The idea is just to get as much as we can out there. We’re trying everything.”