Scientists and government agencies throughout the Northeast are investigating a widespread die-off of an ecologically important fish species known as menhaden, or bunker, that has left the bottoms of local bays and creeks littered with their small silvery corpses this spring.
Scientists say that the usual causes of bunker die-offs — lack of dissolved oxygen in water, or blooms of toxic algae — do not appear to be at fault in the most recent mortality.
Instead, they now suspect that the cause could be a bacterial infection spreading among the fish.
The mortality is not on the scale of some bunker die-offs, which occur fairly regularly and can leave the shorelines of creeks and bays carpeted with dead fish. When several million bunker died in the Shinnecock Canal in November 2016, the raft of dead fish left the shorelines of eastern Shinnecock Bay as much as 3 feet deep in rotting carcasses.
While those events are usually a single, localized mass mortality, typically caused by too many fish swimming into a shallow area and depleting the water of oxygen, the current die-off is affecting fewer fish in each location but is far more widespread.
The bunker deaths were first noticed on western Long Island, in Connecticut and in New Jersey last fall and have surged again this spring, particularly on the East End, as millions of bunker arrived in local waters last month.
Before they die, the afflicted fish can often be seen swimming erratically on the surface, twirling spastically in the water — different than the steady circular swimming motion that schools of bunker exhibit — or beaching themselves.
One state scientist described the scene witnessed in a Long Island bay recently.
“The most notable thing I saw was one fish that had bulging eyes,” he wrote in an email to other researchers. “As the water washed over this fish and it tried to breathe, a little blood was coming out of the gills with each gulp. It didn’t show any signs of external injury.”
Samples taken from dead fish by the New York Department of Environmental Conservation and New Jersey Department of Environmental Protection have shown no signs of a viral infection that has been known to afflict fish and result in similar behaviors among the dying, the DEC has said in its review of the issue.
And scientists from Stony Brook University Marine Science Laboratory have taken water samples from the local creeks where some of the fish were seen dead and have found no signs of depleted oxygen or algae blooms that could’ve killed the fish.
“We did see a potential bloom in the Peconic River sample the previous week, but we looked at five samples this week there wasn’t much of anything there,” said Craig Young, a marine biologist at the marine center. “The dissolved oxygen levels were a little low, but not dangerously low.”
The state says it is pressing on the research into the bacteria that has been seen in many of the fish.
The bacteria found in some of the dead fish collected by the DEC, one of several species of a genus of marine bacteria known as Vibrio, occurs naturally and is not believed to be harmful to humans — though the DEC is still advising people to avoid touching any dead bunker they see.
Several species of Vibrio bacteria, particularly those that are found in shellfish, are known to cause illness in humans who consume raw or undercooked seafood.
The bacteria do not typically harm the fish that are carrying them, however, and scientists are now trying to determine how or if that has changed with the bunker.
“We’ve sent samples to the Stony Brook Marine Animal Disease Lab and they’re doing research on whether this is something new or something we’ve seen before,” said Jim Gilmore, director of the DEC’s Division of Marine Resources. “These bacteria can be cyclical and they’ll be prevalent and then go away for a while and then pop up again.”
Mr. Gilmore said that scientists from the three states where the die-off has been recorded are working together to figure out what the problem is and whether anything can be done to help the fish.
Thus far, there is no evidence that the dead and dying bunker could be infecting other fish or birds that feed on them —potentially threatening the health of other already depleted fish species like striped bass, bluefish and weakfish.
“The simple answer is, we do not know at this point,” he said. “Predatory fish typically eat live fish, and are not picking them up off the bottom and the birds do not seem to be eating them.”
He said it’s possible that crabs would consume the dead fish carcasses when waters warm and said the DEC will be closely monitoring for signs of increased mortality as the season wears on.
It is also possible that the bacteria will wane as waters warm, as some bacteria do.
Bunker are an important prey species for many large fish and marine mammals. Once harvested by the millions of tons each year for their oily flesh that is used in dietary supplements, fish meal and even lubricants, most states have outlawed or greatly curtailed the commercial harvest of bunker since 2012.
As a result, the species’ population has exploded and massive schools of millions of fish now spend each spring, summer and fall in the bays and ocean waters around Long Island. Their resurgence has led to the frequent sitings of lunging and leaping whales, dolphin and sharks off local beaches in summer.
Ed Warner Jr., a commercial bayman and Southampton Town Trustee, said that the bunker schools in Shinnecock Bay had led to a spectacular natural display earlier this spring when thousands of gannets were “dive-bombing” on the bunker. He also said the population of cormorants, another fish-eating bird, in the bay this year is the largest he’s ever seen, presumably because of the huge numbers of bunker.
But now, he said, the bay is dusted with fish carcasses that nothing wants to eat.
“It’s definitely progressed, there’s more and more dying — I would say tens of thousands of pounds every day,” he said. “Everywhere you look, and my friends up in Great South Bay say it’s the same. All you see is silver on the bottom.”