Here’s a statistic, which is a best-guess estimate, but nonetheless difficult to wrap one’s head around: On Tuesday, Oct. 6, there were 5.8 million pounds of menhaden swimming in the bay.

That isn’t too hard to imagine if you looked down from the Pawtuxet River Bridge the following day, or actually, as it turns out, just about any time in the last two weeks. What you would have seen are thousands upon thousands of fish. Most of them were juvenile menhaden averaging about three inches long. Mixed in the schools – so thick that they looked like carpets – were a few adults of about 11 inches. And then there were the predators – the cormorants, sea gulls, terns and bluefish and stripers – gorging themselves.

“The menhaden population has absolutely exploded this year,” says Christopher Deacutis.

Deacutis, supervisor of environmental science for the Department of Environmental Management, said large schools of the fish have been seen up and down the eastern seaboard, and there have been reports of humpback whales feeding off them last month in Long Island Sound. The whales haven’t been reported in Rhode Island waters, but Deacutis suspects the menhaden are the reason why schools of common dolphins have been spotted at the mouth of Narragansett Bay.

Pawtuxet Cove isn’t the only place the fish are plentiful. Large schools are being reported in coves throughout the bay, as well as in deeper waters. DEM contracts for “spotter planes” that are used to estimate the biomass of schools of menhaden. Those estimates have been arrived at through years of working with purse seiners, explains Nicole Lengyel, principal biologist with RIDEM Fish and Wildlife. The weight of a catch is compared to the size of a school, which is then used to estimate the biomass of other schools.

Lengyel said the state usually gets a “spring pulse” of adult menhaden that tapers off in June as they migrate north. There’s usually a second late summer or fall pulse of juvenile fish migrating south in late August and into September. This summer, more menhaden stayed in the bay and the fall pulse picked up at the end of August, running through most of September.

She said estimating the biomass of juvenile menhaden is difficult, as juveniles are not harvested. Estimates for last October put the biomass at about 500,000 pounds, or less than a tenth of what it was last Tuesday.

Lengyel is careful about making comparisons, observing that schools can move quickly, and while the numbers are high one day they can drop dramatically by the next. She’s even more cautious about making any conclusions as to why this year is extraordinary.

The juveniles grow rapidly, nearly doubling length from one and a half inches when first seen migrating into the bay in August to three inches now. Menhaden feed on phytoplankton and zooplankton. They can grow up to 15 inches and have a lifespan of 10 to 12 years, according to NOAA. The fish spawn from March to May and again in September and October. Most menhaden migrate south to the North Carolina capes during the fall and early winter.

The fish are commercially harvested for their oil and for making meal. Lengyel explains regulations are designed to maintain a healthy stock of the fish. Certain areas of the bay are permanently off limits to purse seining, including Greenwich Bay and north of Conimicut Point or the Providence River. She said a biomass of two million pounds must be recorded before seining of the fish is permitted, and once that drops to 1.5 million pounds the fishery is closed. Also, no more than 50 percent of a biomass can be harvested at a time, provided that total doesn’t deplete stocks to 1.5 million pounds or less.

Deacutis was not surprised to hear that there are a few dead menhaden mixed in the schools at the base of the Pawtuxet falls.

“They’re burning all their energy swimming against the current. There’s a high rate of mortality,” he said.

Fish kills are more frequently tied to depleted oxygen levels. In this case, Deacutis said, bay oxygen levels and those at the mouth of the Pawtuxet are well above what would be unsustainable for menhaden. He speculated predators could be a reason for the volume of fish at the bridge and in Pawtuxet Cove as they are taking refuge from bluefish and stripers in the bay.

The sheer number of fish has left an impression on Deacutis, who has worked for the department for more than 30 years.

“I’ve never seen the numbers of this summer,” he said.

That also was the conclusion of area residents looking at the swirling fish Monday morning from the bridge. And confirming that the fish are easy pickings, a cloud of sea gulls was on the wing above the clear waters.