Thousands of dead fish washed up on a mile and a half stretch of beach in South Carolina Tuesday, officials said, at least the second such occurrence in the region in a week.

Roughly 30,000 to 40,000 menhaden fish, 6 to 8 inches long, were spread along the shore from DeBordieu Beach in Georgetown County, S.C., to Pawleys Island, a town on the state's Atlantic Coast, and thousands more were expected, Pawleys Island Police Chief Michael Fanning said.

Similar incidents have happened in the area before, including late last week when hundreds of thousands of the small, oily fish were washed ashore near Masonboro Island, N.C., and last year when an influx of dead starfish were found on the same beaches.

The fish were first spotted by beachgoers taking advantage of the unseasonably warm weather.

"We came down to the beach for the day just to have, you know, a nice day on the beach, smell the fish smell, came down to look for shells and all these fish -- dead," Pawleys Island resident Pat Hawkins told NBC station WMBF in Myrtle Beach, S.C. "It's a shame. I don't know what's causing it."

Officials from the Department of Health and Environmental Control and the Department of Natural Resources visited the area Tuesday and took water samples in an effort to determine what killed the fish.

Marine experts determined the fish died from hypoxia, which occurs when the amount of oxygen in the water drops.

Pawleys Island Police

Thousands of dead fish washed up on a Pawleys Island, S.C., beach Tuesday afternoon.

Mel Bell, director of the Office of Fisheries Management for the South Carolina Department of Natural Resources, said the occurrence was an entirely natural event.

“On Friday we had a new moon (which caused) real high high tides and real low low tides,” Bell told The Sun News. “Probably what happened was a school (of menhaden) got in an area of water on a high tide, in a hole or depression, and at low tide they were trapped and depleted the oxygen in the water. Then, all the fish would suffocate. Then, when the tide came back in, it washed the dead fish out and they washed up on the beach.”

"When it's one species like that, that's usually indicative of a low dissolved oxygen situation because they tend to be more fragile," added Dan Hitchcock, an assistant professor at the Baruch Institute of Coastal Ecology and Forest Science at Clemson University in Georgetown, S.C.

Fanning said the city has no plans to clean up the fish and will let the seagulls and the tide clear the sand. 

“We’re just dealing with it as a force of nature," Fanning said. "There are some residual fish, most of it has gotten washed away, there were a ton of birds down there. If you went down there (Thursday), you’d get more birds than fish.”  

Menhaden fish, typically used by fishermen as bait, are a small, silver fish, whose oil is used in vitamin supplements, lipstick and livestock feed. 

When Haans Galassi lost four of his fingers in a wakeboarding accident in Idaho, he joked, wryly, that they had become fish food.

He wasn’t surprised, then, when the Bonner County sheriff called him up, telling him he had a funny story to tell him.

“I was like: Let me guess, they found my fingers in a fish,” Galassi, 31, told the Spokesman-Review. (The newspaper’s headline was equally flippant: "Lake trout gives fisherman the finger.")

Galassi, of nearby Spokane, Wash., was wakeboarding on Priest Lake in northern Idaho on July 4 when his hand was caught in the cord, fully severing his fingers. His friends rushed him to a resort and he was transported to the hospital by helicopter.

Then, on Sept. 11, a man named Nolan Calvin was fishing on the west side of Priest Lake when he caught a large lake trout. As he was cleaning the fish, he found what appeared to be a severed human finger, according to a press release from the Bonner County Sheriff’s Office.

According to the sheriff’s department statement: “It is unknown, of course, how long the fish actually retained the finger in its digestive track, however, it was noted that the finger was in remarkably good condition at the time of recovery.”

Sheriff’s Detective Sgt. Gary Johnston was particularly impressed.

“You fall asleep in your bathtub or hot tub, you come out and your fingers are all puckered up and prune-like. And it wasn’t like that,” he told the Spokesman-Review.

Calvin, the fisherman, placed the finger on ice, which he turned over to deputies, who were able to derive a decent enough fingerprint to trace back to Galassi. The fish, officials concluded, had traveled about eight miles north of where Galassi lost his fingers.

The investigation concluded, the sheriff’s department offered Galassi back his finger.

Galassi told the Spokesman-Review that he declined.

“I’m like, uhhh, I’m good,” he said.

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J. Miles Cary / Knoxville News Sentinel via AP

Crews using rakes and an industrial vacuum remove dead fish from Butterfly Lake in Knoxville, Tenn., on Monday. Some 10,000 bluegills died.

In lakes and rivers across parched areas of the U.S., heat and lower water levels are reducing oxygen levels -- and killing fish populations by the thousands. 

At one lake in Delaware, up to 6,000 dead gizzard shad and 600 perch were found floating this week.

"Aggravating this summertime problem, increased temperatures lead to warmer water, which holds less dissolved oxygen," state natural resources spokesman John Clark told NBCPhiladelphia.com.

In South Carolina, some 500 fish died at Lake Hartwell.

Slideshow: Summertime living

Celebrating the warm summer months, as schools let out and the cooling off begins

Launch slideshow

"It started Sunday afternoon," local resident Brandi Pierce told NBC affiliate WYFF-TV. "We started seeing ten fish popping up out of the water. Then Monday, it was full." 

Across South Dakota, fishermen have reported thousands of fish kills in multiple lakes and rivers.

And in Tennessee, a fish kill on Butterfly Lake left a horrid stench in one Knoxville neighborhood.

"It's really putrid," Paula Gumpman, president of the local neighborhood association, told the Knoxville News Sentinel. "It's like after a hurricane. Gooky and yucky."

Some 10,000 bluegills were thought to have died, and city workers were tasked with the cleanup even though the lake is on private property.

"It's a public health issue," said Public Service Director David Brace, "and it just smells real bad." 

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Army Corps of Engineers

A sea lion feasts on a salmon along the Columbia River.

Two Pacific Northwest sea lions found shot dead this week, along with at least 18 others in the last two months, make up what one expert calls an "unprecedented" attack on marine mammals protected by federal law.

For years, sea lions along the Columbia River between Oregon and Washington have been gorging on salmon, and experts suspect angry fishermen have retaliated by killing some.

"Unfortunately and not unexpectedly, these deaths coincide with the spring run of salmon," Robin Lindsey, a member of the Seal Sitters Marine Mammal Stranding Network, told msnbc.com.

A few killings are typically seen at the start of each season along the Columbia River, but this year is running about four times the norm, Dalin D'Allesandro, a Portland State University research assistant, told The Oregonian. 

"Since the beginning of April, we've seen a surge in both Steller and California sea lions being shot," said D'Allesandro, who helps the local marine mammal stranding network.

The increase also comes after federal officials in March authorized local authorities to cull up to 92 California sea lions a year in waters just below the Columbia River's Bonneville Dam. Biologists estimate sea lions have eaten between 1.5 and 4 percent of returning salmon.

Related: 8 sea lions found shot to death near Seattle

Jim Rice, coordinator for the Oregon Marine Mammal Stranding Network, is among those who suspect that was also a factor in the dramatic increase.

It "may encourage some people to shoot sea lions" thinking they've got a green light as well, he told msnbc.com.

"Marine mammals have been protected since the early '70s," Rice earlier told The Oregonian. "Prior to that there were actually bounties on the animals and they were hunted quite actively. It is certainly unprecedented within that time frame."

As for the number of sea lions killed in recent months, Rice says the number is probably more than the 20 reported. "There are likely others that never came ashore or have not yet been reported to stranding network responders," he noted.

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