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Environment: Fish gotta swim, and breathe, too

By Charles Swenson
Coastal Observer

They were in the wrong place at the wrong time. And the result wasn’t pretty. Unless you were a hungry gull.

Dead menhaden in the tens of thousands washed up along the shore at Pawleys Island and DeBordieu this week. It put a sudden damper on the trek of beachgoers to the island’s south end to enjoy the unseasonably warm weather.

The fish died from lack of oxygen, but how that occurred is open to conjecture. “It’s some combination of temperature and oxygen shock,” said Dennis Allen, director of USC’s Baruch Marine Lab. “Just what the circumstance were is subject to a lot of guessing.”

Tests of the water by scientists from the marine lab ruled out an algae bloom or a red-tide type event, said Mel Bell, director of the state Office of Fisheries Management.

“We were scratching our heads trying to figure out what’s going on,” he said.

Menhaden travel in dense schools in search of phytoplankton. All the dead fish were 6 to 8 inches long, making them about 2 to 3 years old. The species has an important role in the ecosystem by providing food for larger fish and dolphins.

There have been fish kills associated with cold snaps, but that clearly wasn’t a factor this week, Bell said.

There was a new moon last week with an extreme tidal range. Bell believes the fish found themselves trapped in a depression when the water level dropped and couldn’t get out. They used up the oxygen dissolved in the water.

“They were just traveling along the coast doing what menhaden do and got in the wrong place,” Bell said.

He suggested North Inlet as a likely spot.

But Allen said there is no sign that the event occurred at the inlet, which is the site of much of the lab’s research. “That would be reasonable if there were large pools on the beach,” he said of Bell’s hypothesis.

Allen suggested the school of menhaden may have followed the phytoplankton from inshore waters to the surf zone. The difference in water temperature may have been as much as 10 degrees Fahrenheit. That’s enough to send the fish into shock, either coming or going, he said.

When the fish begin to die, there is an increase in bacteria as they slough off slime, Allen said. The bacteria would further deplete the oxygen in the close-packed school. “It’s one of those perfect storm sorts of thing,” he said.

Another factor was the calm weather, which meant that there was little mixing of the water to offset the difference in temperatures.

“We’ve seen these kinds of kills several times over the decades,” Allen said. “There’s never been a clear smoking gun. It’s always been guesswork.”

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