PORTLAND – Huge schools of small fish have been crowding into coves and onto beaches in southern Maine, prompting fears among some residents that their presence may be caused by unseen pollution.

But experts say the presence of thick swarms of fish is part of a natural – if mysterious – phenomenon.

The movement of immature pogies and herring seems to be part of their migration, which some believe may be moving a little farther north this year because of higher ocean temperatures.

The schools have appeared around September “for probably the past four or five years, almost like clockwork,” said Bruce Joule of the Maine Department of Marine Resources. “And the stock assessment is that people aren’t sure where they are coming from.”

The fish have been so thick around piers in South Portland and Freeport they could be scooped up in nets.

“There was just a ton of them, jumping on top of each other,” said Nadia Chester, a dock hand at Port Harbor Marine in South Portland. “There wasn’t enough room for them.”

In Ogunquit and Scarborough, there have been reports of the dead fish on the water and on the sand above the waterline. For some, those fish are bringing back unpleasant memories of the late 1980s and early 1990s, when adult pogies filled coves and rivers, died and left a big stink as they decomposed.

Those adult pogies got so crowded in some coves and rivers that they used up all the oxygen and died, leaving thick layers of rotting fish on the water. In 1990, the stench drove away tourists and forced coastal residents indoors behind closed windows.

While there have been reports of dead fish this year, the numbers don’t approach those from the earlier large-scale incidents, Joule said.

This year, an abundance of predators rather than a lack of oxygen cause many of those deaths.

Schools of mackerel have been chasing the smaller fish while striped bass and bluefish have been chasing the mackerel. The big fish herd the smaller fish into coves and sometimes push them right onto shore.

Seals and birds sometimes join in the feeding frenzy.

When the fish crowded into the cove at Willard Beach in South Portland last week, residents called state officials and the Friends of Casco Bay.

“There were a bunch of them floating up on the beach and people were calling to see if there were [pollution] issues,” said Nadia McLennan of the Friends of Casco Bay.

But a water sample showed that the water was fine.

“It’s perfectly normal,” she said. “There were just so many fish and the mackerel were feeding,” and that some died.

John Sowles, an ecologist with the state Department of Marine Resources, said other organisms would also be affected if pollution were the cause.

“If it’s a school of the same species and all of similar size, it’s unlikely that it’s chemicals,” he said.