State officials and the local Riverkeeper believe the kill was due to low oxygen related to an algae bloom in the creek on the south side of the Mayo Peninsula.

"This whole creek is one big kill zone," West/Rhode Riverkeeper Chris Trumbauer said yesterday after kayaking around the creek to examine the situation.

Dead fish piled up along both shores of the creek from Whitemarsh Creek to the headwaters by Holly Hill Harbor's community beach.

The Maryland Department of Environment, which had a man on the scene quickly to take water samples, count the dead, and determine a cause, initially estimated 40,000 fish were killed.

Nick Kaltenbach, MDE's natural resources planner, read oxygen levels below two parts per million at the bottom of the creek several hours after the kill likely occurred just before dawn. Fish generally need five parts of oxygen to survive.

"It looks like some sort of algal bloom played a part in this," he said. "I think we'll find some plankton bloom in the sample."

But definitive tests won't be ready for another day or so.

The kill was called in by alert residents Tom Lyons and Elena Maldonado, who found the fish covering the shore of their Overhill Drive home.

"I saw all these dead fish and it's really freaking me out," said Ms. Maldonado, who has lived there for about 12 years. "I've never seen this before. It's so distressing."

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Mr. Lyons said he had seen kills like this in the Long Island area near his native New York City, most of those attributable to larger fish chasing schools of smaller fish into a space so small they use up the available oxygen.

"But that didn't happen here ... It's like a free buffet," he said looking at the dead fish bobbing among his shoreline grasses.

Within a couple of hours, as local waters warmed up from yesterday's abundant sunshine, Mr. Trumbauer noticed a red tint to the water.

"Look there how red it is," he said. "I think its definitely algae related."

He also is confident it was a low oxygen event because all of the dead fish were young menhaden, about 3-inches long. "Menhaden are the most susceptible to oxygen depletion," he said.

Later, after he and Mr. Kaltenbach returned from a boat ride up and down the creek, courtesy of resident Bill Galebach, he said they found young menhaden piled along both shores of the creek and patches of a mahogany colored algae in the main channel of the creek.

The opposite shore from where the fish were initially spotted is the undisturbed shoreline of YMCA Camp Letts that forms the entire western shore of Bear Neck Creek. That pristine wooded peninsula helps filter runoff into the creek.

The opposite shore is almost completely developed. Dead fish were found up by the Holly Hill Harbor community beach, and bobbing 10-deep along the shoreline between and under docks.

Once the wind shifted a little later some of those fish began to float out into the main area of the creek, others sank - providing snacks for crabs.

Mr. Trumbauer estimated the kill was much larger than the one that struck South Creek of the West River earlier this month. There only about 5,000 fish died, caught in a little gut off the creek's main stem in Shady Side.

The first week of the month saw kills on the South River and in South Creek.

Later that week, two die-offs happened in creeks of the Magothy River.

All of those paled in comparison to the 100,000 fish killed on the South River's Aberdeen Creek on Sept. 1.

Yesterday's total brings to roughly 180,000 the fish killed around the Chesapeake Bay this summer. Last year about 307,000 fish succumbed in similar circumstances.

The majority of the kills this year have occurred in Anne Arundel waters.

MDE officials partially attributed that to the heavy shoreline development in the county and nutrient-laden storm runoff that helps feed the algae.

MDE environmental Program Manager Charles Poukish said another factor is the habit of menhaden schools to swim into upper reaches of area creeks, further depleting already low oxygen levels.

There was but one positive thing about the unfortunate incident, Mr. Trumbauer said. "Witnessing a kill like this is an opportunity for people to think about their impact on local waterways."

Curbing nutrient pollution and runoff from roofs, driveways, lawns, pet waste is one way to help prevent these events, he said.