If spring is here, dead fish can't be far behind.

State officials are investigating a wide-ranging fish kill spotted Monday, stretching from Fells Point in Baltimore harbor out into the Chesapeake Bay. Authorities have yet to pinpoint a cause, though warm weather tends to bring a die-off in local waters almost every year.

Dead fish were seen floating off Fells Point in the city and beyond the Patapsco's mouth at Cox's Point in Essex and near Bodkin Point in northern Anne Arundel County, according to Jay Apperson, a spokesman for the Maryland Department of the Environment.

State investigators estimated the kill at "up to 1,000" fish, mainly Atlantic menhaden, said Apperson. But others said the number seemed higher. Educators with the Chesapeake Bay Foundation and the Living Classrooms Foundation also reported seeing dead or dying shad in Baltimore harbor.

"There are fish spread for miles across the river — from the Key Bridge to the Inner Harbor," reported John Tapscott, an educator for the Annapolis-based bay foundation. He was aboard the Snow Goose, the group's workboat, which takes schoolchildren on educational cruises from Baltimore harbor.

James Piper Bond, president and CEO of the Living Classrooms Foundation, said the captains of his educational vessels also reported seeing hundreds of dead menhaden and shad all across the Inner Harbor, though other fish species seemed unaffected.

"I'm sitting here looking over the railing at about 100 dead menhaden," he said from the foundation's headquarters in Fells Point.

Though unusual because it's so early, this fish kill so far pales in comparison to one that smelled up the Patapsco and parts of the bay a little less than two years ago. Officials estimated that as many as 100,000 fish died in that episode in late May 2012. That spring and summer saw 97 fish kills reported statewide, the most recent year for which figures are available.

Though pollution or disease can kill fish directly, most die-offs are blamed on a lack of oxygen in the water, essentially suffocating them. The shortage is often brought on by an algae bloom that uses up all the life-sustaining air in the depths as the algae — tiny aquatic plants — die off and decompose.

Though such fish die-offs are often attributed to "natural" causes, the algae blooms are exacerbated by fertilizer, animal waste and sewage — plant food — being flushed into the water from land by heavy rains.

The state's preliminary investigation has not found evidence linking this die-off to low oxygen levels or algae blooms, said Apperson.

State scientists have seen some algae blooms around the bay already, but breezy, cool weather has generally stirred up the water enough to keep oxygen levels from dropping to dangerous levels for fish, said Bruce D. Michael, resource assessment director for the Maryland Department of Natural Resources.

Educators with the two foundations say they sampled the water after seeing dead fish and found oxygen levels were not too low. A water-quality monitor at Masonville Cove off the Middle Branch of the Patapsco showed similar readings, but it did record a spike Monday in levels of chlorophyll, an indicator of algae growth in the water.

David Flores, the Baltimore Harbor Waterkeeper, said he saw a "pretty extensive algae bloom" a few weeks ago that spread beyond the Inner Harbor.

Others noted that heavy rain nearly two weeks ago turned the waters here and elsewhere around the bay brown with storm runoff, which carried a combination of dirt and those pollutants algae feed upon.

A "pulse" of pollution like that could provide fodder for an algae bloom, said William C. Dennison, a vice president of the University of Maryland Center for Environmental Science. All it would take then would be a few sunny spring days warming the water enough to stimulate aquatic plant growth, he said.

Dennison suggested the strong winds last Wednesday may have stirred up the pollution washed into the water by the recent storm, making it available to algae just as warmer, clear weather arrived at the end of last week.

"That's a recipe for fish kills," he said.

For now, though, all agree there's not enough information to say with any certainty what caused this fish kill.

"Our guys …they're baffled," Bond said of Living Classrooms' staff.

tim.wheeler@baltsun.com