MONTEREY (CBS SF) — Some anchovies died off at the Monterey Marina Thursday and city officials are looking for a cause.

The die-off occurred after a large school of anchovy entered the marina.

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The city’s harbor and marina department turned on an aeration system that helps pump more oxygen into the water, but several hours later some dead anchovies were seen on the surface of the water and on the bottom, according to city officials.

Harbor staff immediately began cleaning up the dead fish, city officials said. The dead fish should not cause any odor in the area or impede recreational activities and boating, city officials said.

The Monterey Marina installed the 13 aerators after a mass die-off of sardines in 1996, city officials said.

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“We are still seeing a large biomass of anchovy alive and swimming around the marina, which is a good sign because it shows that the aeration system is helping,” Harbormaster Steve Scheiblauer said in a statement.

Although the cause of the anchovy die-off is unknown, the harbor’s staff speculates the cause to be disease or stress induced by unusually warm water and changing ocean conditions on the west coast, according to city officials.

A record-breaking algal bloom, which stretches across the North Pacific Ocean, from the Aleutian Islands to southern California, coincides with well-above average sea-surface temperatures throughout the West Coast.  The bloom is also laced with some toxic algae that may have caused some whale deaths in Alaska in June and closed some anchovy and sardine fisheries in California, according to city officials.

Monterey harbor staff are working closely with marine scientists and investigating the incident, city officials said.

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