Algae blooms are suspected of causing the deaths of thousands of fish in Inner Harbor

A “pistachio tide” appeared in the inner harbour on the morning of 3 September. It is suspected that a natural algae bloom caused a mass fish kill in the harbour on Wednesday. Photo courtesy of the National Aquarium.

An estimated 24,000 fish, mostly juvenile Atlantic shad, washed up dead in the waters of the Inner Harbor on Wednesday morning, according to the Maryland Department of Environment.

The department said that as part of its investigation into the fish deaths, it found no evidence of pollution that could have caused the fish to die. Rather, it said in a statement Wednesday, water measurements indicated that a lack of oxygen in the harbor likely killed the fish.

The final days of colder weather may cause the harbor’s water to “turn over,” the department said in a statement, bringing the harbor’s deeper, oxygen-starved waters closer to the surface. Fish that can’t escape that zone of low dissolved oxygen will die.

The fish apparently died between Tuesday and Wednesday, when they were found near Piers 5 and 6.

The National Aquarium echoed the department’s statement, saying in a news release Wednesday that the fish kills were likely the result of “pistachio tides” and “mahogany tides” over the past few days. The release said such tides have nothing to do with water quality, but are “naturally occurring seasonal phenomena, often related to temperature, that occur one to four times a year, lasting several days each time, after which water quality improves.”

Video courtesy of the National Aquarium

The aquarium says the pistachio tide (named for its pistachio-green color) occurs during a temperature inversion, when cold temperatures cause surface waters to sink, “pushing sulfur bacteria living on the bottom of the harbor to the surface.” Once exposed to sunlight, the bacteria perform anoxygenic photosynthesis — that is, they photosynthesize without producing oxygen in the process, but actually use oxygen in the water.

“The result is a bloom of pistachio-green algae that looks and smells strange,” while depriving aquatic animals of oxygen.

The aquarium also said the harbor has seen mahogany tides recently. These are brown algae blooms on hot, dry days that feed on nitrogen and phosphorus. The blooms can block sunlight while also causing dissolved oxygen levels to plummet, depriving aquatic animals of the oxygen they need to survive.

Aquarium staff have recently seen more animals than usual taking refuge in the new Harbor Wetlands exhibit, which is equipped with aerators that help oxygenate the water. The animals began appearing in the exhibit after recent pistachio and mahogany tides, the aquarium said in a statement.

The Department of Environment said the estimated 24,000 dead fish in the harbour were almost all Atlantic shad, but several catfish, white perch and a couple of blue crabs were also seen in the fish kill.