Fish kills follow extreme heat in Baltimore’s harbor and across Maryland

Atlantic menhaden have turned up dead in Fells Point. Golden redhorse suckers washed up along a miles-long stretch of the Potomac River. And this week, in an Ocean City canal, hundreds of horseshoe crabs floated to the surface.

Across Maryland, thousands of fish have died in recent days, leaving a rotting stench and a reminder of the fragile ecosystem in many of the state’s waterways.

The causes likely differ from place to place, but the events all came on the heels of a heat wave that drove temperatures over 100 degrees during the Fourth of July weekend.

The Maryland Department of the Environment reported Wednesday it was investigating a fish kill affecting areas from the Canton waterfront to South Ann Street in Fells Point and across the harbor near the Domino Sugar refinery. The smell of dead fish lingered in Fells Point on Wednesday evening amid the joggers and people mingling at an outdoor café.

The department estimates several thousand menhaden — a small, silvery fish critical as food for larger Chesapeake Bay species — 126 blue crabs and a small number of other species are among the dead.

Theories differ on what caused the event in Baltimore, but experts agree a combination of weather swings and underlying issues such as drought probably threw the harbor’s delicate ecosystem out of whack, depleting the dissolved oxygen that fish and crabs need.

Chris Luckett, who heads fish kill and algal bloom investigations for MDE, said heat stress drove a larger incident in the Potomac, where a new agency report found more than 21,000 dead fish.

Low flows driven by recent drought conditions combined with blistering water temperatures as high as 96 degrees, Luckett said, stressing fish and leaving them susceptible to underlying parasites and bacterial infections.

The majority of fish killed in the Potomac were golden redhorse suckers, but some flathead catfish and other species died, too.

Meanwhile, in the brackish waters of Assawoman Bay in Ocean City, officials this week reported that more than 2,000 dead horseshoe crabs floated to the surface of the town canal, according to the Eastern Shore news station WBOC.

Horseshoe crabs spawn during full moons, a stressful process that often kills them — probably what happened in this case, according to Luckett. The site of this week’s floating crabs, the 94th Street Canal, sees mass horseshoe crab mortality on an annual basis, but Luckett said this week’s deaths were the highest he’s seen there.

Fish kills occur routinely in Baltimore. The onset of cool weather in late summer and early fall brought low-oxygen water from the harbor’s depths to the surface and triggered the deaths of hundreds of thousands of fish.

Two incidents last August wiped out more than 180,000 menhaden, the Maryland Department of the Environment said at the time, and the agency estimated another fish kill the next month killed 25,000. Those kills coincided with an extensive bloom of green sulfur bacteria — a phenomenon known locally as a “pistachio tide” — that turned the harbor a teal green and smothered waterfront neighborhoods in a rotten egg stench for weeks.

That sulfuric smell returned this week.

Although the harbor didn’t turn that intense hue of green, Alice Volpitta, a water quality watchdog with the environmental advocacy group Blue Water Baltimore, said the harbor seems to have experienced another pistachio tide.

Lower overnight temperatures last weekend likely caused low-oxygen waters at the bottom of the harbor to rise to the surface, Volpitta said, bringing a sulfur bacteria bloom with them. That bacteria thrives in low-oxygen waters, which suffocates fish.

Volpitta emphasized that decades of pollution have exacerbated the problem, creating a dead zone at the bottom of the harbor ripe for bacteria and capable of wiping out thousands of fish under the right conditions.

Allison Blood, senior manager of environmental projects at Waterfront Partnership, a downtown booster group that has advocated for swimming in the harbor, agreed that something similar to a pistachio tide played a part in the incident. She said stratification, a stark division between the waters’ surface and depths likely drove the problem.

Blood said extreme heat probably exacerbated a divide between surface and deeper waters, creating an increasingly small zone at the top of the harbor that could support life. That left the ecosystem vulnerable, and a bout of heavy rain seems to have provided a trigger, she said.

What’s clear to Volpitta and Blood is that changes in weather can quickly upset the balance needed for life to flourish in the harbor. After the events of last year, Blood and other harbor experts are watching closely for more shocks to the system this year.

“That definitely is nerve-racking,” she said.

Courtney Knight contributed to this report.