Maryland Fish Kills

In Maryland, a wide range of environmental factors may be related to fish kills.

In May 2009, a fish kill of smallmouth bass and sunfish was reported in the upper Monocacy River. The event was similar to other springtime fish kills in the Potomac River watershed that have occurred since 2002.

Investigations of springtime fish kills in the watershed found a variety of possible factors that might have been involved. Possible causes for fish deaths were listed as: contaminants, damaged skin, gills and internal organs, parasites, and spawning stress. Biologists also discovered a high prevalence of intersex in some species, most notably smallmouth bass.

In July, 2010, a fish kill on the Severn River included bay anchovy, Atlantic menhaden, Atlantic needlefish, and white perch. Staff from the Maryland Department of the Environment (MDE) investigated the kill and estimated the number of dead fish to be 1000 or more.

MDE reported that a heavy algae bloom was present at the time of the kill. The investigation concluded that the cause of the fish kill was low dissolved oxygen induced by the large bloom of non-toxic algae.


In July, 2010, Maryland's Department of the Environment and the Department of Natural Resources investigated a large fish kill in Deep Creek Lake.

The 2010 Deep Creek Lake fish kill involved hundreds of dead fish and invertebrates including yellow perch, walleye, smallmouth bass, brown bullhead, largemouth bass, bluegills, chain pickerel, Northern pike, and crayfish.

Research indicated that unusually high temperatures in the lake's upper water column set the stage for a parasite protozoan infestation. According to scientists, cold water species of fish were primarily affected by the parasite infestation.

In late December, 2010 and January 2011, a fish kill in the Chesapeake Bay involved roughly two million fish. The fish were almost exclusively small (3-6 inch) spot fish. The fish kill occurred from the Bay Bridge to Poplar Island.

The Maryland Department of the Environment investigated the kill and found that cold water stress, combined with a large population of fish was the most likely cause of the kill.

Winter fish kills involving spot have occurred at least twice before in Maryland. In late January 1976, records show that about 15 million spot died of winter stress in the bay. A smaller number of spot deaths were recorded in January 1980

Fish kills also occur in Maryland coastal bays, most often in dead-end canals. Due to a lack of flushing action in canals, algae blooms can lead to hypoxic or anoxic conditions. Fish kills can occur in dead-end canals when schools of fish enter and become trapped in lethal water conditions. During the summer of 2001, approximately 3,000,000 Atlantic silversides entered a canal in West Ocean City and apparently became entrapped during low tide overnight. The fish became concentrated by low water, exhausted all available oxygen, and died.

Maryland residents can report fish kills and algae blooms at 1-866-MDE-GOTO or 877-224-RBAY. Reports of dead or dying fish in the Monocacy or Potomac River can be reported by contacting the Maryland Safety and Environmental Hotline (877) 224-7229 or the DNR-Fisheries Service at (301) 898-5443 with the time, date, location, fish species, and approximate number of affected fish. Photographs and a description of any unusual behavior or water conditions are also helpful.