Early Events Could Forecast Busy Summer Season of Fish Kills for DNREC
On Tuesday, DNREC staff responded to a fish kill involving an estimated 30,000 young menhaden in Dirickson Creek, a tributary of Assawoman Bay near Fenwick Island. The cause was the same as two earlier fish kills in the Inland Bays this year: not enough dissolved oxygen in the water.
These three events could be just the beginning of what promises to be a busy year for fish kills, Fisheries Manager and Fish Kill Coordinator Craig Shirey said today. “Last year we had very, very few fish kills in the Inland Bays, but this year seems to be shaping up very differently,” Shirey said. “We’re not seeing a significant difference in water quality this year. What we are seeing, however, is a bumper crop of menhaden.”
Atlantic menhaden provide a vital link in the food chain of the Inland Bays and adjoining ocean, serving as a protein-rich food source for striped bass, bluefish and a host of other species. Menhaden are rich in oils and are also processed for food supplements. Adults spawn in the ocean in early spring, and their fry swim into the Inland Bays and other estuaries, where they grow large enough to return to the sea at the end of the summer.
Young menhaden travel in huge, dense schools, seeking rich areas of plankton and algae to feed upon – and, Shirey explained, the larger the school, the more dissolved oxygen in the water the fish need. “During sunny summer days, aquatic plants make oxygen. When the sun goes down, they stop making oxygen and begin consuming it. Levels can drop a little or a lot, depending on demand. So if you have a school of thousands of menhaden as well as other aquatic organisms using up the oxygen, nighttime levels can drop too low for their survival. The result is a fish kill,” Shirey explained.
An algae bloom can complicate this equation. Bright sunny days encourage algae growth, contributing to the oxygen supply, and then consuming it at night. “Plus, the algae can die, drop to the bottom and decompose, consuming even more oxygen,” Shirey said.
A number of other factors can also contribute to or cause fish kills, including toxic species of marine microorganisms. However, none of the fish kills that have occurred in the Inland Bays this year appear to have been caused by any toxic species, he added.
Although the Inland Bays rise and fall with the tides, these salty inland waters have only two inlets from the ocean – one at Indian River and one at Ocean City. Those inlets do not generate the flushing movement necessary to circulate the water and evenly distribute the oxygen in it, especially in still water coves, as well as creeks such as Dirickson, Pepper and Blackwater, where this year’s fish kills have occurred, Shirey added.
Fisheries biologists routinely monitor the status of fish and other aquatic species, while environmental laboratory scientists from the DNREC Division of Water Resources keep an ongoing record of water quality in the Inland Bays as well as other waterways throughout the state. In the Inland Bays area, DNREC also works with volunteers in the Citizens Monitoring Program for sampling and other information gathering.
Between 1981 and 2007, DNREC has documented 59 fish kills in the Inland Bays, with only four in May and June, noted Marine Biologist Dr. Robin Tyler, one of the scientists who currently monitor water quality in the area. “What these three early kills mean for the rest of the season is hard to say. However, with the unpredictable weather and early heat wave, the number of citizens complaints we’ve already had about algae and what appears to be a very large class of juvenile menhaden for 2008, I feel we could see more than usual this summer,” Tyler added.
Area residents who observe an unusual number of dead or dying fish are encouraged to report their observations, including an estimate of how many fish are involved and what species if they can tell. Depending on the situation, Fisheries biologists, Fish and Wildlife Enforcement, environmental scientists and/or volunteers will respond to check out a fish kill. To help determine the cause of the event, responders will estimate the number of dead fish, note the species, location and other conditions, and, when necessary, take water samples.
“Fish kills are sometimes a natural occurrence. However, human activities ranging from changes to the shoreline to lawn fertilizer runoff providing extra nutrients in the water can have a bearing on these events. Our job is to monitor them, determine causes and watch for potentially preventable problems such as pollution,” Shirey said.
To report a suspected fish kill, please call 302-739-9914 from 8 a.m. to 4:30 p.m. Monday through Friday and 800-523-3336 weekends and after hours.