Though some local residents are concerned about potentially toxic organisms in the water, last week’s hot, sunny weather is considered a likely culprit in the fish kill that left approximately 30,000 juvenile Atlantic menhaden dead in Dirickson Creek near Old Mill Bridge Road between Sunday, June 15, and Thursday, June 19.
According to Fisheries Manager and Fish Kill Coordinator Craig Shirey of Delaware Department of Natural Resources and Environmental Control’s Division of Fish and Wildlife, “It was probably the bright, hot, sunny days” that led to an algal bloom that resulted in low oxygen levels in the water and eventually killed the fish.
“The weather that week very hot, and it really cranked up water temperatures over a short time,” he said this week, noting that rainfall events also have the ability to influence algae growth by increasing the flow of nutrients into the water.
In the case of Dirickson Creen, Shirey said the fish kill was “probably a result of an algae bloom that consumed most of oxygen in the water at night.”
Like most plants, algae produce oxygen and consume carbon dioxide during the day but consume oxygen and produce carbon dioxide at night, leading to reduced dissolved oxygen levels after nightfall. If the oxygen level in the water drops too far, fish die. The algae itself can complicate the situation if it dies and then falls to the bottom and decomposes, consuming even more oxygen.
Still waters are more prone to this problem, since the inland bays do not have inlets with sufficient flushing power to distribute oxygen more evenly.
The fish kill at Dirickson Creek was just one of three fish kills in the area on the same day. Shirey said there was one reported in Lewes and a small one reported in a lagoon off the Rehoboth Bay, near Dewey Beach. The latter report, he said, concerned the deaths of “a few crabs,” which he said typically means fewer than a dozen.
Toxic algae found, but not ‘of concern’
The recent boom in fish kills has led to concerns from some members of the public that toxic algae may be a cause and may pose a health threat to humans. Shirey said that appeared not to be the case in the Dirickson Creek fish kill.
“There were some species of potentially toxic algae,” he acknowledged of the water sample sent for testing at the Citizen Monitoring Program at the University of Delaware’s College of Marine Studies, which routinely tests samples taken in the area by program volunteers. “Their densities were not high enough to be of concern,” he reported of the toxic varieties of algae found in the sample.
According to Shirey, the harmful algal bloom or HAB report on the sample collected at Dirickson Creek showed a bloom of non-toxic gyrodinium instriatum at a concentration of 11.5 million cells per liter — the oxygen-depleting killer apparently responsible for the death of the menhaden but not considered a threat to human health.
Also found in the sample was potentially toxic heterosigma akashiwo, at 1.6 million cells per liter — a concentration deemed to be “well below the threshold of concern.” When found in large surface concentrations, heterosigma akashiwo is known as a “red tide.”
The sample also showed a variety of organisms at “low cell densities,” including two other potentially toxic algae organisms: chatonella and karlodinium, both also deemed to be at concentrations “well below the threshold of concern.”
Lower levels of these dinoflagellates are frequently cited as a threat to marine animals, such as fish and shellfish.
Nutrient levels contribute to blooms
“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.
As Shirey noted, blooms in the population of many varieties of algae have been connected to increases in nutrient levels in the water — something DNREC has been trying to curb for decades, including with new regulations in the proposed Inland Bays Pollution Control Strategy that Gov. Ruth Ann Minner has pledged to have in place before the end of her term in early 2009.
“The department has long fought the battle to try to curb nutrient enrichment in the Lower Bays, in particular, and throughout the state,” Shirey emphasized. “It’s not a battle that’s easily won,” he noted. “People are going to have to take steps to curb the nutrient enrichment.”
Nutrients often make their way into the area’s creeks and inland bays through run-off of fertilizers, including chicken manure laid or sprayed on farmland, as well as lawn fertilizers used by home owners, golf courses and others maintaining turf grass and gardens.
Individual septic systems have often been cited as culprits in nutrient-rich run-off and are under pressure for elimination by both state and county agencies, in favor of central sewer systems that better prevent nutrients from ending up directly in nearby streams and bays.
Public encouraged to report fish kills
Between 1981 and 2007, DNREC 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.
That may mean extra vigilance is needed by residents and visitors alike — particularly on hot, sunny days that contribute to algal bloom — as they keep an eye out for signs of something amiss with the water and creatures in which they swim, boat and fish.
Area residents who observe an unusual number of dead or dying fish are being 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.
Shirey said initial results of testing are usually available within a couple of hours, including identification of dinoflagellate species involved and a calculation of their density. He noted that it could take longer to get full test results if a toxin is suspected.
“If there was a toxin in the water associated with these critters,” he said, “it would have to be sent off to a lab out of state, and that would take a while.”
While at least one intrepid kayaker traversed the fish-filled waters of Dirickson Creek last week during the fish kill, Shirey said he could not make a definitive recommendation to avoid waters where a fish kill was suspected, since the circumstances causing fish to die may not indicate a risk to human health.
“That’s a tough one,” he said. “People should use some amount of common sense and caution in an area that is obviously demonstrating an unusual water-quality issue. But I can’t say people should clear out of the area or anything like that.”
Shirey acknowledged that the fish killed in last week’s event were not cleaned up.
“We do not clean up the fish,” he said of the agency’s policy on fish kills. He said a clean-up of a fish kill in Rehoboth Beach’s Silver Lake this spring was done by Rehoboth Beach itself but was not, strictly speaking, necessary.
“We leave it to natural ways of cleaning up, such as seagulls or crabs,” he said. “These were relatively small menhaden, 1 to 2 inches, so there was not a lot of biomass there,” he added of last week’s fish kill.
Shirey said the fish kill had been restricted to the one area of Dirickson Creek where it crosses Old Mill Bridge Road and was ongoing for several days. He said oxygen in the water seemed to have returned to normal levels by Friday, June 20. He said he didn’t believe the decaying fish posed any risk to human health.
To report a suspected fish kill, call (302) 739-9914 from 8 a.m. to 4:30 p.m. Monday through Friday and (800) 523-3336 on weekends and after those hours.