Contact Joanna Wilson, DNREC Public Affairs, 302-739-9902.
First reported fish kill of summer fits familiar
pattern of high heat and low dissolved oxygen
REHOBOTH BEACH (July 5, 2012) – On July 4, DNREC responded to reports of a fish kill in Rehoboth Beach’s Silver Lake that initially involved an estimated 1,500 gizzard shad 2 to 4 inches long along with 800 white perch the same size, plus a few bluegills and a largemouth bass. Today, DNREC scientists continued to investigate the cause of the fish kill, which had broadened overnight to include both juvenile and adult fish of those same species – with approximately 5,000 to 6,000 dead gizzard shad of all sizes and 600 adult white perch, plus adult blue gills and largemouth bass observed today.
Surface water testing on both days by fisheries biologists indicated that dissolved oxygen levels in the water have been low enough this week in Silver Lake, a freshwater impoundment, to be lethal for fish. These conditions repeat a familiar pattern for fish kill occurrence – high temperatures and an excessive microscopic algae bloom result in low oxygen levels in shallow water.
“As long as these conditions persist, there’s a chance of more fish kills in Delaware,” said John Clark, DNREC Fisheries Section administrator. “Aggravating this summertime problem, increased temperatures lead to warmer water, which holds less dissolved oxygen. So seeing more fish kills this year as the heat continues would come as no surprise.
“We can’t predict when fish kills will occur, but we know what usually contributes to them, and a low oxygen level is at the top of the list,” Clark said. “Bright sunny days encourage also algae growth. Like other aquatic plants, algae contribute to the oxygen supply by day, and then consume oxygen at night. Plus, algae can die, drop to the bottom and decompose, which consumes even more oxygen from the water.”
DNREC Division of Fish and Wildlife biologists routinely monitor the status of fish and other aquatic species, while environmental laboratory scientists from DNREC’s Division of Water keep an ongoing record of water quality in waterways throughout the state.
Anyone observing an unusual number of dead or dying fish in Delaware ponds, rivers or other waterways are encouraged to report their observations, including an estimate of how many fish are involved, and, if known, the species of fish. Depending on the situation, DNREC staff – including fisheries biologists, Fish and Wildlife Enforcement, and environmental scientists – will respond to investigate 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 also take water samples.
To report a suspected fish kill, please call the Fisheries Section at 302-739-9914 from 8 a.m. to 4:30 p.m. Monday through Friday, and DNREC’s citizen complaint hotline at 1-800-662-8802 after hours and on weekends.
Vol. 42, No. 254