delware environmental institute

DNREC News: First reported fish kill of summer fits familiar pattern of high heat and low dissolved oxygen

DNREC News: First reported fish kill of summer fits familiar pattern of high heat and low dissolved oxygen

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 on July 5.

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.