Intense heat and low oxygen levels are the likely causes of a large fish kill that closed down Silver Lake in the city of Dover earlier this week.


Intense heat and low oxygen levels are the likely causes of a large fish kill that closed down Silver Lake in the city of Dover earlier this week.

Hundreds of dead fish could be seen floating from the spillway at the St. Jones River dam in Silver Lake Park to the footbridge late Tuesday night and Wednesday morning, Dover Public Information Coordinator/ Kay Dietz-Sass said. By end of Wednesday, most of the spill had been cleaned up by the city and its environmental contractors, with the  Delaware Department of Natural Resources and Environmental Control providing guidance.

City officials closed Silver Lake on Wednesday and reopened it on Thursday.

City Manager Scott Koenig said the city reached out to DNREC for its guidance as well as to Dover consulting firm Compliance Environmental Inc.

“All entities worked very well with one another to rectify the issue,” he said.

DNREC Fisheries biologists and environmental scientists investigating found that conditions at Silver Lake fit a familiar pattern for fish kills: high temperatures, low water levels, little rainfall and an algae bloom that led to lethally low dissolved oxygen levels in the water.

DNREC estimated the kill entailed 3,150 gizzard shad juveniles and adults, along with 888 bluegills, 180 white perch and 110 catfish, plus a few black crappie, American eels, hog chokers and largemouth bass.

City of Dover officials informed DNREC Tuesday that they planned to shut off the floodgate to the Silver Lake spillway that was draining the lake in an attempt to avoid a major fish kill in the lake itself, said Dietz-Sass, also the emergency management coordinator for the city. Silver Lake is a popular fishing spot for largemouth bass, as well as bluegill and other sunfish, black crappie, white perch and catfish.

“As proactive measure, the city opened the valve at Silver Lake to lower the water level in the big lake allowing more water over the spillway to add oxygen for fish there,” Dietz-Sass said. “We were trying to prevent the inevitable.

“The hotter the water, the less dissolved oxygen the lake will hold,” she added. “Other factors could be sediment buildup from runoff and bacteria blooms. It certainly did not help the situation that temperatures have been in excess of 90 degrees for many days over the past couple of weeks.”

Dover Mayor Carleton Carey Sr. said city crews and the contractor, Clean Harbors, worked to clean up the dead fish on Wednesday.

The city will continue to monitor the situation in the lake and react accordingly, he said.