It’s a warm summer day, and you’re taking advantage of the weather to go fishing. The state has a number of ponds, lakes, bays, rivers and other bodies of water and so you head to your favorite spot to cast your line.

But when you get there, you’re met by an unsettling sight: dead fish floating on the water’s surface. Perhaps it’s only a few fish or maybe there are deceased fish as far as the eye can see.

Either way, it raises questions. Is it safe to fish here? Should I report this die-off? What killed these fish, anyway?

As part of its duties of overseeing Delaware’s natural resources, including wildlife, the Delaware Department of Natural Resources and Environmental Control responds to events like this. Known as fish kills, these die-offs can occur in bodies of water as disparate as an ocean or a neighborhood stormwater basin.

A fish kill can consist of a small enough number that you can count the dead fish on your fingers, or it can include literally millions of dead aquatic residents.

“To be classified as a fish kill, the number of dead fish has to be more than would naturally die at one time,” said Edna Stetzar, a fisheries program manager with DNREC’s Division of Fish and Wildlife.

Fish kills often result from low levels of dissolved oxygen in the water, although the cause cannot always be determined. According to DNREC’s databases tracking fish kills, of the 502 events recorded over the course of several decades, more than half were at least likely due to low dissolved oxygen. Other causes include pesticide spraying, contaminants, water levels being too low, spawning stress and pathogens.

Though there is some variation based on species impacted, the U.S. Environmental Protection Agency states dissolved oxygen levels are considered “stressful” for fish at below 5 milligrams per liter and generally cannot support fish once they fall below 3 milligrams per liter.

Dissolved oxygen can result from a number of causes, including dense algae or pollution depleting oxygen levels. Sediments stirred up during dredging can also disrupt dissolved oxygen, as can stagnant water and hot weather.

Unfortunately, DNREC scientists aren’t always able to determine what killed the fish, and even if they can identify oxygen levels as the cause, they may not be able to pinpoint what prompted the levels to drop.

Factors like whether the deceased fish are of all the same species and whether amphibians are among the dead can help rule out some causes. Larger fish need more oxygen, for instance, so a lake that has many sizable fish floating on the surface while smaller ones swim about beneath can indicate the issue is the levels of dissolved oxygen.

“If literally everything is dead including salamanders and turtles, then it’s probably chemical or pollutant in nature, so there’s certain clues we look for,” Stetzar said.

Stetzar emphasized the importance of contacting DNREC to report dead fish as soon as possible so the Department can respond quickly. A list of phone numbers indicating who to call depending on the body of water and the day and time is available at de.gov/fishkills.

Dissolved oxygen levels can dip, causing fish to die, and then recover within a span of a few hours, meaning DNREC might not be able to determine the cause of death even for a fish kill that occurred earlier that same day.

“It’s important to note that we have a fish kill response network that involves other sections of DNREC, primarily the enforcement part of DNREC (Environmental Crimes, Natural Resources Police, State Park Enforcement Officers) as they are often the first to arrive at a fish kill or to receive a report from the public after normal working hours/holidays,” Stetzar said. “We also occasionally send water or fish samples to DNREC’s Division of Water, Environmental Laboratory Section, when trying to determine a cause. If agricultural pesticide spraying is suspected, we work closely with the Delaware Department of Agriculture’s Pesticides Section. They assist with identifying the source and testing the fish and water samples we collect, sometimes jointly.”

DNREC’s fish kill response is supported in part by federal sport fish restoration funding.

Fish kills are most common in summer, with a majority of the 502 instances recorded by DNREC occurring in June, July or August. They can happen virtually anywhere in the state there is a body of water, though Sussex County is the most common site for fish kills given it borders the Delaware Bay and Atlantic Ocean and surrounds the Inland Bays and Nanticoke River.

“A lot of the fish kills are associated with these dredged canal systems or manmade lagoons where there really just isn’t much flow or there was dredging that occurred,” fisheries biologist Devon Scott said, noting he visited Rehoboth Bay multiple times in 2024 for reports of dead fish. He coordinates responses to fish kills in the Inland Bays with volunteers from the University of Delaware.

While nobody likes the sight or smell of dead fish, the best thing DNREC can do in the event of a fish kill is generally to let nature take over, experts said. Predators and bacteria ensure that dead fish typically break down quickly, although there may be a lingering odor in the immediate aftermath.

We let nature take its course and let the scavengers eat,” Stetzar said. “Of course, if there was a dangerous pollutant involved, we would take care of that.”

While some may worry about the impact of fish kills on the environment, Stetzar said surveys months after the events have shown populations of species like bluegill and bass to still be thriving in various bodies of water.

In fact, fish kills have even been enlightening to the Department.

“We might not know certain species were in an area until we have a fish kill, so we get fish population data from it,” Stetzar said.

DNREC’s fish kill database contains an array of species, including some that aren’t actually fish like freshwater mussels, crabs and tadpoles. There’s even an entry for sandbar sharks. Some of the most common species include Atlantic menhaden, gizzard shad, the aforementioned bluegill and bass and types of catfish and carp.

Ultimately, though much still remains unknown about what causes fish kills and why, there are certain steps Delawareans can and should take. People are urged to avoid using too many herbicides; read the label before simply dumping the whole bottle into your backyard pond. Also keep in mind the weather, with an eye toward not applying chemicals when it’s especially hot and dissolved oxygen levels tend to be lower.

Perhaps the most important thing to remember is simply to act quickly if you spot a collection of dead fish. Contact DNREC and provide as much detail as you can.

It may still be safe to fish in a body of water that’s just experienced a die-off. But, if you smell a foul odor or seemingly every animal in the waterway is now deceased, it’s probably a good idea to find a new fishing hole for the time being.

Related Topics:  animals, fish, fish and wildlife, fish kill, fishing, health, health and science, outdoor delaware, science