Inland Bays see record number of fish kills in 2021

Even before a Dec. 28 accident dumped thousands of gallons of untreated wastewater in Rehoboth Bay, 2021 was serving as a reminder there’s still lots of work to do related to water quality, because Delaware’s Inland Bays saw a record number of fish kills in the past year.

According to Dr. Marianne Walch, Center for the Inland Bays science and restoration coordinator, as of late October, 15 fish kill events were reported in the Inland Bays this year. The previously reported high was 11 in 2000, with the typical amount being one to three events per year, she said in an email Dec. 28.

All together, nearly 2 million individual fish were reported killed in 2021, with the number of deaths ranging from 6 to 1.5 million fish, said Walch. There were likely more small kills because they go unreported after the dead fish are quickly washed away by the tides or eaten by gulls, blue crabs and other scavengers, she said.

Walch said the state’s Division of Fish and Wildlife has the lead role in Delaware for investigating reports of dead or distressed fish, but CIB has been tracking incidents since 1981, and the University of Delaware’s Citizen Monitoring Program provides support.

A majority of the fish – an estimated 1.7 million, primarily juvenile menhaden – were killed between June 10 and June 17 in the Arnell Road canal near Rehoboth, due to extremely low oxygen conditions, said Walch.

Walch said most fish kill reports came from the upper Indian River, and residential canals in Rehoboth Bay and South Bethany. However, she said, there were also reports from other tidal creeks and even some of the more open areas of Rehoboth and Indian River bays this year.

Walch said there are a number of possible reasons for the fish kills – algal toxins, toxic pollutants, parasites or disease – but by far the majority of kills in the Inland Bays are attributed to low oxygen caused by large algae blooms. Anything below 4 milligrams per liter of dissolved oxygen in the water is extremely harmful to many different bay organisms, she said.

“The CIB’s water-quality monitoring has shown that in the upper Indian River during the months of July and August, dissolved oxygen drops to nearly zero every night, and often stays below 4 mg/L for several hours at a time,” said Walch. “It is almost certain that many organisms including blue crabs, shellfish and bottom-dwelling fish, such as flounder, are being harmed by this.”

The majority of fish kills reported in the bays involve Atlantic menhaden, which are especially susceptible to low levels of dissolved oxygen in the water, said Walch. However, she said, small numbers of blue crabs, gizzard shad, American eel, and Atlantic silversides were involved in some reports.

Over the past few years, there have been a number of positive changes for the water quality of the Inland Bays – for example, introduction of shellfish aquaculture and removal of the Rehoboth wastewater treatment plant’s discharge pipe – but nonpoint source loads of nitrogen remain far in excess of healthy limits in all three bays, said Walch, adding that phosphorus loads have also exceed healthy limits in most recent years.

“In the past five years, water quality in the upper Indian River has been particularly bad,” said Walch. “Many sources contribute to this, including fertilizers and runoff from both agricultural and developed areas, on-site waste treatment systems, and legacy nutrients in groundwater that may take many years to reach the bays.”

Looking forward, CIB Executive Director Chris Bason said there is a lot that can be done to address these types of water-quality problems. Environmental policy is key here, he said.

There is an important opportunity at the county level right now, as Sussex County Council considers changes to its ordinance affecting buffers between new developments, and wetlands and waters. Buffers prevent pollution from entering the water.

“The most effective buffers are wide and forested,” said Bason in an email Dec. 28. “With so much deforestation in the watershed right now, it is really important that trees near the water are protected.”