Uptick in Chesapeake Oyster Mortality

Spotty oyster mortality appears in Bay

Oysters in waters from Tilghman Island south are turning up dead, and there could be any number or a combination of several reasons why.

The mortality is spotty, said Maryland Department of Natural Resources Shellfish Division Director Chris Judy, but still something watermen should keep an eye on and inform the department when they pull up dead oysters.

DNR is conducting its fall oyster survey. Meanwhile, it has been getting reports from watermen about this oyster season being a worse harvest year than last season.

The mortality levels depend on the bar, Judy said. A lot of areas have similar mortality rates compared to last year, he said.

“We’ve been to oyster bars were the mortality is well over 30 percent, which is high,” he said. “But in the very next bar, it looks fine.”

Sounds like an infectious disease to me.

There are a few theories about the causes of the mortality.

The most obvious of which, Judy said, is it has been very dry and salinity levels are up, and have been for more than a year. High salinity levels can cause a spike in disease, and when disease increases, mortality can follow, he said.

A reasonable theory; two diseases in particular plague oysters in Chesapeake Bay, Dermo and MSX, both infections by protozoans. Both do better in high salinity conditions.

Another theory points to low dissolved oxygen in the water, which can suffocate the oysters.

However, anoxia and low dissolved oxygen was of average extent this year, so that doesn't seem likely.

Another theory is they are dying because it has been warmer than usual lately, and because of that, oysters have been spawning longer and are weakened for an extended period of time.

However, the same oysters do well from Maine to Florida, so temperature alone is unlikely to be an important factor.