VIMS team discovers probable cause of croaker deaths

According to Vogelbein, dying croaker submitted for evaluation to the Aquatic Animal Disease Diagnostic Laboratory at VIMS appear healthy externally (Figure 1) except for extensive bleeding from the gills (Figure 2). Microscopic evaluation (histopathology) of the gills indicates extensive hemorrhage (bleeding) and degeneration of the respiratory tissues (Figures 3, 4) associated with a bacterial infection (Figure 5).

"The damage to the gills is so extensive that respiration is impaired and the fish are likely dying of suffocation, blood loss, or both," says Vogelbein.

Identification of the bacterium has so far been hampered by the organism's failure to grow in culture, and its failure to react with the traditional gram stains used to classify bacteria. However, a universal molecular probe for bacteria designed and applied by VIMS fish pathologist David Gauthier clearly labels the organisms as bacteria. It also clearly indicates that the bacterium is an intracellular pathogen specifically infecting the respiratory epithelial cells of the secondary gill lamellae (Figure 6). Efforts to culture and identify the pathogen are underway. The researchers have also begun evaluating the tissues with an electron microscope to gain a better understanding of the mode of infection.

Reports of dead and dying fish were first brought to the attention of the VIMS investigators in early July. Samples of freshly dead croaker were initially obtained through sampling efforts by the Virginia Department of Environmental Quality and from Dr. Mark Luckenbach, Director of VIMS Eastern Shore Laboratory in Wachapreague.  “The dead fish are all large croaker, 4 years old and older “ indicated Luckenbach.

The consensus at the time was that these fish were the result of a net dump by the menhaden fishing fleet. Subsequent reports came from charter boat captains plying the coastal waters off of Virginia's Eastern Shore beginning on Friday, July 30. "As the day went by, we heard more reports from the south," said Luckenbach. By Saturday, he had reports of dead croaker from the beach at the Chincoteague National Wildlife Refuge and then from Ocean City, Maryland. By Sunday, reports were coming in from Northern Maryland and the Delaware shore.

According to the Richmond Times Dispatch, estimates from Maryland and the Delaware portion of the Eastern Shore placed the number of dead fish at more than 1,000,000 during the July 31 weekend. Luckenbach did not estimate the number of dead croaker on the uninhabited barrier islands of Virginia's Eastern Shore, but says that the shores near Wachapreague Inlet "were absolutely littered" with dead fish that same weekend.

On the following Wednesday (August 4), Luckenbach received a report of dead croaker from a trawler captain 8 miles off of Chincoteague Island. Other reports of recently dead and dying croaker came on Thursday August 5 from a trawler 6-8 miles off of Ocean City, Maryland, and from near Cape Charles on August 31. On August 26, millions of adult croaker were reported to be dying near Cocoa Beach and Cape Canaveral, Florida. Sporadic accounts of dead and dying croaker continue.

The current research contradicts earlier reports from Maryland officials suggesting that the croaker were dying due to an upwelling of deep cold water associated with the passage of large storms through the region during late July. Anecdotal reports put offshore bottom water temperatures during this period at ~40 degrees Fahrenheit. Temperatures along the shore measured in the high 60s to low 70s.

"Croaker are particularly susceptible to cold water," notes Luckenbach.

Vogelbein notes that infectious diseases in fishes are almost always modulated by environmental factors. "Microbial organisms capable of causing disease in fishes are always present in the environment. Only when fish become stressed by a changing environment will they generally break out with disease. However, which specific environmental factors are modulating the croaker disease outbreak are not known," he says.

Upwelling of cold water cannot be ruled out as a predisposing factor. However, it does not explain the numerous early reports of croaker dying within the relatively well-mixed waters of Chesapeake Bay. Further, the region is experiencing one of the wettest years on record. Thus freshwater runoff may also be playing a role in coastal fish-disease events. Other unknown environmental factors may also be playing an important role.

VIMS scientists have ruled out several potential causes of the croaker deaths. Drs. Kimberly Reece, Jeff Shields, and Larry Haas have screened water samples from affected areas for Pfiesteria and Pfiesteria-like organisms, as well as the toxic alga Alexandrium chatonella, but all tests have come back negative. Efforts to understand this unusual disease outbreak continue at VIMS.