The deep lesions found on many fish killed during Pfiesteria outbreaks in the Bay in 1997 contained fungal infections which, according to a recently published paper, can originate independent of the potentially toxic algae.
A number of Virginia and Maryland scientists have maintained that the deep wounds found on dead menhaden during the outbreaks, often referred to as “Pfiesteria-like lesions,” had originated days or weeks before the fish kills took place.
Now, a paper in the Journal of Aquatic Fish Health by Yasunari Kiryu and colleagues at the Virginia Institute of Marine Science and the U.S. Geological Survey’s Fish Health Laboratory in Leetown, WV, reports that the fungus could induce wounds on laboratory fish totally absent of Pfiesteria or other agents.
Another paper in the March issue of the journal confirmed that the fungus is Aphanomyces invadans, the same species that has been linked to fish kills in the Southeast Pacific. That paper also confirmed that the same fungus has infected menhaden along the East Coast from Delaware to South Carolina.
Scientists involved with Aphanomyces research say their results do not mean that Pfiesteria did not kill fish in 1997; only that some of the most severe lesions can form independent of the harmful algae.
“The fungus itself is pathogenic,” said Wolfgang Vogelbein, of the Virginia Institute of Marine Science. “When it comes into contact with a fish, it will infect the fish and ultimately cause the development of those lesions. We have been able to do that in the absence of Pfiesteria, and basically any other environmental stressors as well.”
Vogelbein said monitoring has continued to show that menhaden with Aphanomyces lesions show up around the Bay, sometimes in large numbers, where there is no evidence of Pfiesteria. “We can now say with great certainty that this is a primary pathogen, and it is the causative agent of those lesions,” he said. “That is a big deal.”
Fungi are widespread in the environment. But typically, they are secondary invaders of an already existing sore — that is, they don’t cause wounds themselves.
As a result, the possibility that a fungus was actually the initiator of the deep lesions found in the Pfiesteria kills was initially discounted by many. But the VIMS work shows that Aphanomyces invadans is one of a number of aquatic fungi that can induce wounds on their own.
Their recent paper said that, in the laboratory, about a third of the healthy menhaden exposed to Aphanomyces developed deep lesions and that 24 percent of the population died after 21 days.
But the study also showed that Aphanomyces was opportunistic like other fungi and would quickly infest existing sores. Among fish that had damaged scales or other wounds, mortality was 100 percent when exposed to Aphanomyces.
Robert Magnien, a biologist with the Maryland Department of Natural Resources who has coordinated the state’s Pfiesteria work, cautioned that just because the fungus induced wounds in the lab does not mean that the same processes for initiating lesions occur in the wild. “What we’re really all looking for is what initiates the lesion process in wild fish,” he said.
Magnien said that menhaden are a particularly fragile fish, and that any used in laboratory tests could be stressed, possibly predisposing them to infections. “These are among the most sensitive fish in the Bay,” he said. “Whenever you put a menhaden in a fish tank, it is under stress and it is not like the wild fish.”
Nonetheless, Magnien said, “there really is a lot of common ground” between the fungus and Pfiesteria research.
Magnien agreed that the fungus might cause infections in the wild, and that some menhaden with lesions undoubtedly have no link with Pfiesteria.
On the other hand, he said Pfiesteria could — in some cases — play an initiating role in making the fish susceptible to Aphanomyces infections.
Laboratory work with Pfiesteria has indicated that it can cause ugly external abrasions on fish which can turn into bloody wounds. Magnien said it is possible that wounds caused by an initial Pfiesteria exposure would allow the fungus to invade.
Vogelbein agreed that “anything that predisposes an animal to a skin infection like that would certainly enhance the ability of Aphanomyces to infect and kill the fish. What that means is that we still can’t rule out that Pfiesteria, in some instances, doesn’t play some initiating role in the development of certain menhaden lesions. But we have no evidence that this occurs in the field.”
Aother possibility is that fish infected with Aphanomyces — for whatever reason — might lead to fish kills associated with Pfiesteria.
Laboratory research with Pfiesteria indicates it is drawn to exposed flesh and body fluids. Therefore, a school of menhaden with a number of fish containing deep Aphanomyces lesions that leak fluids into the water could stimulate Pfiesteria in the area, possibly leading to a fish kill.
“That’s not terribly speculative,” Magnien said. And, he said, it could explain why — in Maryland — there is a correlation between large numbers of lesioned fish and the detection of Pfiesteria in the water.
“We have done a rigorous statistical analysis from a number of different approaches, and you definitely find that the more lesioned menhaden you have out there, the better your chance of finding Pfiesteria,” Magnien said. “But it’s not a one-to-one relationship.”
In Virginia, though, where scientists have historically found less evidence of Pfiesteria or related organisms, researchers say they see little link between Aphanomyces and Pfiesteria.
VIMS scientists have been monitoring menhaden for several years in the Great Wicomico River, where large numbers of lesions are periodically seen, and where they observed a kill of an estimated 100,000 fish, many with lesions, last September.
According to VIMS researchers, no Pfiesteria piscicida was found in the water, and only very low levels of other Pfiesteria-like organisms have been seen, even during the fish kill.
Scientists working on the river say lesion events — including one starting just before the September fish kill — took place within a week or two of a significant rainfall after a prolonged dry period. That leads them to believe that zoospores from Aphanomyces are either being washed off the land, or that something in the runoff stimulates the fungus in the water to release infective spores.
VIMS scientist Howard Kator said the filter-feeding menhaden are lured to the low-salinity headwaters of the river by extremely dense algae blooms created by a combination of high nutrient levels and shallow, quiet water.
Then, according to a hypothesis being developed, when it rains, the salinity is lowered so the fungus can infect the fish — something that happens only when salinities are less than two parts per thousand. Once infected though, the fungus burrows through the skin of the fish and can survive at much higher salinities, Kator said.
VIMS scientists believe the September fish kill was caused by a large concentration of algae and a large number of fish all crowded into a shallow area — factors that combined to deplete the oxygen in the water.
The fungus seems to be primarily observed in menhaden. The reason for that could be that menhaden remain in dense schools, which allows the fungus — once it becomes established in a few individuals — to produce spores that spread to others, said Vicki Blazer, of the USGS Fish Health Laboratory, who was the lead author of the paper linking the fungus here to that found in the Pacific
She said fish infected in the Southeast Pacific are also schooling species. Also, Blazer said, the relatively thin skins of menhaden allow the fungus to more easily penetrate, reaching the muscle.
She said the Aphanomyces caused wounds that are unusually deep, and would almost certainly result in death — if not by the infection itself, then by making it susceptible to something else. “When you have large numbers of fish with these kinds of lesions, they are not going to respond well to any kind of stress,” she said.
In fact, scientists say large numbers of infected menhaden are probably never seen outside of fish kills because the weakened fish are quickly picked off by sea gulls and other scavengers.
While observing lesion events on the Great Wicomico, Kator said huge numbers of sea gulls are seen grabbing menhaden out of the water, while turkey vultures and other scavengers lined the shore. “We figured something on the order of one sea gull was hitting the water every 10 seconds,” he said. “The sky was just full of them. That’s a lot of fish that are being cropped out.”