Tuesday, June 23, 2009
Virus is likely cause for dead carp
Mark Taylor is outdoors editor at The Roanoke Times.
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As they have been the past couple of springs, biologists in the Forest regional office of Department of Game and Inland Fisheries have been inundated with reports of sick and dead fish.
But many of this year's calls and e-mails have been different from the reports in 2007 and 2008.
Instead of reporting sightings of sick or dying fish on the James River, the callers were reporting dead carp at Smith Mountain Lake.
Carp draw hardly any interest from recreational anglers at the lake, but biologist Scott Smith said the office actually has gotten more calls on dead carp than it did in the winter and early spring of 2003 when the lake experienced a major die-off of striped bass.
"But part of that may be that there are more people on the water," Smith said.
It could also be that observers worry that the dead carp might be indicative of a water quality problem or a large-scale fish disease that could spread to popular sport fish.
Smith said it's unlikely that either of those scenarios are the case.
Because the problems seem to be isolated pretty much exclusively to carp, the likely culprit is a virus.
"Since it's so species-specific, it's not a water quality issue," Smith said. "There are a couple of viruses that affect carp, goldfish and koi."
Dan Wilson, the biologist who manages the lake's fishery, has collected some dead carp and sent the fish off to a lab for testing.
The two possibilities considered most likely are spring viremia of carp, and koi herpes virus.
"Both of those have been implicated in koi and carp kills around the world," Smith said.
Testing for specific viruses is fairly straightforward so the office hopes to have results back by the end of this week.
Dead carp have been found throughout the lake, but the severity varies.
At Bridgewater Marina, where visitors enjoy feeding popcorn to a large school of big carp, the fish have not yet been affected.
"We would know immediately," said owner Roy Enslow, who added that he has been watching -- and smelling -- for problems since he started hearing reports of the carp kill. "Can you imagine [the smell]?
"One guy said he had 15 to 20 dead in his cove. He had to leave his house."
Biologists don't yet know the extent of the kill.
Most dead fish have been in the 3- to 6-pound range, while larger and smaller fish seem to be avoiding the worst of it.
"It won't completely eliminate carp at Smith Mountain Lake," Smith said. "And they come back really fast. Even if you wiped out 95 percent of them, in three years you'd never know."
Smith said it appears things seem to be slowing down with the kill as the lake continues to warm. This is not unusual with viruses, which tend to thrive in relatively narrow water temperature windows.
River kills seem less severe
While the Smith Mountain Lake carp kill was not expected, Smith did expect to get some reports of fish kills on the James River this spring.
Those reports have come in, but not at the rate they did in 2007 and 2008.
"It's been quiet," said Smith, who added a caveat. "But part of it is that the river really hasn't been fishable since April.
"When nobody is out on the river, even if fish are dying or have lesions, you don't get reports."
Smith said he has been able to get out a couple of times between extreme high water events for sampling.
Because of high, murky water, the teams weren't as effective with their sampling as they would have liked. But they were still able to collect fish, and were encouraged by what they found. Or, rather, what they didn't find.
"Instead of finding 30 to 40 percent of the fish with lesions like before, we were seeing 10 percent," he said.
Smith said problems on the Shenandoah River system, which is in its sixth year of springtime kills, also appear to be relatively minor this year.
As in the past couple of years, the fish kill has hit the upper James River and its Cowpasture and Jackson river tributaries, and also touched the lower reaches of Craig Creek.
There haven't been any indications that problems have spread to other systems, including the Roanoke or New River watersheds. Even the Maury River, a James River tributary, seems to have stayed trouble-free.
Scientists continue to search for the cause of the problems. Some researchers have established a potential link between the kills and the Aeromonas salmonicida bacteria.
That cool water bacteria is not a newcomer to Virginia waters, but typically doesn't cause widespread fish kills. It traditionally has had its most significant impact in trout and salmon hatcheries, Smith said.
The link to cool water might explain why the problems seem to peak when waters are chilly in the spring -- a time when smallmouth bass are also stressed from spawning -- and typically fade in the summer as the streams warm.
Smith said researchers at a U.S. Geological Survey lab in West Virginia are continuing to study the possible link to the bacteria, with an answer possible by the end of the summer.
Yet even if that link to the river kills is confirmed, it will leave scientists searching for more answers, such as what other contributors may be causing the bacteria to thrive and do more damage than it should be.
Some who have been following the river fish kills have been wondering if maybe the high water minimized problems this spring by somehow diluting whatever in the water might be causing the kills.
In previous years scientists haven't been able to draw any correlation between flow levels and the severity of kills.
The high flows may have had one effect, though.
Smith said in years past many of the fish collected in electroshock samplings have had hooking wounds in their mouths.
This year, few have.
"The fish got a season off," Smith said. "If the water ever goes down, the fishing should be really good."
