Tuesday, July 9, 2019

Will Snakeheads Kill the Bay After All?

Ten-year-old Dustin Stem worked with his brother Ryan
 to land this 10.68 lb. snakehead, the tournament winner.
  (Dave Harp)
Snakeheads may be wreaking ecological harm, after all
The species is a native of Asia and has no natural predators in the mid-Atlantic’s freshwaters. Fishery managers in Virginia and Maryland have long warned that the snakehead’s voracious appetite and prolific spawning ability could spell ecological disaster.

After Channa argus appeared in the Chesapeake watershed in 2002 and spread to most of its major rivers, little evidence emerged to support those fears. A new fish population analysis conducted in the same waters as the June fishing tournament, though, suggests that snakeheads may not be off the hook after all.

Newhard and a DNR biologist named Joseph Love surveyed the Blackwater and Little Blackwater rivers in southern Dorchester County, MD, during a three-year period that ended in 2007. Both waterways traverse Blackwater National Wildlife Refuge, a 28,000-acre expanse considered an important winter sanctuary for geese, ducks and swans.

At the time, the researchers were looking into the ecological impacts of a small dam constructed to block saltwater. A few years later, snakeheads found their way into the rivers. The two men realized they had a unique opportunity to see how life changed in the waters after the invaders’ introduction.

Their latest sampling, conducted from June 2018 to May 2019, provides the strongest argument yet that snakeheads can upend a Chesapeake ecosystem. And the effects might not just be limited to two rivers on the Eastern Shore, the researchers said.

At the Blackwater refuge, Newhard and Love used baited, fish-trapping nets to catch more than 45,000 fish the first time around. Using the same methods more than a decade later, they only counted a little more than 7,600, a decline of more than 80%. Where did all those fish go? It’s possible many were eaten or muscled out of their territory by snakeheads, Newhard said. “I like to say there’s only so much energy to go around,” he said.

A four point study. Three before, and one after. Anyone who has observed the year to year variation in fisheries would have to conclude that the findings, while suggestive, are not ironclad.
Snakeheads registered eighth in abundance in the latest survey, but that figure likely obscures the species’ influence on the ecosystem, Newhard said. The study didn’t involve weighing the fish, but if it had, snakeheads “would easily be in the top five,” he added. Newhard doesn’t want to put all the blame on snakeheads, though. Last year’s record-shattering rainfall led to low dissolved oxygen levels in the water. That would cause many fish to flee — but not snakeheads, which can tolerate meager amounts of oxygen and even breathe out of water for short durations, he said. “I’ve had them sit in a refrigerator for three days and still be alive,” Love said. “They’re not your typical fish.”
Just your typical fish that breathes air, and walks on land, albeit not especially well.
Another finding points more emphatically at snakeheads as the cause: There were stark declines in the types of species snakeheads prefer to eat. White perch were once the most dominant species by far, accounting for nearly 40% of all those netted. By 2018–2019, barely 10% were white perch. The abundance of sunfish, another snakehead staple, also dimmed in parts of the system. Meanwhile, two species not typically found on the snakehead’s menu — common carp and gizzard shad — emerged as the top two most common fish.
 But these are two fish well known to tolerate poor water quality.
The results are particularly concerning because of the study’s location, Newhard said. The Blackwater area is one of the few freshwater marshes in the Chesapeake region and historically has served as an important nursery for perch species and other fish.

The two researchers don’t expect to finish analyzing their results until the end of the year. But they say the early findings make a case that snakeheads are making their presence felt.

“I think Blackwater as a nursery has changed and is degraded,” Love said. “We cannot determine if snakeheads actually caused the changes or if some other effect we did not measure did, but in light of the science done on the species to date, we will have a justifiably strong argument.”
As the article notes, we're stuck with them now. And it's too late to go back and get long-term data on fish populations before the arrival their arrival. We may never be able to establish that long term changes have occurred in the fish populations, and if they do, whether those were caused by or merely correlated with the snakeheads.

The Wombat has Rule 5 Sunday: Baba Fumika up and running at The Other McCain.

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