Posted: October 4, 2019

There is an unwanted guest lurking in Pennsylvania's northern watershed. It has a taste for endangered mussels, and plans for expansion.

Round Goby

Round Goby

In 2013, when Jay Stauffer learned that round gobies had been discovered in Lake LeBouef, he proposed using a toxic chemical to poison all the fish in the lake to remove the invasive species--that's how calamitous he regarded its introduction into the upper reaches of northwestern Pennsylvania's unique French Creek watershed.

"That sounds draconian, I know, but the native fish species would have come back," says the Penn State Distinguished Professor of Ichthyology. "It would have been the only way to avoid what I believe is an unfolding ecological disaster."

Unfortunately, that did not happen, and now the gobies have moved down into French Creek. "We will never be rid of them," says Stauffer. "We missed an opportunity."

Stauffer and his colleagues are investigating the biology of the round goby in an effort to understand how they are impacting mussels, and possibly oysters, and how they might be controlled.

Missing Mussels

The threat posed by the round goby--a small, extremely prolific invader from Europe--is mainly to endangered freshwater mussels in French Creek, one of the last strongholds for two species of mussel. French Creek flows from southwest New York state about 117 miles to the Allegheny River at Franklin, Pennsylvania. It is the most species-rich stream in Pennsylvania and is nationally recognized for its biodiversity, with more than 80 species of fish and 29 species of freshwater mussel.

French Creek
French Creek

Four of the mussels in French Creek are listed under the Endangered Species Act: northern riffleshell, snuffbox, clubshell, and rayed bean. Northern riffleshells and clubshell mussels are considered critically imperiled and have lost 95 percent of their historic global range--but they appear to have stable populations in French Creek.

Predation by the round goby, however, is likely to destroy that stability, warns Stauffer. His research group confirmed that growing numbers of the round goby, which do not grow longer than 10 inches or so in Pennsylvania streams, are eating native mussels in French Creek.

Originating from the Black and Caspian seas, round gobies were introduced into the Great Lakes by the release of ballast water from large trans-Atlantic cargo ships around 1990. Able to live in saltwater or freshwater, they were first found in Lake Erie in 1995, where they are now one of the most common of all fishes in the lake. From there, they were likely transported in bait buckets carried by fishermen to the 72-acre lake on LeBoeuf Creek, a tributary of French Creek in Erie County Pennsylvania.

"It is not an overstatement to say that French Creek is a one-of-a-kind waterway, and that gobies being introduced to that watershed could end in severe harm to the ecosystem," Stauffer says. "They quickly moved downstream and now that they are in French Creek, we are trying to gauge how much danger they pose to native species and how much damage they will do. We think that this is going to be a huge problem."

Gauging Gobies

To document the diet of round gobies in the French Creek watershed to determine whether consumption of native, freshwater mussels is occurring, researchers collected round gobies in the summer months of 2016 using a kick seine in four locations. They then dissected the fishes and closely examined their stomach contents. Gobies were separated into categories based on length so researchers could determine if their diet changed with increased size and age.

Their findings, which were published in American Midland Naturalist, showed that native mussels were crushed and consumed by gobies of all lengths in French Creek. Supported by the Pennsylvania Sea Grant College Program through the National Oceanic and Atmospheric Administration--with help from Elizabeth Boyer, associate professor of water resources--it was the first study focusing on the ecological impact of gobies on native mussels in a stream environment in the United States. Many previous studies have looked at gobies' impact on the biomass and food chain in the Great Lakes.

Round Goby
Round Goby

The introduction of gobies into French Creek and the specter of them consuming endangered mussels is a worst-case scenario, notes lead researcher Casey Bradshaw-Wilson, now assistant professor of environmental science at Allegheny College, who was a doctoral student in Stauffer's group when the research was started.

"While human alteration to stream systems in North America has pushed these mussel species toward decline, many species in French Creek are thriving," she says. "The introduction of round gobies into the French Creek watershed poses a serious threat to native mussels, both directly through their consumption of juveniles and indirectly through goby-caused decline of the fishes that mussels use as hosts to grow and be transported during an early stage of their life."

Some of the mussels in French Creek only have one host, she adds, and if the gobies wipe out that fish, then the mussels are not going to be able to reproduce.

Bradshaw-Wilson has studied darters, a group of small fishes impacted by round gobies, in both Lake Erie tributaries and French Creek. Gobies outcompete darters and eat similar food items, she has learned, and because darters are a host for the endangered mussels during the first stages of their life, that has ramifications for the endangered species. "There is a darter species that only lives in the upper reaches of Allegheny River [which includes French Creek] that biologists are worried about in the face of the goby invasion," she says. "It is the spotted darter, and French Creek is one of a few places that it is found in the world."

Gobies spread so quickly in stream environments, Bradshaw-Wilson explains, because they spawn from May to August, and there is a constant stream of eggs and larvae in the water column. Eventually they colonize downstream areas.

In stream ecology classes that Bradshaw-Wilson teaches at Allegheny College, she includes projects in French Creek that involve sampling for gobies to see how fast they are spreading. Anglers are probably responsible for some of the dispersal.

"Bait fishermen, unfortunately, view them as an important baitfish for catching smallmouth bass, and although it is illegal to transport live gobies, they are found sometimes in bait shops around Lake Erie because it can be difficult to separate minnows from gobies. And if an angler dumps his bait bucket when he is done fishing, you can have a goby introduction."

Next Steps

Bradshaw-Wilson's future research, coordinated with several doctoral degree students in Stauffer's lab--Kyle Clark, Sara Mueller, and Josh Wisor--will be aimed at determining which endangered mussels are most at risk because right now researchers don't know which species gobies are eating. "When they are so young and tiny, and I'm looking at them under a microscope, I just can't identify the mussel species," says Bradshaw-Wilson. "We are planning a collaborative project with my lab and Jay's lab at Penn State looking at the DNA of gobies' stomach contents to figure out exactly what mussel species they are eating."

The results of the current research--as ominous as they are for endangered mussels in French Creek--portend that gobies will wreak havoc on shellfish related to another watershed that also starts in New York and flows through Pennsylvania, Stauffer says. And although that river, the Susquehanna, is almost 200 miles and a three- hours' drive away, the researchers predict that gobies will get there in fishermen's bait buckets.

Clubshell Mussel
Clubshell Mussel

The Susquehanna River empties into the Chesapeake Bay, which has been called a world-class oyster-making machine. With tons of fresh river waters pouring into it daily, a constricted mouth, and shallow waters, that estuary is one of the largest bodies of ideal oyster habitat on earth. If round gobies get there, they could ruin a multi-million-dollar oyster industry.

"The threat to the oyster beds in the lower Susquehanna River, and really all of the Chesapeake, is real--I think it will happen in my lifetime," Stauffer says. "Someone is going to use gobies as bait in the Susquehanna, that is going to happen. From there, it is just a question of time until they get into the bay."

By Jeff Mulhollem