Non-native, invasive crayfish are spreading through streams, rivers, lakes and ponds across Pennsylvania, replacing native species and destroying the natural habit in those waters. And, while the invasion has been documented in many locations, Central Pennsylvania shows up clearly as the primary war zone on maps recording invaded waters.
"The Lower Susquehanna Drainage is infested at this point," David Lieb, invertebrate zoologist, told the Pennsylvania Board of Fish and Boat Commissioners on Monday. Rusty crayfish, native to the Ohio River Basin, have taken over the river and the streams of the Lower Susquehanna.
The invaders bring with them increased metabolic rates and increased growth rates, meaning "they have to eat more. So, the impact on the things they're eating is more," he explained. "We have all levels of the food chain impacted. You end up having a lot of exotic crayfish and not so much of the other things" that previously inhabited the stream, river, pond or lake.
Crayfish are generalist omnivores, eating plant matter, animals and the eggs of animals.
When crayfish invade a new ecosystem, the first impact is a replacement of the native crayfish species that naturally inhabit that water. The aquatic plant community is the next thing to go, and in some waterways that decline has been as much as 90 percent.
Next populations of mussels, snails, caddisflies, midges and other invertebrates decline as the exotic crayfish eat them. Amphibian populations laying their eggs in the water that's been invaded, and then the fish, "especially the nest builders, are impacted.
Invading crayfish also bring unnatural crowding to their new home. While the norm for native crayfish species might be one or two per square foot of stream bottom, an invasive species like the rusty crayfish may be found at more than 20 per square foot. "They can literally stack up on top of each other," he noted.
While the situation sounds dire, Lieb explained, there is hope for waters where the invaders have not yet invaded. "There's still time for preventive measures," said Lieb. And, prevention is simply a matter of encouraging humans to not release any crayfish into any waters of the state.
Crayfish are not spread on the legs of waterfowl, on the hulls and props of boats or on anglers' gear and boots. They also cannot crawl across the land masses separating the waters.
"They've arrived in Pennsylvania through human intervention," he explained. "To get between drainages they have to have our help."
The primary routes for the invaders to find their way into new waters have been as fishing bait, biological and education supplies, aquarium and pond stock, and aquaculture. After the buyers have used the crayfish they need, they often dump the rest into the nearest waterway.
And, when they arrive in the new waterway, they take over fast. For example, Lieb said, in the Manatawny Creek in southeastern Pennsylvania, the crayfish population changed from all native in 2006 to all exotic invasives in 2011.
In the internationally famed trout stream, Spruce Creek, near State College, the Allegheny crayfish, which is native to western Pennsylvania, is the invader. Lieb described it as "a relatively benign invader." Under even that best case scenario, the stream has seen a 70 percent loss of its native invertebrates, which are the basis of the entire food chain in the stream.
To many people "a crayfish is a crayfish" and they question whether it matters what species of crayfish is roaming the bottom of a stream or river. However, Lieb stressed, "exotic crayfish and native crayfish are not equivalent."
Among many impacts, the invaders generally are not a good fit for the existing food chain. They grow more quickly out of the smaller stages of their lives, when they are most vulnerable to predators like fish, and they tend to carry larger claws to defend themselves. Combined with their replacing the native crayfish, the larger size earlier and the larger claws results in fewer crayfish for other predators to eat, even with more crayfish now inhabiting the stream.
Across the state, four non-native crayfish species from outside Pennsylvania have been introduced into state waters and three species native to one part of Pennsylvania have been introduced to waters outside their natural range.
Other waters with documented non-native crayfish take-over are Yellow Breeches Creek in Cumberland County; Spring Creek, the home of the world-famous Fisherman's Paradise near State College; Penns Creek in Centre and Snyder counties; and Valley Creek in the southeastern corner of the state.
Commission staff plans to bring a proposal to the commissioners at their October meeting to ban all possession and transportation of any crayfish unless the head of the animal has been removed.