By Tim Campbell
The Lake Winnebago sturgeon spearing season is in full swing, and sturgeon news is everywhere. Talking about aquatic invasive species when everyone’s thinking sturgeon seems like a losing strategy, so let’s try a “if you can’t beat them, join them!” mindset and discuss a few ways that lake sturgeon are affected by aquatic invasive species. This way, you’ll have something interesting to think about while staring at the green glow coming from the hole in the lake, waiting for a sturgeon to swim by.
Round goby: While research has suggested that round goby prey upon fish eggs at a similar rate as native benthic fish, their high abundances within the Great Lakes Region has hindered lake sturgeon reproduction in some places. One such place is the St. Clair River, the river that drains Lake Huron into Lake St. Clair. On lake sturgeon spawning reefs in the St. Clair River, round goby density has been documented at 25 fish/m2. While lake sturgeon reproduction had been observed on those reefs, Michigan Department of Natural Resources researchers were observing an egg hatch rate of less than 1 percent, likely due to egg predation by round goby. Researchers put eggs in “predator exclusion chambers” on the reef and saw the hatch rate jump to 16 percent. Luckily, the Winnebago System is currently round-goby free and hopefully this efficient egg predator doesn’t count the lake as its newest conquest.
Sea Lamprey: The salmonids of the Great Lakes tend to be the prey of choice for sea lamprey, but that doesn’t mean a lamprey won’t try to latch onto a lake sturgeon. The bony scutes the sturgeon “wears” as armor instead of normal scales offers sturgeon some protection from lamprey attacks, but the toothy predator can find the fleshy areas of the lake sturgeon (the underside of the fish, the base of the fins) and the attacks can be fatal. Young lake sturgeon are especially vulnerable to sea lamprey attacks. The Winnebago system is currently free of sea lampreys, thanks to a barrier at Rapide Croche Lock and Dam on the Fox River. This barrier has been effective at keeping the sea lamprey at bay, as well as trapping other invasives, including the round goby, in Lake Michigan. Throughout the rehabilitation of the Fox River lock and dam system, this barrier will remain in place so that aquatic invasive species cannot naturally disperse upstream to Lake Winnebago. Anglers and recreational boaters could still move invasive species above the Rapide Croche barrier, so it is important that any boater using the system practice the big three clean boat steps – clean, drain, dry. Another option for boaters trying to navigate around the Rapide Croche Lock will be a system that lifts and cleans boats as they move upstream. Wisconsin Sea Grant’s very own Phil Moy is involved in this project, and between the barrier and the boat wash station, Lake Michigan’s unwanted invaders likely won’t be advancing up the Fox River any time soon.
Zebra Mussels: Juvenile sturgeon like to hang out in sandy and silty habitats. This allows them to easily vacuum up the small macroinvtebrates that they depend on for food. Expansive beds of zebra mussels can complicate this search for food. The small crevasses created by zebra mussel beds give macroinvertebrates plenty of places to hide, significantly affecting a juvenile sturgeon’s foraging success. When those sturgeon grow up, however, zebra mussels can become a food source. In the St. Clair River, one diet study found zebra mussels to be the most abundant food item in lake sturgeon stomachs. However, one can imagine that all of those hard, crunchy shells aren’t the most nutritious thing for a sturgeon to eat—certainly not as good as a lake sturgeon’s normal diet of soft-bodied macroinvertebrates. Many Great Lakes fish prefer a small, shrimp-like organism called Diporeia which has an energy density four times higher than that of zebra mussels! With that discrepancy in caloric goodness it is easy to see how a fish that consumes mostly zebra mussels will grow slower and not be as healthy as a fish that feeds mostly on soft-bodied invertebrates. While many may see a silver lining in the fact that lake sturgeon and other fish can consume zebra mussels, lake sturgeon don’t consume nearly enough zebra mussels to eradicate them from the environment. The same is true for any species that consume zebra mussels. Including round goby and freshwater drum. Zebra mussels end up being junk food for fish that can consume them, and make it harder for the fish that can’t eat them to find preferred food. Not a good thing, especially for lake sturgeon.
Viral hemorrhagic septicemia (VHS): Now for some good news! While many fish are susceptible to VHS, no lake sturgeon have tested positive for the VHS virus. This is obviously great news for the lake sturgeon, as it has plenty of other things to worry about while on its road to recovery in much of the Great Lakes region.
With those AIS and sturgeon tidbits to occupy your thoughts, best of luck and safe travels to everyone heading out to the ice of the Winnebago System!
