The zooplankton never saw it coming. Well, perhaps it would be more correct to say that they never smelled it coming. These tiny, eyeless water creatures recognize predators by their scent, and zooplankton in the Upper Midwest have never added the spiny water flea to their stink list. The results have been catastrophic.
"The word I use is blindsiding," says limnologist W. Charles Kerfoot, a professor of biological sciences at Michigan Tech. "When Bythotrephes longimanus was introduced here from northern Europe 30 years ago, the native species were totally oblivious to it."
They still are, which is why the spiny water flea, aka Bythotrephes (pronounced BITH-oh-TREH-feez) is devouring its way through the Great Lakes and into the surrounding inland waters. As a result, this half-inch-long predator with a spikey tail is on the verge of disrupting an entire ecosystem from the bottom up.
Kerfoot recently completed a three-year study in the region that shows just how widespread these carnivorous crustaceans have become. His group sampled the waters in dozens of sites from New York to northwestern Minnesota; they found 83 sites that were infested.
The scientists checked out Lakes Michigan, Superior and many interior lakes and lingered two years in Voyageurs National Park, on Minnesota's border with Canada, to examine Bythotrephes' long-term effect on the native zooplankton. Three of the park's lakes -- Rainy, Kabetogama, and Namakan -- had significant densities of the spiny water flea. Compared to nearby lakes with little or no Bythotrephes, they had far fewer of the native zooplankton that support a food web topped by pike, bass, walleye -- and of course, fishermen.
They also compared their measurements with data from as far back as 2001 and found that some native zooplankton species had nosedived, whereas the entire foodweb experienced declines ranging from 39 percent to 80 percent. In addition, they documented similar crashes in the populations of native predatory zooplankton species that feed on other zooplankton, probably because the Bythotrephes are eating up their food supply.
"Bythotrephes is having as much impact on the plankton communities as quagga mussels have had in Lake Huron and Lake Michigan," said Kerfoot. "We expect it will have cascading effects up to the fish, but right now, we can see a major collapse of the plankton community."
So how did the spiny water flea get here, and what makes it such a bad actor in the ecosystem?
Bythotrephes is no big deal in its native waters. "The zooplankton communities in Scandanavia and northern Europe have adjusted to it," Kerfoot said. "They have evolved together, and the prey species the Bythotrephes eats have developed devices to protect themselves, like helmets and spines." North American zooplankton like many Daphnia and Bosmina species, which are favorites on Bythotrephes' menu, have similar hardware but "fail to sense the presence of the exotic and don't deploy the defenses."
Since it can only live in fresh water, the spiny water flea could never have crossed the Atlantic Ocean on its own. "It came in ballast water, no question," Kerfoot said. Like many aquatic invasive species, it was transplanted by ships loading ballast in one part of the world and dumping it in another, in this case, the Great Lakes. Since Bythotrephes needs cool conditions, it has not gained a footing in more southerly waters. But it is having a field day in a band of inland lakes stretching from eastern Ontario to northern Minnesota and in the cooler Great Lakes.
Ocean-going vessels can't be blamed directly for Bythotrephes in Voyageurs National Park, however. "It is transported to inland lakes by recreational fishing," said Kerfoot. The spiny water flea has an enormous spine with barbs that attach to all types of surfaces (fishing lines, nets, anchor ropes), and unless boats and fishing gear are thoroughly cleaned, they can carry spiny water fleas and their resting eggs between lakes, infecting one after another.
Bythotrephes resting eggs are exceptionally hardy. "We discovered that the eggs are special: big, round and thick shelled," said Kerfoot. "They go through fish guts intact."
That means that minnows taken from Bythotrephes-infested waters and used for bait elsewhere can poop viable eggs into the new lake. Spot-tailed shiners are especially good at spreading spiny water flea eggs, but there's a solution: if minnows are held for 24 hours, they will pass all the eggs and be safe to use as bait. "Then all you need to do is clean out your live well and bait bucket at home, before you go fishing," Kerfoot said.
If Bythotrephes fit into the North American food web, it might not be such a problem. However, it developed impressive armor back home in Eurasia, and to the small North American fish who would normally eat it, Bythotrephes is a fully armed pincushion.
The spiny water flea is mostly long, pokey tail with a three or four barbs sticking out the bottom for good measure. It's famous among anglers for forming tangled, sticky gobs on fishing gear and anchor lines. What's merely a nuisance for people is a disaster for small fish.
"My students were first to show that the tail barbs protected them against fish. When the fish try to eat them, they get stuck by the barbs," said Kerfoot. "The spine can poke right through the wall of the stomach."
Right now, there's no way to get Bythotrephes out of infested lakes. But boaters and anglers can stop it from spreading. The techniques are straightforward and familiar, similar to the ones used against other aquatic hitchhikers like the VHS virus, which threatened trout populations:
• After boating and before you leave the launch, remove all plants, animals and any other organic materials and dispose of it on dry land. Don't take it home with you.
• Drain all water from bilge, live wells, ballast tanks, etc. before leaving.
• Dry everything thoroughly before you put your gear in another lake. This usually takes about five days. If you want to use your boat sooner, clean all surfaces with hot (over 104 degrees F.) water, a high-pressure hose, or a disinfectant, like a household bleach solution.
• Don't transfer fish directly from one lake or stream to another, either whole or cut up for bait. To assure that minnows are not carrying Bythotrephes eggs, keep them for 24 hours and then transfer them to a container with clean water. Or, avoid lake spot-tailed shiners and use river emerald shiners, which don't eat Bythotrephes or its eggs.
Some states, such as Minnesota, have launched "Stop Hitchhikers" campaigns to raise awareness and have installed spray units at public boat launches, where boaters can clean their vessels and bait buckets. California inspects vessels and assesses fees based on boat size; the money goes to programs to fight exotic pests.
Measures like this can be successful, Kerfoot notes. Isle Royale National Park is one of the few regions in the Upper Midwest where his team found no Bythotrephes in inland lakes. There, boaters must disinfect all vessels before putting them in water, and anglers are limited to artificial bait. In addition, motorized craft are banned.
"Once people get behind an effort to stop hitchhikers, it's really effective," Kerfoot said. "People do care. Unfortunately, that doesn't stop the odd Typhoid Mary. In some places along Highway 41 in Upper Michigan's Keweenaw Peninsula, every lake we tested with a boat ramp had Bythotrephes."
Nevertheless, all the dedicated, responsible fishermen in America can only do so much. The primary highway for invasive aquatic species, the ballast tanks of ocean-going vessels entering the Great Lakes, is still open. Though exotics have been invading the region since the St. Lawrence Seaway opened in 1959, many freighters still release contaminated ballast water into the system.
"They can pick up something in the southern Great Lakes and pollute Thunder Bay or Duluth," said Kerfoot. "We absolutely have to cut that conduit off."
The study of Lakes Superior and Michigan was funded from National Science Foundation OCE-9726680, OCE-9712872 and OCE-9712889. Geographic survey sampling and studies in the national parks during 2008-10 were funded by a grant from the National Park Service Natural Resource Preservation Program GLNF CESU Task Agreement No. J6067080012.
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