We're being invaded. Thankfully the culprits aren't carrying guns or threatening our national security. The invasion is nonetheless serious, ongoing and at this point there is little we can do to prevent or control future invasions. A surprising number of nonnative species have been accidentally (or inadvertently) introduced into the Great Lakes. Most make the trip from a salt to fresh water environment inside the ballast water of commerce ships that travel from the Atlantic Ocean into the Great Lakes.
The journey from salt to fresh water is apparently a painless experience. Some species do so well in the Great Lakes that they quickly grow in numbers large enough to compete with native species.
This invasion is not exactly new or newsworthy. Nonnative species have been invading the Great Lakes for many decades. What is newsworthy is that some of these invasions, while far from planned, have actually benefited the sport fishing scene. Others are having such a profound impact that they are actually changing the food web on the Great Lakes which could lead to significant problems down the road.
Frankly, the jury is still out on whether nonnative invaders are good or bad, but one thing is consistent. These unwanted introductions are forcing changes in the Great Lakes no one could have predicted.
The changing face of the Great Lakes has researchers, fishery biologists and anglers alike concerned. Since no one has a crystal ball tuned to the future, there has been a lot of speculation as to what impact nonnative invasions will have on the long term health of sportfishing. Plenty of gloom and doom opinions have been bantered about, but the fact remains that everything associated with nonnatives has not been bad.
"The Great Lakes has probably experienced more nonnative species introductions than any other freshwater environment on Earth," says Fred Snyder, Extension Specialist for Ohio Sea Grant a division of Ohio State University. "Just a few of the nonnative species currently living in the Great Lakes include the rainbow smelt, gizzard shad, alewife, round goby and zebra mussel. As the populations of these nonnatives increases they begin to impact on both the native and nonnative species around them. Our job is to try and predict how these changes will impact on recreational fishing efforts."
ZEBRA MUSSEL
The tiny zebra mussels has without question had the most impact on the Great Lakes of all the current nonnative species. Small in size, but large in numbers, the zebra mussel is widespread throughout the entire Great Lakes system. These mollusks collect on just about anything hard in the water. Rocks, gravel, water intake pipes and even boat hulls become convenient homes for zebra mussels.
"A zebra mussel feeds by filtering microorganisms from the water," says Snyder. "In doing so the zebra mussel has actually restructured the entire food web in the Great Lakes."
Zebra mussels filter so much plankton from the water that certain species of baitfish are having a difficult time finding adequate amounts of food. Gizzard shad, which favor plankton rich waters, are a prime example of a species that is struggling to adjust to the environmental changes brought on by zebra mussels. As plankton levels drop, shad numbers have also decreased.
"Zebra mussels have reduced the amount of plankton in the water and this in turn means that the waters of the Great Lakes are also becoming clearer," explains Snyder. "Increased water clarity allows better light penetration and triggers a number of other environmental changes. Clear water encourages weed growth to expand to deeper depths, creating improved fish habitat for species like smallmouth bass, northern pike and muskie."
Not every species is benefiting from this change in the food web and increase in water clarity. The environmental conditions that are gripping the Great Lakes have not had a positive impact on walleye fishing. Walleye in Great Lakes waters fare best in plankton rich environments that support huge amounts of forage fish.
Lake Erie is still without question the World's richest walleye fishery, but the population of walleyes on Erie has steadily declined over the past two decades as a direct result of changes in the food web. This realization might be devastating for walleye anglers if it weren't for a touch of irony. Zebra mussels created an environment less suitable to walleyes. Ironically, zebra mussel numbers are now so high that the waste excreted when they feed on plankton is increasing the phosphorus levels in Lake Erie's Western Basin. Increased levels of phosphorus in turn causes algae blooms to form, reducing water clarity in the process and making an environment more suitable for the types of forage fish walleye favor. Walleye anglers on Lake Erie could be seeing a return to the blue/green waters and booming walleye populations of the 1970's and 1980's.
ROUND GOBY
The round goby is a bottom orientated fish that seldom grows to more than a few inches long. An aggressive little critter, the goby has largely replaced an important native species known as the sculpin. "Anyone who has tried to fish live bait near bottom in the Great Lakes has undoubtedly come to know the annoying little goby," says Snyder. "These aggressive fish nibble on live baits to the point that it's difficult to catch walleye, bass or other species."
The goby makes bottom fishing presentations frustrating, but in the same token they are also providing an important forage base for species including smallmouth bass that feed mostly near bottom. "Research suggests that the goby may actually be making it possible for young-of-the-year smallmouth to increase winter survival rates," says Snyder. "Because young-of-the-year smallmouth are big enough to feed on young-of-the-year gobies, smallmouth head into winter with the extra fat reserves required to insure survival."
The drama between the goby and smallmouth bass runs even deeper. Evidence has shown that adult gobies eat bass eggs. The suggestion is that increasing goby populations could spell disaster for smallmouth bass in the Great Lakes. This is unlikely because both species seem to be doing a pretty good job of keeping each others numbers in check. One thing is for sure. Fishing tackle manufacturers have caught on to making lures that look like gobies!
In case you're curious, gobies don't eat walleye eggs because walleye spawn in shallow water early in the year when gobies are largely found in deep water. By the time that gobies start invading shallow water, walleye eggs have hatched and moved on.
Gobies are currently more of a nuisance than threat to sport fishermen, but trouble could be brewing in the future. Gobies eat a wide variety of foods including zebra mussels. While gobies can't eat enough zebra mussel to keep them under control, they are eating enough of these mollusks to significantly increase chemical contaminants in their flesh. Because zebra mussels filter the water, they quickly bioaccumulate chemicals like mercury. This mercury is passed right up the food chain when gobies eat zebra mussels and perch or bass in turn eat the gobies.
Fortunately, more than 80% of the bass caught in the Great Lakes are not kept for food, but rather are released. Yellow perch however feed heavily on gobies and these fish are routinely kept for the table. Walleye also feed on gobies, but these bottom orientated baitfish make up only a small percentage of the walleye's more diverse dietary requirements.
The sportfishing outlook on the Great Lakes remains bright despite the many nonnative species that have invaded the lakes in recent years. Current forage types and water clarity conditions are favoring smallmouth bass, largemouth bass, yellow perch, muskies and northern pike. The overall populations of these fish are at current high levels.
Walleye numbers have declined a little in recent years, but a strong Lake Erie year class in 2003 will soon produce a much needed boost. The 2003 walleye year class is the largest ever recorded on Lake Erie. Steelhead and salmon numbers are stable, largely because population levels are controlled by stocking not natural reproduction.
"The interesting thing is that walleye are about the only popular species that have experienced notable population declines in recent years," says Snyder. "Walleye numbers are down from historic highs in the 80's and walleye catch rates are also down proportionately. Amazingly, the number of walleye taken per angling hour has actually stayed the same. Despite the fact that fewer fish are available to them, modern walleye anglers are still catching as many fish per hour. That means that fish aren't the only thing evolving on the Great Lakes these days."
