Asian carp

Asian carp in Chain Lake | Photo by Thad Cook

More than one-third of Lake Erie’s biomass could become Asian carp in the next century if an invasion of the dreaded invasive fish occurs in the Great Lakes as many predict.

The latest study from almost a dozen Canadian and American researchers concludes that fish throughout the aquatic food web would be devastated should silver and bighead carp become established in the Great Lakes. The report, published this week in the Transactions of the American Fisheries Society Journal, argued everything from planktivores like emerald shiners and gizzard shad, to fish-eating predatory fish such as walleye and rainbow trout would show signs of decline, in some cases by as much as 37 percent.

Not all native fishes would decline as a result of an increased Asian carp presence. If the modelling is accurate, smallmouth bass — which prey on juvenile fish — could see an upswing once young Asian carp become more plentiful.

Silver and bighead carp, two invasive fish from China that have been overrunning US rivers from Louisiana to Minnesota since the 1970s, are the dominant fish in size and volume throughout many ecosystems they inhabit. Both are highly efficient filter feeders, sucking up phyto- and zooplankton from the water faster than native species (who also rely on these microscopic insects and plants for food) can ingest them. But unlike most fish native to the Great Lakes, Asian carp are highly opportunistic feeders: both have been shown to switch food sources by surviving on detritus and bacteria when other food sources are scarce. Native fish simply cannot do this.

Both species have so successfully colonized the Mississippi and Illinois rivers that more than 90 percent of all living matter in these waters is Asian carp.

Yet there’s a silver lining to the report’s findings, albeit, a dark one. The fact that mathematical modelling and structured expert judgement capped the projected biomass at 34 percent is, in a perverse way, good news. While the study confirmed their detrimental impact on forage and predatory fish, the degree of damage was less than the Mississippi and Illinois rivers and far less than many predicted.

“Our results suggest that the lakewide impacts on the Lake Erie food web will not be as great as some have feared,” according to Hongyan Zhang, lead author of the report and a researcher at the University of Michigan. “Fortunately, the [biomass] percentage would not be as high as it is today in the Illinois River, where Asian carp have caused large changes in the ecosystem and have affected human use of the river,” she said in a release.

But not so fast. Zhang and her fellow scientists ran a series of models to gauge the potential effects that silver and bighead carp could have on Lake Erie’s food web. To obtain a broad range of potential impacts, researchers structured the model to account for several variables: whether the carp would turn to detritus in the absence of plankton, what impact nutrient loading (a common problem in the warm waters of western Lake Erie) would have on carp feeding and whether Asian carp would turn to eating sport fish larvae if phyto- and zooplankton volumes drop.

This last variable is crucial. “We have found no evidence in the published literature that Asian carp consume larval fish or fish eggs,” Zhang notes. However, based on research conducted by Canadian biologist Becky Cudmore and the US Geological Survey, many experts have suggested that Asian carp probably do feed on fish larvae given the flexibility of their diets and their proximity to spawning grounds.

So while the overall prognosis isn’t as terrible as many thought, it all rests on Asian carp avoiding fish larvae as food. There’s no record of this happening, but it’s cold comfort to many who know the invasive fish best. Should larvae end up a staple food for the invasive fish in Lake Erie, expect populations of walleye and yellow perch to decline dramatically, researchers warned.

Lake Erie, the most productive of the Great Lakes’ $7-billion fishery, was chosen as the first modelling subject. Live Asian carp, as well as traces of their environmental DNA, have already been found in the western basin of Lake Erie near the Sandusky and Maumee rivers. The research team intends to run similar studies to project the impacts of Asian carp on lakes Ontario, Huron and Michigan in future.

The results of future studies can’t come soon enough — 2015 saw a string of bad news on the carp front. Bumper crops of silver and bighead carp were born in the Upper and Lower Mississippi River systems and the Illinois River. Meanwhile, the fish moved 106 kilometres closer to Lake Michigan via the Illinois River. There, they threaten to bypass electric fences and other obstacles built to stop them in the Chicago Area Waterway System. Asian carp now swim approximately 122 kilometres from the Great Lakes.

Andrew Reeves is the Editor-in-Chief of Alternatives Journal. Overrun, his book about Asian carp in North America, will be published in Spring 2019 by ECW Press. His work has also appeared in the Globe & MailSpacing and Corporate Knights. Follow him on Twitter.

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