A new study suggests that experts are divided on whether Asian carp would thrive in Lake Erie, but they appear more certain that an established carp population may not have much of an effect on some of the fishery’s most important species.
The study pulls together the best guesses of 11 Asian carp and Lake Erie fishery experts on how large an established population of bighead and silver carp might grow, as well as what effect those fish might have on the abundance of walleye, yellow perch, gizzard shad and rainbow smelt.
The experts’ opinions on how large an established Asian carp population could grow varied widely. Their compiled projections ranged from just 1.5 metric tons of Asian carp biomass per square kilometer up to a whopping 25 metric tons per square kilometer.
For reference, that upper bound is greater than the biomass of all of Lake Erie’s walleye, yellow perch, gizzard shad and rainbow smelt put together, said Marion Wittmann, they study’s lead author and a post-doctoral associate at the University of Notre Dame’s Department of Biological Sciences.
The ranges for walleye and the forage fish species, on the other hand, were much more narrow. And while experts predicted that walleye would probably decline slightly, there’s also a chance the species could benefit from eating carp and gain biomass. Yellow perch were projected to either stay the same or increase.
It’s also significant, Wittmann said, that none of the experts predicted any shifts for any species that would be more significant that what the lake has already experienced in the past two decades. That’s a span that includes the introduction of quagga mussels and the round goby, as well as other challenges to walleye productivity.
“That translates to: ‘Bighead and silver carp could take off, or they could not. But we’re fairly confident that the effects to these other four fish really won’t be worse than anything we’ve seen,'” Wittmann said.
If the results seem uncertain or contradictory, with boom-or-bust ranges and “could get better, could get worse” predictions, that was largely the point. The researchers asked the experts to give a range of numbers for each species rather than just one number because it’s important for managers working on Asian carp prevention programs to understand the uncertainty tied to any particular prediction, Wittmann said.
“If someone hands you a report or a study and it’s got this single number on it, it’s like, ‘Who in their right mind can actually estimate a single number, especially when it has to do with fisheries?
“Everybody knows — especially anglers know — that it’s uncertain,” Wittmann said. “You know there are going to be more fish some years and some time of year than others, but you won’t ever know exactly how many there are.”
Still, to keep uncertainty as low as possible, the researches weighted the experts’ responses using a method called “structured expert judgment,” which was created by study co-author Roger Cooke, a fellow with D.C.-based nonprofit research organization Resources for the Future.
The method is rare in the fisheries and conservation biology fields. Studies published using structured expert judgment typically have dealt with industrial issues like the probability of engine failure or bacterial infections in chicken processing plants, Wittman said.
Each expert was given a set of quiz-like calibration questions before they gave their responses on biomass. The better they performed on the first set of questions, the more heavily their biomass estimates weighed into the aggregated group projection.
That’s not to say than any of the experts were out of place in the lineup. The list included researchers From the U.S. and Ontario that work at universities, agencies and the commercial fishing industry. They all knew the lake and its fish well.
“We wanted people who are in Lake Erie, who touch the water and get it,” Wittmann said.
The study was published online by the journal Conservation Biology.