by Steve Carpenter
Every moment, our brains make predictions of what we will next encounter in our lives based on our lifetime of experiences. As our experiences are updated, our brain adjusts and accounts for the new information in its next predictions. The discovery that brains engage in co-evolving cycles of predictions and actions is one of the great accomplishments of neuroscience.
In a microcosm, it represents the way science is made. Scientists routinely make predictions to evaluate ideas. Usually those predictions are wrong and, through these mistakes, we make the ideas better.
Occasionally, however, a scientist is confident enough to make a public prediction and hits the nail on the head. Center for Limnology director emeritus, Jim Kitchell and his colleagues, Don Stewart and Larry Crowder, made such a prediction in 1981, and now their paper is honored as a Fisheries Classic by the American Fisheries Society.
During the 1950s, Lake Michigan was a severely degraded ecosystem. Lake Trout had been decimated by overfishing and invasive sea lamprey. Two invasive fishes, alewife and rainbow smelt, were abundant. Although these forage fishes were small enough to be eaten by lake trout, in the absence of lake trout their populations exploded. Booms in alewife were followed by busts when the fishes died in droves and their carcasses littered the beaches.
Opportunity to control the forage fish populations came in 1965 when development of a selective toxicant brought sea lamprey under control and allowed the re-introduction of lake trout. In addition, managers stocked increasing numbers of non-native salmonids including chinook and coho salmon and rainbow trout. The booming salmon populations attracted anglers, supported a lucrative sport fishing industry, and consumed the invasive forage fishes, apparently a win-win-win situation.
Noting that “forage for salmonids in Lake Michigan is finite”, Kitchell and colleagues asked the radical question “How many predators are too many?” Using the brand-new tool of bioenergetics modeling, the scientists showed that salmon could consume nearly all the alewife by the late 1970s. This startling observation led to a key prediction: continued stocking of exotic salmonids would lead to decline of alewife populations and decline of salmonid growth rates. To maintain the valuable salmonid fishery, stocking rates would have to adapt to fluctuations in forage fish abundance. As if that was not radical enough, Kitchell called on managers “to view the entire Lake Michigan salmonid stocking program for the experiment that it is”, invest more in monitoring fish stocks, expect surprises, and use the booms and busts of fish stocks as an opportunity to learn and adapt.
The paper ignited controversy. Some folks agreed that salmonid stocking should be moderated. Others could not accept that predation could control an eruptive species like alewife in one of the world’s largest lakes. No consensus emerged, but stocking rates were decreased slightly while research intensified on forage fish and predator-prey interactions.
By the early 2000s, it was clear that Kitchell, Crowder and Stewart were right. Alewife declined in Lake Michigan, and completely collapsed in Lake Huron. Climate and other invasive species played a role, but severe predation by salmonids had driven their demise. Decline of this formerly-reviled invader was followed by declining production of salmonids. Unfortunately, the salmon decline caused significant economic losses for coastal communities dependent on sport fishing.
Instead of simply adding a piece to the puzzle of Lake Michigan’s fishery, the authors showed us the full picture and accurately predicted that basic bioenergetics would constrain predator-prey interactions, and that stocked predators could control prey even in the vast complex ecosystem of Lake Michigan. And the paper offered important cautions which, if followed, might have forestalled the decline of alewife and salmon in Lake Michigan. It is extremely rare for a scientific paper to be so right, which makes it no surprise that this prescient paper has been named a classic by the American Fisheries Society.
by Steve Carpenter