By Jeff Schaeffer, AFS Co-Chief Science Editor
The upper Laurentian Great Lakes were separated physically from the Atlantic coast by Niagara Falls, but constructed ship canals allowed marine invaders to penetrate the system. Invading Sea Lampreys Petromyzon marinus decimated native piscivores during the 1940s, which allowed Alewife Alosa pseudoharengus to proliferate to nuisance levels of abundance by the 1960s. Fisheries managers attacked the Alewife problem by introducing Pacific salmon Oncorhynchus spp. This worked, and created a wildly popular sport fishery that had not existed previously.
This was a classic “be careful what you wish for, because you may get it” conundrum. There was some evidence that Alewives were being controlled, and there was tremendous stakeholder pressure to maximize salmon populations because the novel sport fishery was actually revitalizing coastal communities. Indeed, Lakes Michigan and Huron resembled a cornucopia in that there was no apparent upper limit in terms of how many predators could be sustained.
However, in 1981, Don Stewart, Jim Kitchell, and Larry Crowder published a Transactions paper that suggested there was an upper limit to how many predators could be sustained. Using bioenergetics modeling (a rather novel concept at that time), they suggested that stocking programs had decoupled predatory demand from prey production, with possible destabilization of the predator-prey system and ultimately strong potential effects on Alewife biomass, with most consumption by Chinook Salmon O. tschawytsha.
Their work created a firestorm in that it was the basis for a wide range of subsequent applications of their method to evaluate other predator-prey systems (many of those manuscripts appeared in TAFS), but in the Great Lakes it began a controversy that raged for the next 25 years. There were some managers and researchers who began calling for more judicious stocking rates, while others expressed utter disbelief that a stocking program could actually impact prey dynamics within one of the largest freshwater ecosystems on the planet. No real consensus emerged, but a regional approach developed in which both predators and prey were assessed annually, stocking rates were reduced somewhat from initial levels, and predator-prey dynamics were studied intensively. This approach sustained the salmon fishery until 2004 when Alewife collapsed in Lake Huron, with concurrent decline in Lake Michigan. While there is still some debate regarding the role of climate and invasive effects as contributing factors, salmonid predation was implicated strongly as being severe enough to serve as an absolute slate-wiper for Alewife, with both stocked and feral fish contributing to a high predatory demand. And while salmon controlled a malignant invader, there was an economic burden placed on coastal communities, especially in Lake Huron where fishing effort declined sharply in many areas. On the other hand, Alewife absence was followed by strong recruitment of several native fishes that had experienced chronic low abundance for decades in the presence of Alewife.
We love this paper because it was so prescient, and it stimulated a broad range of new research efforts that led to great insight into how models can be useful in understanding predator-prey dynamics. And it began a compelling story about a bold management strategy that led to initial great success, but eventually experienced the most feared and unlikely outcome.
REFERENCE
Stewart, D. J., J. F. Kitchell, and L. B. Crowder. 1981. Forage fishes and their salmonid predators in Lake Michigan. Transactions of the American Fisheries Society 110(6):751-763. dx.doi.org/10.1577/1548-8659(1981)110<751:FFATSP>2.0.CO;2
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