|Alewife, a herring, average 10 inches in length|
-- second in a series --
Researchers have found that Lake Huron cannot provide a salmon sport fishery any longer because the alewife population on which the salmon fed has crashed. It seems likely that the same sequence is repeating itself in Lake Michigan.*
The best course now may be to restore native species like lake herring, lake trout, walleye and whitefish.
Scientists studying the subject are Sara Adlerstein-Gonzalez, Yu-Chun Kao and Edward Rutherford.
Alewives, a herring species, invaded the Great Lakes 60 or 70 years ago through the Welland Canal. They became the main food source for the salmon, steelhead, brown trout and hybrids that have been introduced in Lake Michigan and Lake Huron over the past 50 years.
The new study is the first attempt to use a food-web modeling approach to assess the various factors behind the 2003 collapse of Lake Huron alewives and the implications for future fish populations there. The total weight or ‘biomass’ of alewives in Lake Huron plunged by more than 90 percent between 2002 and 2003, and the exact causes of the collapse are still debated by anglers and biologists.
Some researchers have suggested the alewife collapse was mainly due to too much predation by Chinook salmon and native lake trout. Others say it likely resulted from a drop in food availability tied to the explosive spread of zebra and quagga mussels starting in the late 1980s.
The computer simulations in the new study show that the collapse was caused by a combination of predation and food limitation—and that predation alone would not have caused the crash. The spread of the non-native mussels, coupled with declining levels of the nutrient phosphorus entering the lake from rivers and streams, were essential factors, according to the new study.
Predation peaked in the 1980s, then remained constant until the alewife population crashed in 2003. Filter feeding by staggering numbers of invasive mussels reduced nutrients, including phosphorus, which had already been depleted in farm runoff as a result of improved agricultural practices.
[Chinook] Salmon are declining at an alarming rate [in Lake Michigan] according to biologists and the Department of Natural Resources.
“Because there’s less prey, the capacity to support the current Chinook salmon just isn’t there,” [Jay] Wesley said. “So one way we can address that is to lower the Chinook stocking. That will help reduce predation pressure on the remaining prey.”
As for invasive mussels, biologists are hoping they’ll reach a peak, allowing algae and plankton back into the water…
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