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Low Likelihood Of Chinook Salmon Resurgence In Lake Huron
Waiting for Lake Huron’s Chinook salmon fishery to come back? Don’t hold your breath.
According to a study led by scientists at the University of Michigan, the fishery will very likely never return to its glory days. Why? The lake simply can’t support the predatory fish’s main food source, the alewife.
Researchers made the find through a computer-modeling study that let them simulate a number of the lake’s dynamics. They suggest resource managers should focus on restoring populations of native fish, including lake trout, whitefish and walleye, and move on from chinook stocking in Lake Huron.
The effort also yielded findings for Lake Michigan, which researchers predict will likely experience an alewife collapse similar to the one that occurred in Lake Huron. From there, its Chinook salmon fishery would also be likely to collapse.
Scientists say the results serve as a reality check for those who continue to pressure resource managers to stock Chinook salmon in Lake Huron. They add the findings may also be seen as good news for native fish species and the restoration of the entire Lake Huron ecosystem.
The 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.
First came increased predation of alewives, due initially to heavier stocking of Chinook salmon and later the result of increased natural reproduction of salmon and a drop in sea-lamprey mortality. Predation of Lake Huron alewives by Chinook salmon likely peaked in the mid-1980s and then remained roughly constant until the alewife collapse, according to the new simulations.
Beginning in the 1990s, quagga mussels spread quickly at a time when the level of phosphorus flowing into the lake from rivers and streams was dropping in response to nutrient abatement programs initiated in the 1970s. Mussels in Lake Huron’s Saginaw Bay compounded the problem by sucking up and storing nutrients near the shore, preventing them from making it into Lake Huron’s main basin.
The loss of essential nutrients in the main basin reduced the amount of algae at the base of the Lake Huron food web. Zooplankton, tiny animals that feed on algae and that provide food for small fish such as alewives and rainbow smelt, suffered.
At the time, alewives and rainbow smelt were the two most important prey species for Chinook salmon in Lake Huron. The new computer simulations show that rainbow smelt suffered significant declines before alewives did, dropping 78 percent by 2002. Deprived of a favorite food, Chinook salmon began to rely more heavily on alewives, and this increased predation hastened the alewife population collapse.
This sequence of events can be used to assess the likelihood of an alewife and Chinook salmon collapse in lakes Michigan and Ontario, researchers believe. And there are already similar warning signs, including decreasing nutrient loads, a decrease in soft-bodied, bottom-dwelling invertebrates due to the mussels and decreases in rainbow smelt.
A paper summarizing the findings is posted online in the journal Ecosystems.
Top image: Chinook salmon in California’s Tuolumne River. (Credit: U.S. Fish and Wildlife Service)