(2) Mussel Invasion, Alewife Collapse Doomed Salmon in Lake Huron

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.

http://ns.umich.edu/new/releases/23602-lake-huron-s-chinook-salmon-fishery-unlikely-to-recover-due-to-ongoing-food-shortage

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.
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*  UPDATE:

[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…

http://fox17online.com/2016/04/25/dnr-lake-michigan-salmon-are-declining-at-an-alarming-rate/

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(1) Alien Mussels Dominate Lake Bottoms, Scramble Ecology

--- first in a series ---

Hello, invasive zebra and quagga mussels. Goodbye: introduced salmon, invasive alewives, native Diporeia, diatoms and on and on.

Diporeia

Invasive quagga mussels have out competed their cousins, zebra mussels, at the bottom of Lake Huron and Lake Michigan. Quaggas tolerate colder, deeper water and thrive on mud bottoms. Zebras prefer to attach themselves to less abundant hard surfaces.

Both are efficient filter feeders and prolific breeders. Small quaggas can filter a quart of water daily. Billions of them cover the bottoms Lake Huron and Lake Michigan.

Diatoms are single cell plants, a form of algae, that make up part of the quagga’s diet. Diatoms envelop themselves in a hard shell of silica taken from the water.

The process is useful to mussel researchers as a marker of or proxy for total algal density, i.e. an extrapolation of the quantity of all algae in the water.

Diatom blooms occur in the spring. As the plants absorb silica and sink to the bottom, a reduction or drawdown of silica in the water is measurable. Less silica means more diatoms; more diatoms implies more algae in general.

In 2008, EPA researchers found that silica in the water was 80% lower than 30 years earlier, indicating a similar reduction in algae and coinciding with the proliferation of quaggas, which began in 2004 in Lake Michigan.

“By filtering out the algae, the mussels are robbing other organisms of the food they need to survive. Of particular concern is the plight of Diporeia, a tiny shrimplike creature that was one of the pillars supporting the base of the Great Lakes food web. Nearly every fish species in the Great Lakes relies on Diporeia at some point in its life cycle. But Diporeia populations have crashed in lakes Michigan and Huron, and the change is already impacting Great Lakes commercial fisheries and the sport-fishing enterprise.”

http://michigantoday.umich.edu/a7981/

It’s worth noting, however, that in Lake Huron, where the salmon/alewife-heavy ecology crashed first, native species like lake trout have rebounded and invasive gobies have entered the picture.

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