CHICAGO, July 8 (Xinhua) -- The ongoing warming of Lake Michigan in the United States increases its susceptibility to Asian carp, in part by reducing the capacity of quagga mussels to act as an ecological barrier against the voracious algae-eating fish, according to a study led by researchers from the University of Michigan (UM).

Focusing on the two species of Asian carp, bighead and silver carp, the study looks at how three factors, namely climate change, nutrient management and the arrival of zebra and quagga mussels, have affected Lake Michigan's vulnerability to bighead and silver carp, which currently reside in adjacent watersheds but not in the lake itself, according to the study published Tuesday in the journal Biological Invasions.

The simulations of historical conditions showed that Lake Michigan would have been more hospitable to Asian carp in the 1970s and 1980s, when levels of the nutrient phosphorus were much higher and zebra and quagga mussels had not yet been established. The simulations show that bighead carp would have grown roughly two to five times faster in the 1980s, compared to current conditions, with higher phosphorus levels being more influential than the absence of the mussels.

When the researchers simulated the levels of climate warming expected in coming decades, Asian carp habitat improved in two fundamental ways: the fish fed more actively in warmer waters during the spring, and the annual summer "stratification period", when lake waters separate into three distinct layers due to temperature and density differences, started earlier and lasted longer, limiting the time that top-feeding Asian carp and the bottom-dwelling mussels would compete for food in the upper portions of the water column.

Great Lakes surface temperatures are already warming faster than the global average increase, with Lake Michigan's surface waters warming at the rate of roughly half a degree Fahrenheit per decade. The duration and extent of winter ice cover across the Great Lakes is also shrinking, and climate models suggest the summer stratification period in Lake Michigan and other northern temperate lakes will grow longer and start earlier in the years ahead.

Earlier stratification would benefit Asian carp by blocking the mussels' access to food in surface waters and would also open up previously unsuitable offshore habitat to the fish earlier in the year, according to the new modeling study. In the UM-led study's simulations, warm years roughly doubled the volume of suitable habitat available to bighead carp in the spring and increased the volume of suitable springtime habitat for silver carp by nearly one-third.

Phosphorus levels emerged as the most influential of the three factors analyzed in the study of Asian carp habitat suitability across Lake Michigan. The study's "High P" scenarios resulted in the longest growing seasons for Asian carp, the greatest quantity of suitable habitat and the highest final fish weights.

The impact of Asian carp on already invaded ecosystems, as well as the proximity of the invasion front to Lake Michigan, has elevated concerns about the potential invasion of the Great Lakes via the Chicago Area Waterway System, the man-made connection between the Illinois River and Lake Michigan.

In 2019, the U.S. Army Corps of Engineers sent Congress a plan to install carp defenses about 40 miles from Lake Michigan in Illinois. The project, which could cost more than 800 million U.S. dollars, is awaiting congressional approval.

"Our new research demonstrates that the presence of an already entrenched food competitor, the mussels, has a limited capacity to act as a barrier to a bighead and silver carp invasion, and that climate warming and nutrient enrichment could further diminish the mussels' ability to affect Asian carp habitat," said study lead author Peter Alsip of the Cooperative Institute for Great Lakes Research.

In the 1970s, the United States imported Asian carp from China to resolve the problem of plankton. In the early 1990s, Asian carp got into the Mississippi River and since then they have expanded and multiplied rapidly. Enditem

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