Tuesday, August 27, 2013

Fish Populations are Changing in Lake Superior

After posting the blog update a week or so ago about how climate change is impacting Lake Superior, I decided to research whether or not there is any evidence that the warmer temperature of the lake water is impacting or changing fish populations.  Unfortunately, the answer is yes.  Newly published research has found that Lake Superior's warming water is probably already affecting its most abundant big fish.
 Increasing water temperatures over the last three decades have made conditions more favorable for Chinook salmon, walleye and lean lake trout but less favorable for cold water-loving siscowet lake trout.  
 Unlike the lean lake trout, obesity is a natural state for siscowet trout that has a tendency to develop more body fat (40-70 percent) than lean lake trout (about 10 percent).  
 The study, using a mix of computer modeling and temperature measurements, estimates that fatty siscowets have lost about 20 percent of their historic habitat because of the temperature changes that have already occurred.   Although the larger lake trout has increased in numbers relative to their drastic decline due to the Lamprey eel during the 1950s and 1960s, their recovered population may now be threatened by the warmer temperatures as well as by the decrease in their preferred food source -- there is less smelt and other feeder fish in the lake. 
 The research was conducted by the University of Wisconsin-Madison, funded by Wisconsin Sea Grant as well as the University of Minnesota Duluth's Large Lakes Observatory who found that average Lake Superior surface water temperatures increased almost five degrees F (2.5 degrees C) between 1979 and 2006, among the most dramatic examples of climate change in North America.   Since then, the rapid warming phenomenon has not only intensified in Lake Superior but also been found in other big lakes, including Lake Baikal in Russia. In 2010 research showed that the average temperature taken at three Lake Superior buoys was the highest in the 31 years of records.  Lake Superior deserves attention because these are some of the biggest temperature changes that have been seen anywhere on the planet.
Siscowet prefer temperature over 39 degrees Fahrenheit (4 C), while lean lake trout, the more sought-after species by anglers, prefer about 50 degrees Fahrenheit. Researches found that the number of days with preferred temperatures and the amount of water available within the preferred temperature range has increased significantly for lean lake trout, salmon and walleye.  Meanwhile, higher water temperatures have forced siscowets to move farther from shore.  Lean lake trout and salmon gained about 11,583 square miles of habitat in their preferred temperature zone while walleyes gained a whopping 19,305 square miles. But siscowet trout lost about 3,861 square miles, or nearly 20 percent, of their historical habitat.
Overall, the fish harvest in all the great lakes has drastically declined over the last few decades.


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