Trends in Great Lakes Mysis Populations from 2007 to 2011

Mysis were collected as part of the EPA’s Great Lakes monitoring program.  A 1x1 m net was deployed at any station that was visited at night from 2007 to 2011.  The net was lowered to within 2-3 m of the bottom and pulled to the surface, duplicate tows were conducted.  Mysis were counted and measured and densities calculated.  Lake Ontario generally had the highest densities among lakes, followed by Michigan, Superior, Huron, and Erie. In Lakes Superior, Michigan, and Ontario, there were fluctuations in populations over the 5-yr period, but no apparent dramatic changes in abundance.  However, in Lake Huron, Mysis populations in spring were 10-15/m2 during 2007-09, increased in 2010 to 44/m2, then declined to 5/m2.  In summer densities were higher, but still mimicked the spring trends, since populations were 20-30/m2 during 2007-2009, increased to 56/m2 in 2010, then declined to 20/m2 in 2011, suggesting quagga mussels/fish predation may have depressed populations during 2011.  Comparisons with Lake Huron Mysis abundance data from 1971 indicated densities were considerably lower during the early 2000s.  There was a rough linear relationship between Mysis densities and depth in all lakes; more were usually collected in summer than spring.  Length-frequency histograms showed an abundance of 4-5 mm individuals in spring, and a fewer larger organisms, while in summer some 4- mm individuals were still found, but the spring cohort was now much larger 7-9 mm.  Few Mysis were caught in Lake Erie and all were small <7 mm.