Mysis diluviana is an important mid-trophic level omnivore in many lakes, but studies of Mysis in Lake Champlain are rare. We used an unpublished 1975 study as a baseline to test for changes in contemporary Mysis populations in Lake Champlain. Invasive zebra mussels (Dreissena polymorpha) and alewives (Alosa pseudoharengus)were first reported in Lake Champlain in 1993 and 2003, respectively. Based on the negative relationships of these two species with Mysis and Mysis prey in the Great Lakes, we hypothesized a decline in Mysis in Lake Champlain since 1975 and tested this by repeating the 1975 study in 2012. We found a nearly ten-fold decrease in mean density (± SD) between 1975 (712 ± 373 individuals/m 2 ) and 2012 (82 ± 48 individuals/m 2 ; p = 0.002). Despite the decline, Mysis growth rates appeared similar between the two studies, although fecundity significantly increased by 3 embryos/female in 2012 (p = 0.002).Mysis vertical distribution appeared similar in both years, while the horizontal distribution appeared limited to deeper bathymetric strata in 2012 compared to 1975.Data from a long-term monitoring program from 1992 to 2008 indicate the decline occurred abruptly in the mid-1990s, which coincided with zebra mussel establishment although a direct link between the two is not evident. Alewife did not invade Lake Champlain until 2003 and can be ruled out as a contributing factor to the decline. We hypothesize that the combination of predation by alewife and smelt and shifts in the planktonic food web may prevent Mysis from recovering to pre-1995 densities.

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