Trophic relationships among northern Lake Huron forage fish and zooplankton in 2007

The Lake Huron offshore food web has changed dramatically over the last 15 years.  In particular, there have been substantial declines in the abundance and biomass of offshore fish and zooplankton populations.  We sampled zooplankton monthly and major forage fish during four months near two ports in northern Lake Huron from May – October 2007.  We examined the diets of bloaters, rainbow smelt, ninespine sticklebacks, threespine sticklebacks, slimy sculpins and deepwater sculpins, captured from depths of 37 m and 91 m.  Zooplankton abundances and biomass peaked in July and were lowest in May.  Typically, calanoid copepods dominated the zooplankton assemblage throughout the study (≈ 60%), which was reflected in the diets of the fish numerically.  The diet composition varied among fish types both temporally and spatially, and numerical trends were not always consistent with trends in biomass (Mysis, chironomid larvae, and crustaceans dominated biomass alternately).  However, the biomass of food per fish stomach reflected the size of the fish.  Diet overlap varied amongst the fish species throughout the study and reflected preferred habitat.  Values were lowest in July consistent with highest zooplankton densities.  Food web patterns were consistent with patterns observed for the Lake Huron main basin since 2003.