Zebra Mussel (Dreissena polymorpha) Effects on Diet and Condition of Early-Stage Fishes in the Hudson River

Invasion of zebra mussel (Dreissena polymorpha) in the Hudson River in the early 1990s sharply reduced phytoplankton biomass and markedly altered estuarine energy flow. Previous studies suggest that early-stage fishes of several species were negatively impacted by the invasion; one such species was the American Shad (Alosa sapidissima).  The objective of the present study is to assess feeding success indicated by condition index of shad over multiple years, including pre-invasion and post mussel invasion years. We hypothesized reduced feeding success and thus lower bodily condition in zebra mussel impacted years. Analysis to test this hypothesis is possible thanks to 21 years of early-stage fish samples collected as part of the Hudson River Utilities’ long-term monitoring program. Results varied with developmental stage. We found that condition of post yolksac larvae varied among years, such that condition was relatively high in some pre-invasion years and relatively low in some post-invasion years. Condition of post-metamorphic young-of-year shad was not clearly affected by year. We are conducting analyses of gut fullness and diet composition to complement analyses of condition.