Changes over three decades in feeding success of young American Shad Alosa sapidissima are influenced by invading zebra mussels Dreissena polymorpha

We analyzed the feeding ecology of young American Shad Alosa sapidissima over a 25 yr period in the Hudson River Estuary, USA, where population declines have been attributed in part to food web alterations associated with invasive zebra mussels Dreissena polymorpha. We hypothesized that feeding success was positively associated with the standing crop or abundance of lower trophic level organisms and negatively associated with indicators of competition for resources, including the grazing rate of zebra mussels. Based on reports of feeding by larval American Shad on zebra mussel veligers, we considered an alternative hypothesis that zebra mussels have had a positive effect on feeding success. To test hypotheses, we employed 2 metrics of feeding success: amount of material in the gut, and condition. Hypotheses were tested separately for 2 early life stages of American Shad: post yolk-sac larvae and postmetamorphic juveniles. In larvae, short-term feeding success was positively associated with chlorophyll a concentration and negatively associated with conspecific abundance. In juveniles, short-term feeding success was negatively associated with conspecific abundance. Condition of larvae was positively associated with chlorophyll a concentration and with copepod abundance; condition of larvae and juveniles was negatively associated with cladoceran abundance. Our results on feeding success generally support the inference of previous studies that zebra mussels reduced the availability of essential prey for early-stage American Shad, perhaps contributing to recruitment losses in the Hudson River population of the species.