Abundance, biomass, and macrophyte consumption by rudd in Buffalo Harbor and the Niagara River, and potential herbivory by grass carp

The invasive rudd and grass carp consume aquatic macrophytes, thereby altering important habitat, creating novel trophic pathways, and adversely affecting indigenous species. We estimated the abundance and biomass of rudd and their consumption of macrophytes in Buffalo Harbor (northeastern Lake Erie) and the upper Niagara River during 2012-2013. We also estimated consumption of macrophytes by hypothetical populations of grass carp equal in biomass to rudd populations. Using mark-recapture methods, we estimated that 2571 (95% CI: 2362-2821) rudd were present in Buffalo Harbor and 142,957 (95% CI: 135,127-151,751) were present in the upper Niagara River. Biomass of rudd was estimated at 1.89 mt (95% CI: 1.73-2.07) in Buffalo Harbor and 100.21 mt (95% CI: 94.72-106.38) in the upper Niagara River. Using observed water temperatures and published consumption rates, we estimated that (1) rudd could have consumed 98 mt of macrophytes in Buffalo Harbor and 5210 mt in the upper Niagara River, and (2) hypothetical populations of grass carp equal in biomass to rudd could consume 96 mt of macrophytes in Buffalo Harbor and 4941 mt in the upper Niagara River. Rudd abundance and biomass are substantial in these two waters, and consumption of aquatic macrophytes by rudd may threaten existing aquatic habitat and restoration projects. Annual standardized surveys of fish and macrophyte assemblages would provide data necessary to monitor rudd populations, detect invasion and reproduction by grass carp, and assess habitat changes resulting from herbivory by these two invasive fishes.