Top predators are relevant indicators of the ecological status of a system and can have a high impact on food webs. But top predators are difficult to include in network analyses because their biomass in ash free dry weight or carbon content ismissing. Regression equations were determined for the relationships between fresh weight and dry weight, ash free dryweight, carbon and nitrogen contents respectively for six of the most abundant bird species in theWadden Sea (Calidris canutus, Limosa lapponica, Haematopus ostralegus, Chroicocephalus ridibundus, Larus canus, Anas penelope) and harbor seals (Phoca vitulina). The relationships for all specieswere interpreted as linear through the origin. Carbon content vs. freshweight ratios for birds ranged from0.16±0.01 to 0.22±0.02. Carbon content vs. fresh weight ratio was 0.17± 0.02 on average for harbor seals. This work highlights that the biomass of top predatorswas often over- or underestimated in previous studies. The determined conversion factors will be useful for future studies to generate more realistic food web models.