Herring and Sprat Abundance Indices Predict Chick Growth and Reproductive Performance of Common Terns Breeding in the Wadden Sea

Herring (Clupea harengus) and sprat (Sprattus sprattus) are the key prey resources of common terns (Sterna hirundo) breeding in the Wadden Sea. Breeding success of the terns has been below average since 2002, coinciding with exceptionally low herring recruitment and sprat abundance. Time series of herring and sprat abundance in the North Sea and in the Wadden Sea were analyzed to explain long-term breeding success and chick development at two common tern breeding colonies. North Sea herring recruitment and sprat abundance in the Wadden Sea explained the largest part of common tern breeding success, both as single variables and in a multiple regression approach. Breeding success showed stronger correlations with herring recruitment indices derived from the North Sea region compared to the Wadden Sea. Also, herring and sprat abundance data explained more variability in breeding success than of more directly responding measures such as growth rate and maximum weight of chicks. Despite spatial and temporal incoherences between fish surveys and the common tern breeding season, breeding success of common terns reflected the abundance of their key prey fish beyond their foraging range and breeding season. We argue that the ecological connectivity between large- and small-scale herring abundance and the responsiveness of common tern breeding success is strong enough to establish a fish-seabird indicator system to be potentially valuable in monitoring and conservation.