Common Tern Foraging: Seasonal Trends in Prey Fish Densities and Competition with Bluefish

We present a method for making direct quantitative measurements of population dymamics of fish species preyed upon by seabirds. We studied foraging behavior of Common Terns (Sterna hirundo) in relation to density of fish prey of Long Island, New York. We used an echo sounder to record the abundance of small prey fish and large predatory fish near a tern colony. Prey fish abundance was highest when terns were feeding young chicks and declined sharply later in the season, coincident with the arrival of large numbers of predatory bluefish (Pomatomus saltarix). Mean depth of occurrence of prey fish was shallower after the arrival of bluefish. Terns fished in areas with high concentrations of prey, but there was no correlation between numbers of feeding terns and prey density. Number of feeding terns was inversely correlated with prey depth, however. Terns were strongly attracted to predatory fishes feeding near the surface. We suggest that in addition to concentrating prey fish near the surface, predatory fish may have been important as competitors.