Foraging behavior of a generalist marine top predator, Japanese cormorants (Phalacrocorax filamentosus), in years of demersal versus epipelagic prey

Abstract

To examine the behavioral adjustment of a generalist marine top predator to variability of their prey, we studied the foraging behavior of Japanese cormorants (Phalacrocorax filamentosus) breeding at Teuri Island, Hokkaido, in years of contrasting demersal and epipelagic prey composition. We used radio telemetry and ship-based surveys to determine behavior and at-sea distribution during three summers (1996–1998). The cormorants fed on epipelagic anchovy (Engraulis japonicus) and sandlance (Ammodytes personatus) in 1998 (year of epipelagic diet), while they fed on benthic rock fish (Sebastes spp.) and flatfish (Pleuronectidae) and nearshore-living naked sandlance (Hypophychus dybowskii), as well as epibenthic greenling (Hexagrammidae) in 1996 and 1997 (year of demersal diet). Cormorants engaged in larger feeding groups, visited more feeding sites, and stayed at each feeding site for a shorter period in the year of epipelagic diet than in the years of demersal diet. The cormorants made long foraging trips and fed in the mainland coastal habitat, distant from the colony, in the years of demersal diet. Individual radio-tracked birds fed over the wide area between the islands and mainland, in the year of epipelagic diet, while most individuals specialized in mainland or island coastal habitats in the years of demersal diet. Behavioral adjustment of Japanese cormorants might allow them to exploit both unpredictable epipelagic and predictable benthic prey efficiently.