The response of Grey Herons Ardea cinerea to changes in prey abundance

Capsule Herons responded to the lower abundance of a preferred fish by switching prey. Aims To investigate how Grey Herons respond to the decline in abundance of a preferred fish, the Round Goby Neogobius melanostomus. Methods Grey Herons Ardea cinerea breeding in the Gulf of Gdansk area (Baltic Sea) were the study population. Regurgitated food and pellets were analysed. Biomass and abundance of fish was compared between 2000-02 (when the population of Round Goby was increasing) and 2008-09 (when the Round Goby population stabilized). Results The abundance and biomass of Round Goby preyed upon by herons decreased from between 95 and 99% in 2000-02 to between 38 and 56% in 2008-09. Energy-rich salmonids, not recorded in 2000-02, were an important diet component in 2008-09 (42% of biomass). Small-sized fish abundance was higher in 2008-09 (37%) than in 2000-02 (9%). Conclusions Grey Herons in 2008-09 adapted to the lower availability of Round Gobies by prey switching to other available fish - abundant but small-sized Three-spined Sticklebacks Gasterosteus aculeatus and less abundant but more profitable salmonids (easy to catch in the case of hatchery-reared, tagged individuals). Diet composition suggests that Grey Herons in 2008-09 exploited more diverse foraging areas (dry habitats, woods, garden ponds) than in 2000-02.