Selective predators or choosy fishermen? Relation between fish harvest, prey availability and great cormorant (Phalacrocorax carbo sinensis) diet

Abstract

The increase in numbers and range of the continental subspecies of great cormorant Phalacrocorax carbo sinensis in Europe has led to a growing number of conflicts with commercial fisheries and recreational angling. We investigated the dietary preferences (fish species and size classes) of wintering cormorants in two freshwater basins in Northern Italy, using pellet analysis and data on fish community composition. First, we compared cormorant diet (biomass of fish species consumed) with the annual harvest by professional fishing (biomass/year). Second, we compared diet with a measure of prey availability: direct fish sampling with nets of different mesh size. Cormorants preyed mainly on cyprinid fish. A comparison of pellet analysis and commercial fishing data showed that some fish species consumed by cormorants did not occur in the commercial harvest. Using only species that occurred in the diet and in the commercial harvest, there was no relationship between the amount of fish of different species taken. A positive correlation between pellet analysis data (consumed) and data from scientific fish sampling (available) showed that cormorants exploited fish resources on the basis of prey availability. Comparing the size of fish collected from nets and fish consumed by cormorants also showed that birds were selective with respect to prey size, picking mostly small individuals (< 110 g). We conclude that potential effects of cormorants on commercially important fish will mainly depend on habitat type (fish ponds, natural deep lakes, others), and that in the natural lakes in Lombardy cormorants mainly concentrate on species which are of limited value for commercial fishing.

Keywords:

• Diet composition
• fish community
• food choice
• prey availability
• prey–predator relationship

Acknowledgements

This paper is dedicated in remembrance of Guido Tosi, our colleague and friend. We are grateful to Cesare Puzzi and Stefania Trasforini, working for the GRAIA Institute for fish research and management, who organised haul fish samplings in the Varese study area. Constructive comments by three reviewers greatly helped to improve the manuscript. We appreciate the technical assistance of Carlo Romanò, fishery technician of Como Provincial Administration; we also thank the environmental technician staff of Varese and Lecco Provinces. Peter Lurz from Edinburgh revised the English. The work was financed by a research grant of Insubria University.