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The Use of Coded Wire Tags to Estimate Cormorant Predation on Fish Stocks in an Estuary

Abstract: 
The European conflict between the increasing population of cormorants Phalacrocorax carbo sinensis and the fisheries has remained strong over the last two decades. One of the main obstacles to a resolve or mitigation of the conflict is lack of documentation of the effect of the birds¿ predation on the fish stocks. Naturally, such documentation is difficult to produce, due to the complexity and openness of the ecosystems (lakes, coast-areas, rivers). Tagging and releasing fish with coded wire tags, to recover tags from cormorant pellets can be a viable method to measure the impact of cormorants on fish populations. This study focused on the predation in a shallow estuary, where nearly 100,000 fish were tagged and more than 10,000 cormorant pellets were collected in the 3-year study period. A total of 112 tags were recovered from the collected pellets. The analyses of tag recovery data showed a considerable cormorant predation on tagged Flounder, Eel and Salmon smolts. The method and the obtained results are discussed and considerations on sampling strategy and data analyses are presented. Results from eel, flounder and salmon are shown. The basic assumptions are discussed and recommendations for study design are given.