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RESEARCH ARTICLE
Contents Vol 61(3)

The use of coded wire tags to estimate cormorant predation on fish stocks in an estuary

Niels Jepsen A E , Reinhard Klenke B , Per Sonnesen C and Thomas Bregnballe D
+ Author Affiliations
- Author Affiliations

A European Commission – Joint Research Centre, Institute for Environment and Sustainability, Building 48C(TP 483), Via Enrico Fermi, ISPRA, I-21020, Italy.

B Helmholtz Centre for Environmental Research – UFZ, Department for Conservation Biology, Permoser Str. 15, D-04318 Leipzig, Germany.

C DTU-Aqua, Vejlsovej 39, 8600 Silkeborg, Denmark.

D National Environmental Research Institute, Department of Wildlife Ecology and Biodiversity, Grenåvej 14, Dk-8410 Rønde, Denmark.

E Corresponding author. Email: nj@aqua.dtu.dk

Marine and Freshwater Research 61(3) 320-329 https://doi.org/10.1071/MF09038
Submitted: 25 February 2009  Accepted: 21 August 2009   Published: 29 March 2010

Abstract

One of the main obstacles to resolving the conflict between an increasing population of cormorants, Phalacrocorax carbo sinensis, and the fishing industry is the lack of documentation of the effect of the birds’ predation on fish stocks. Tagging and releasing fish with coded wire tags followed by intensive cormorant pellet sampling may be a viable method to measure the impact of cormorants on fish populations. To test this new method, we studied cormorant predation in a shallow estuary, where nearly 100 000 fish were tagged and more than 10 000 cormorant pellets were collected over a 3-year study period. A total of 112 tags were recovered from the collected pellets. Analyses of tag recovery data indicated considerable cormorant predation on tagged flounder, eel and salmon smolts, but the method did not deliver high-quality documentation, mainly because of limitations in pellet sampling. We conclude with recommendations to enhance the value of this method.

Additional key words: conflicts, eel, flounder, Phalacrocorax carbo sinensis, smolt.


Acknowledgements

The study was supported by the 5th EU-framework project FRAP, the national Danish fishing licence fund and Ringkøbing County. Several technicians and students from Ringkøbing County, DTU-Aqua and NERI assisted with the fieldwork. All handling and tagging of live fish were conducted in accordance with the guidelines described in permission from the Danish Ministry of Justice (2005/561-987). The forest district of Oksbøl gave permission to enter the cormorant colony for sampling. Brendan Ebner and an anonymous referee are sincerely thanked for very detailed and constructive comments.


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