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Licensed control does not reduce local Cormorant Phalacrocorax carbo population size in winter

Abstract

Cormorants Phalacrocorax carbo have increased on European freshwaters, creating conflicts with fishing interests. As a result, control measures have been implemented in several countries, although their effect on the English population has yet to be determined. Wetland Bird Survey data was used to derive population growth rates (PGR) of non-coastal Cormorant populations in England. PGR was analysed in relation to control intensity at different scales (5- to 30-km radius) from 2001 to 2009 in order to determine (1) the extent to which control intensity (proportion of the local population shot per winter) was associated with site-level population change, and (2) whether potential effects of control intensity were evident on Special Protection Areas (SPAs). There were no clear differences in PGR when comparing sites which had experienced control versus sites where control had never been carried out. The few significant relationships between control intensity and Cormorant PGR detected were mostly positive, i.e. population growth was associated with higher control intensity. Control intensity was not related to Cormorant numbers in SPAs. Positive associations with control may arise because control is reactive, or because non-lethal effects cause greater dispersal of Cormorants. These results provide no evidence that Cormorant removal at local scales is having an effect on longer term (i.e. year-to-year) population size at a site level. They also suggest that control measures have not affected national population trends, although a better understanding of site use and movements of individual Cormorants needs to be developed at smaller scales (including those due to disturbance caused by control measures) to more fully understand processes at larger scales. Further research is also needed into the extent to which lethal and non-lethal effects of control on Cormorants are having the desired impact on predation rates of fish, and so help resolve the conflict between Cormorants and fisheries.

Zusammenfassung

Genehmigte Bestandskontrollen führen nicht zur Reduzierung lokaler Kormoranbestände Phalacrocorax carbo im Winter

Durch die Zunahme der Kormoranbestände in europäischen Binnengewässern sind Konflikte mit Interessen der Fischerei entstanden. Als Folge davon wurden in verschiedenen Ländern bestandsregulierende Maßnahmen eingeführt, obwohl ein Effekt auf die englische Population bisher nicht festgestellt werden konnte. Zur Berechnung von Populationswachstumsraten (population growth rates PGR) binnenländischer Kormoranvorkommen in England wurden Daten aus Wasservogelzählungen herangezogen. Für die Jahre 2001 bis 2009 wurden PGR im Hinblick auf die Intensität der Maßnahmen in verschiedenen räumlichen Skalen (5 km bis 30 km Radius) berechnet, um folgendes zu bestimmen: (1) das Ausmaß in dem die Maßnahmenintensität (Anteil der geschossenen Individuen an der lokalen Population pro Winter) assoziiert war mit gebietsbezogenen Populationsveränderungen und (2) ob die Maßnahmenintensität mögliche Einflüsse auf EU-Vogelschutzgebiete (special protection areas SPAs) hat. Beim Vergleich der Gebiete mit und ohne Durchführung bestandsregulierender Maßnahmen konnten keine klaren Unterschiede in PGR nachgewiesen werden. Die wenigen signifikanten Unterschiede zwischen Maßnahmenintensität und Kormoran PGR waren meisten positiv, d.h. das Populationswachstum stand im Zusammenhang mit einer höheren Maßnahmenintensität. Die Maßnahmenintensität stand nicht in Beziehung mit Kormoranzahlen in SPAs. Positive Folgen bestandsregulierender Maßnahmen können entstehen, da solche Kontrollmaßnahmen reaktiv wirken oder nicht-letale Effekte eine größere Dispersion der Kormorane bedingt. Diese Ergebnisse belegen nicht, dass Kormoranabschüsse auf lokalen Ebenen einen gebietsbezogenen Langzeiteffekt (d.h. in darauf folgenden Jahren) auf die Populationsgröße haben. Sie deuten auch darauf hin, dass Kontrollmaßnahmen landesweite Populationstrends nicht beeinflussen. Obgleich ein besseres Verständnis von Gebietswahl und kleinräumigen Bewegungen von Kormoranen (einschließlich solcher, die durch Störungen aufgrund von Kontrollmaßnahmen verursacht werden) erreicht werden muss, um die großräumigen Prozesse besser bewerten zu können. Weitere Untersuchungen sind nötig, um zu klären, in welchem Ausmaß letale und nicht-letale Auswirkungen bestandsregulierender Maßnahmen bei Kormoranen tatsächlich einen gewünschten Effekt auf Fisch-Prädationsraten bewirken. Nur so kann der Konflikt zwischen Kormoranen und der Fischerei gelöst werden.

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Acknowledgments

This work was funded through the Food and Environment Research Agency (Fera) by the UK Department for Environment, Food and Rural Affairs (Defra). Our thanks go to Tim Andrews and Ashley Smith from Defra and Graham Smith from Fera for their help and comments, and to members of the fish-eating birds policy review group and to Rhys Green and Sarah Dove (RSPB) for their comments. We are also indebted to Thomas Bregnballe, who provided highly constructive suggestions on a previous draft, and to Verena Keller for providing additional references. Spatial data on numbers of birds licensed to be and actually shot and advice on policy were provided by Natural England. Site-specific data on Cormorant numbers were obtained from the Wetland Bird Survey (WeBS), a joint scheme of the British Trust for Ornithology (BTO), Royal Society for the Protection of Birds (RSPB) and Joint Nature Conservation Committee (JNCC), in association with Wildfowl & Wetlands Trust (WWT), that aims to monitor non-breeding waterbirds in the UK.

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Correspondence to D. E. Chamberlain.

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Communicated by F. Bairlein.

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Chamberlain, D.E., Austin, G.E., Newson, S.E. et al. Licensed control does not reduce local Cormorant Phalacrocorax carbo population size in winter. J Ornithol 154, 739–750 (2013). https://doi.org/10.1007/s10336-013-0938-3

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Keywords

  • Culling
  • Disturbance
  • Fisheries
  • Human–wildlife conflict
  • Population growth rate
  • SPA
  • Wetland Bird Survey