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1 March 2018 Breeding Population Sizes, Distributions, and Trends of Pelagic, Double-Crested, and Brandt's Cormorants in the Strait of Georgia, British Columbia, 1955–2015
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Abstract

After initial colonization of the Strait of Georgia in the late 19th and early 20th centuries, nesting populations and number of colonies of Pelagic Cormorant (Phalacrocorax pelagicus) and Double-crested Cormorant (P. auritus) increased in abundance until 1987, but subsequently both species declined by 2000. To update population status, we conducted surveys of 45 Pelagic Cormorant and 24 Double-crested Cormorant nesting colonies in 2014, and supplemented this survey with recent and historic counts conducted from 1955 to 2015. We modeled changes in population size over 6 decades to measure long-term population changes, and found strong non-linear trends that varied widely among colonies. Using years when all known colonies in the Strait of Georgia were surveyed, we documented that the Pelagic Cormorant population increased from 961 to 2234 nests in 1959–1974, remained fairly stable (approximately 2250–2450 nests) from 1974 to 1987, decreased after 1987 to approximately 1100 nests in 2000, and then rose slightly to approximately 1400 nests in 2014. The Double-crested Cormorant population increased from approximately 250 nests from 1959–1960, to approximately 1900 nests in 1987, before decreasing to approximately 600 nests in 2000, and remaining at this level through 2014. Many smaller colonies of both cormorant species no longer exist, and the majority of Pelagic and Double-crested Cormorants currently nest on Mandarte Island, Mitlenatch Island, and 3 bridge locations (Second Narrows, and Burrard & Granville) in Vancouver. The main factors apparently causing decline after 1987 are increased predation and disturbance by Bald Eagles (Haliaeetus leucocephalus), a potential reduction in availability of forage-fish prey, and human disturbance. Remaining large colonies offer some protection from eagle impacts (cliff habitats or man-made structures) or alternate avian prey for Bald Eagles, such as Glaucous-winged Gulls (Larus glaucescens) at Mandarte and Mitlenatch islands. In 2013–2015, small numbers of Brandt's Cormorants (P. penicillatus) nested at Mandarte Island. Continued monitoring is needed to track changes in populations and nesting locations and to inform management of cormorants in the Strait of Georgia.

Harry R Carter, Trudy A Chatwin, and Mark C Drever "Breeding Population Sizes, Distributions, and Trends of Pelagic, Double-Crested, and Brandt's Cormorants in the Strait of Georgia, British Columbia, 1955–2015," Northwestern Naturalist 99(1), (1 March 2018). https://doi.org/10.1898/NWN17-11.1
Received: 5 April 2017; Accepted: 1 November 2017; Published: 1 March 2018
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