Changes in Cormorant Populations in the Strait of Georgia, British Columbia, 1955-2015

Between 1987 and 2000, nesting populations of Pelagic Cormorant (Phalacrocorax pelagicus; PECO) and Double-crested Cormorant (P. auritus; DCCO) declined in the Strait of Georgia, BC. This northern section of the Salish Sea is a rapidly urbanizing area, and piscivorous birds are important indicators of ecosystem health. To update population status, we conducted a complete survey of 35 PECO and 23 DCCO colonies in July 2014 and opportunistic surveys of some colonies between 2001 through 2015. The PECO population decreased from ~2100-2400 nests in 1959-1987 to ~1100 nests by about 2000, and then rose slightly to ~1600 nests by 2015. The DCCO population increased from ~200 nests in 1959 to ~2,000 nests in 1987, before decreasing to ~600 nests in 2000, and then remained at this level through 2015. Many smaller colonies no longer exist and the majority of PECO and DCCO currently nest in three locations: Mandarte Island, Mitlenatch Island, and bridges in Vancouver. The main factors affecting population changes from about 1990 to 2015 include reduced prey availability, increased Bald Eagle (Haliaeetus leucocephalus) predation, and use of man-made structures for nesting at certain colonies. In 2013-2015, small numbers of Brandt’s Cormorants (P. penicillatus) nested at Mandarte Island. This is the first reported nesting of Brandt’s Cormorants nesting in the Strait of Georgia