Chatham Island shag  (Phalacrocorax onslowi)

It is not clear why the Chatham Island shag population is declining so rapidly, but it has been noted with interest and concern that the largest and most successful breeding colonies of this sea bird are found in areas that are not disturbed by the various introduced species living on the Chatham Islands. The colonies of Chatham Island itself are plagued by humans, cattle, pigs, sheep, cats, dogs, weka (Gallirallus australis) and brush-tailed possums (Trichosurus vulpecula). Disturbance is thought to cause the birds to trample on their nests; breaking the eggs before abandoning the site. Breeding sites have also been abandoned following the arrival of the fur seal (Arctocephalus forsteri) on Star Keys Island. Small numbers of birds are shot or caught in crayfish pots, and it is thought that changes in the marine environment may have affected the availability of fish species (2).

A census of the Chatham Island shag was taken in 1997 and a follow-up was done in 2003-2004 as part of the Chatham Island Shag and Pitt Island Shag Recovery Plan that was brought into effect in 2001. It recommends that the population as a whole is surveyed every five years, but that a closer eye is kept on the colonies of Chatham Island, with a census each year. Authorities are hoping to come to an agreement with local land and stock owners that would enable fences to be built around breeding colonies. Dog owners need to be educated as to the effect their pets can have on the Chatham Island shag and eco-tourists must be similarly supervised. It may be necessary to gain legal protection for all colonies (2).