Divergent trends in bank cormorants Phalacrocorax neglectus breeding in South Africa's Western Cape consistent with a distributional shift of rock lobsters Jasus lalandii

Abstract

In South Africa's Western Cape Province, numbers of bank cormorants Phalacrocorax neglectus breeding at 11 localities decreased from above 500 pairs during 1978–1987 to 350 pairs from 1995 to 2006. The most northern colony (Lambert's Bay) was extinct by 1999 and there were substantial decreases at the two largest colonies (Malgas and Dassen Islands), also north of Cape Town. There was long-term stability at three smaller colonies near Saldanha and fluctuating growth at Robben Island near Cape Town, where the colony was twice affected by oil spills. Between Cape Point and Cape Hangklip, two colonies were monitored and their numbers increased. Farther east, numbers at the Dyer Island group showed a slight decrease. These trends are consistent with a reduced abundance of West Coast rock lobsters Jasus lalandii in the north and an expansion of this resource to the east, but not yet as far as Dyer Island. Rock lobsters were the most important item in the diet of bank cormorants to the north of Cape Town.