Influence of Cape fur seals on jackass penguin numbers at Sinclair Island

Both jackass penguins and Cape fur seals breed at Sinclair Island. A concrete wall 1m high was built around the penguin colony in 1940 to protect the penguins from the fur seals. Penguins and seals within the enclosure were counted on aerial photographs taken in 1967, 1971, 1976 and 1977and compared with published counts for 1948 and 1956. In 1967 the penguin count was one quarter of that for 1948 and 1956. Since then it has decreased more slowly. Few seals were within the penguin enclosure in 1956 but by 1967seals outnumbered penguins there. The latter increase is attributed to the removal of three gates in the wall during that period. The penguin counts for Sinclair Island are compared with those for two nearby islands without seal colonies (Plumpudding & Pomona).Since the penguin population on these two islands did not decrease dramatically until after 1969, the earlier decrease at Sinclair is attributed to interference by seals. Recommendations are made for the protection of this small penguin colony.