THE ROLES OF INTRODUCED RATS AND COMMERCIAL FISHING IN THE DECLINE OFANCIENT MURRELETS ON LANGARA ISLAND, BRITISH-COLUMBIA

I examined tbe decline of Ancient Murrelets (Synthliboramphus antiquus ), a small, burrow-nesting seabird, at Langara Island. Tbe island's se abird colony was historically one of the largest colonies of Ancient M urrelets in British Columbia-perhaps in the world-with an estimated 20 0,000 nesting pairs I reviewed historical information and compared the results of surveys from 1981 and 1988 that employed the same census p rotocol. The extent of the colony, a potential index of population siz e, declined from 101 ha in 1981 to 48 ha in 1988. Burrow density incre ased during the same period, however; suggesting that the colony had c onsolidated. In 1988, the population estimate was 24,200 +/- 4000 (S.E ) breeding pairs compared to 22,000 +/- 3700 in 1981. In 1988, 29% of the burrows that were completely searched contained bones of Ancient M urrelets. Bones were most common in burrows located in abandoned areas of the colony and were least common where burrow occupancy was high. The discovery of adult Ancient Murrelets killed in their burrows by in troduced rats, combined with the high proportion of burrows with bones , suggests that rats (Rattus rattus and R. norvegicus) have contribute d significantly to the decline of the population. In addition, the pre sence and activities of a salmon-fishing fleet in the 1950s and 1960s may also be linked to the decline of the Langara Ancient Murrelet popu lation during that period because these fisheries are known to have ca used heavy mortality through fatal light attraction and drowning in gi ll nets. The combined effects of ongoing predation by introduced rats and-to a lesser extent--previous, episodic fishery-induced mortality a re probable causes for tbe population decline.