Nine years after the Exxon Valdez oil spill: Effects on marine bird populations in Prince William Sound, Alaska

We compared post Exxon Valdez oil-spill densities of marine birds in Prince William Sound from 1989-1991, 1993, 1996, and 1998 to pre-spill densities from 1984-1985. Post-spill densities of several species of marine birds wer e lower than expected in the oiled area of Prince William Sound when compar ed to densities in the unoiled area. These negative effects continued throu gh 1998 for five taxa: cormorants, goldeneyes, mergansers Pigeon Guillemot (Cepphus columba), and murres. Black Oystercatchers (Haematopus bachmani) a nd Harlequin Ducks (Histrionicus histrionicus) exhibited negative effects i n 1990 and 1991. Loons showed a weak negative effect in 1993. Black-legged Kittiwakes (Rissa tridactyla) showed relative decreases in 1989, 1996, and 1998 which may have been caused by shifts in foraging distribution rather t han declines in populations. Glaucous-winged Gulls (Larus glaucescens) show ed positive effects in most post-spill years. Murrelets and terns showed re lative increases in 1993, 1996, and 1998. Generally, taxa that dive for the ir food were negatively affected, whereas taxa that feed at the surface wer e not. Effects for some taxa were dependent upon the spatial scale at which they were analyzed. Movements of birds and the mosaic pattern of oiling re duced our ability to detect oil-spill effects, therefore our results may be conservative. Several marine bird species were negatively affected at the population level and have not recovered to pre-spill levels nine years afte r the oil spill. The reason for lack of recovery may be related to persiste nt oil remaining in the environment and reduced forage fish abundance.