Ocean warming and long-term change in pelagic bird abundance within the California current system

Abstract

As a result of repeated sampling of pelagic bird abundance over 3 X lo5 km2 of open ocean 4 times a year for 8 yr we report that seabird abundance within the California Current system has declined by 40"0 over the penod 1987 to 1994 This decline has accompanied a concurrent, long-term increase in sea surface temperature The decline In overall b ~r d abundance is largely but not entirely, a consequence of the 90% decline of sooty shearwaters Puff~nus grlseus, the numeilcally dominant species of the California Current Seabirds of the offshore waters we sampled showed a different pattern from s e a b ~r d s of the shelf and slope waters Leach s storm-petrels Oceanodroma leucorl~oa, the commonest species offshore, significantly ~n c l e a s e d during 1987 to 1994, while sooty shearwaters and other inshore specles declined Thus the clearest pattein that emerges from our data is one of gradual but persistent changes in abundance that transpire at time scales longer than 1 yr Nevertheless we did find evidence of change at shorter time scales (weeks and months) that may relate to the El Nino episode of 1992 to 1993 P~onounced positive anomalies of abundance of brown pelicans Pelecanus o c a d e n t a ~i s and Heeiman S gulls Larus heerman! In fall 1991, and black Oceanod~onia melania and least 0 i n ~c r o s o n ~a storm-petrels in late summer 1992 likely reflect northward d ~s p e r s a l following reproductive failure in the Gulf of California