UC Santa Cruz

Our aim was to examine the foraging behavior of northern elephant seals Mirounga angustirostris during the 1997-98 El Nino and compare it to foraging in others years. Given their deep diving and spatial distribution, their immediate response to a severe El Nino was expected to give insight into the timing, scale and magnitude of El Nino Southern Oscillation impacts on a large marine predator. Time-depth records and Argos-linked satellite tracks were obtained from adult females departing on post-breeding foraging migrations from 1990 to 1999, including females foraging during the 1998 El Nino. Rates of mass gain and trip duration were recorded for females from 1983 to 1999. Movement tracks of females in 1998 were similar to those observed in non-El Nino years. Rate of mass gain at sea was 0.29 +/- 0.36 kg d(-1) in 1998, the lowest measured since 1983. Marked declines in the mass gain rate of females were noted in severe El Nino years, but not in moderate El Nino years. Females increase spring foraging trip duration to compensate for decreases in foraging success. In 1998, the frequency distribution and temporal pattern of dive shapes suggested reduced residence time in prey patches and increased travel time between patches and these parameters showed a strong relationship with rates of mass gain. Our data confirm that the immediate ecological impact of the 1997-98 El Nino was not limited to the near-shore coastal margin, but extended far out into the North Pacific Ocean.