Response of seabirds to thermal boundaries in the tropical Pacific: the thermocline versus the Equatorial Front

Our 10 yr study is the first to associate seabirds with oceanic boundaries, or fronts, in tropical oceans. It addresses the semi-permanent Equatorial Front, hereafter 'front', a vertical boundary lying between the South Equatorial Current (SEC) and North Equatorial Countercurrent (NECC) in the eastern tropical Pacific (ETP). We also consider the subsurface, horizontal thermal boundary created by the thermocline, Our study area included the front and the north and south boundaries of the SEC and NECC respectively, between 100 and 145 degreesW. Our data were based on 45 north-south crossings of this area. The intensity of the front differed with season and phase of the El Nino Southern Oscillation (ENSO). Densities of planktivorous seabirds, higher at the front than in the SEC or NECC, increased with front intensity (i.e., during autumn/La Nina). Densities of piscivorous seabirds were unaffected by the front, but were related to depth and intensity of the thermocline, Piscivore densities were highest in the NECC, and higher during spring than autumn. Furthermore, they were higher in the western part of the study area, i.e., when and where the thermocline was deepest and most stratified, We suggest that the difference in patterns observed for the tropical planktivorous versus piscivorous seabirds reflects processes that affect the distribution and availability of their respective prey. Zooplankton are concentrated in the surface layer by physical processes that structure the front. Prey fish, on the other hand, are made available by foraging tuna, which drive them to the ocean surface. As a result, the distribution of the piscivorous seabirds is closely tied to that of schooling surface-feeding tunas, that are most abundant in the NECC. Other studies have shown that tunas spend most of their time near the thermocline. We suggest that this, along with the patterns observed among the piscivorous seabirds, indicate that the tuna's prey aggregate near the thermocline. Specifically, most tropical squid and small fishes perform vertical movements to the surface at night. A deeper thermocline, such as that of the NECC, is likely to be attractive to these animals for hiding during the day (when the tuna and birds forage) because of the lack of light at depth, and because of the enriched O-2 content of the cold-water layer lying immediately below the warmer surface layer. Hence, by searching on a horizontal plane near the boundary created by a deep, well stratified thermal structure, schooling tuna, and thus piscivorous seabirds, are likely to find concentrations of prey more readily than equally abundant prey dispersed throughout a less stratified water column.