Foraging in Guanay cormorant and Peruvian booby, the major guano-producing seabirds in the Humboldt Current System

Abstract

The Humboldt Current System is the most productive upwelling system globally in terms of fish and sustains huge populations of guano-producing birds as well as the world's largest single-species fishery targeting the Peruvian anchovy Engraulis ringens. Peruvian boobies Sula variegata and Guanay cormorants Phalacrocorax bougainvillii are the most abundant seabird species in the system and feed mainly on anchovies. By using high precision GPS and time-depth recorders, we compared the horizontal and vertical movements of the 2 species to examine whether segregation occurs between their foraging areas and whether there is any evidence for prey depletion close to large colonies. A simultaneous acoustic cruise in the foraging area of the 2 bird species estimated the vertical and horizontal distributions of anchovy schools. During the study period in austral spring 2008, the 2 bird species foraged in the same area, at relatively short distance from the colony (average, 19 to 20 km). Both species foraged at similar times of the day, although Peruvian boobies tended to leave earlier in the morning and return later in the evening than did the cormorants. Foraging trips were similar in duration, distance covered and range. Peruvian anchovies were abundant and schools occurred at shallow depths (median, 7 m; range, 3 to 34 m), mainly to the north and west of the colony where foraging bouts of the 2 species overlapped extensively. Cormorants are mainly pelagic surface divers, diving at shallow depths (median, 4 m) but reaching up to 32 m depth, and are thus able to track all the depths at which anchovies occurred during the study period. Peruvian boobies are plunge divers able to reach occasionally 10 m (median, 2 m), and are thus able to reach only shallow anchovy schools. No sex-specific differences in horizontal or vertical movements occurred between males and females in the 2 species. We suggest that the absence of significant differences between horizontal movements of boobies and cormorants, the relative short duration of their trips and the high number of chicks fledged were probably explained by the high abundance and accessibility of anchovies. Differences may be exacerbated during years of low anchovy abundance or accessibility, especially El Nino years.