Long days, long trips: foraging ecology of female rockhopper penguins Eudyptes chrysocome chrysocome at Tierra del Fuego

Abstract

In rockhopper penguins Eudyptes chrysocome, the relationship between foraging effort and the daylight period is unclear. We studied the foraging ecology of female southern rockhoppers that were brooding chicks at Bahia Franklin (Staten Island) in December 2000. We compare our data with data presented for other subspecies and locations, analyzing the features of their foraging ecol- ogy and its relationship with the daylight period around Staten Island, the population size of differ- ent breeding locations and the marine environment. Southern rockhopper penguins performed longer trips (about 30 and 60% longer for daily and overnight trips respectively) and between 5 and 7 more overnight trips (53%) than the northern and eastern subspecies. Mean dive depth and dive duration were 28.9 ± 24.6 m and 79 ± 30 s respectively. The diving rate (30.4 ± 5.0 dives h -1 ), and the proportion of trip duration underwater (66.2 ± 5.9%) confirmed the high foraging effort of rockhop- per penguins. During the study period foraging trip duration increased, but dive parameters did not vary. Penguins traveled as much as 20 km to their foraging waters, either in shelf waters and/or in waters of a close shelf-break and slope. The diet was dominated in numbers by euphausiids, amphipods, cephalopods, fish larvae and juveniles, all prey species commonly found in Subantarctic waters. The long trips could be explained by the long twilight period and by diel vertical migration of prey, confirming the phenotypic plasticity of penguins in response to different marine environments. The extended foraging trip duration may contribute to sustaining the high level of the breeding population (167 000 breeding pairs) at Bahia Franklin, and account for differences in the recent his- tory of different breeding colonies in the SW Atlantic Ocean.