Foraging performance and reproductive success of Humboldt penguins in relation to prey availability

During the austral summer 1998/1999, we compared foraging ecology and reproduction between chick-rearing Humboldt penguins Spheniscus humboldti at 2 colonies in Chile. At Pan de Azúcar in northern Chile prey availability was assumed to be lower than at Puñihuil, 1500 km further south, according to oceanographic data. Fifteen adult penguins were equipped with temperaturedepth recorders and stomach-temperature-loggers in order to investigate foraging effort and foraging success in relation to differences in prey availability. There were no differences between colonies in diving behaviour. At Pan de Azúcar, foraging trips were longer than at Puñihuil (mean duration 36.5 ± 7.8 h vs. 19.0 ± 8.7 h), resulting in higher energy expenditure per foraging trip (on average 8092.7 ± 2216.7 kJ vs. 3934.5 kJ, range: 3071 to 7930 kJ). Catch per unit effort at Pan de Azúcar equalled 2.2 ± 1.4 g min–1 under water, which was significantly less than 10.0 ± 7.3 g min–1 at Puñihuil. As a result of higher energetic costs and lower foraging success, the gross foraging efficiency per foraging trip (GFE) was lower at Pan de Azúcar than at Puñihuil (on average 0.94 ± 0.52 vs. 4.61 ± 3.4 [kJ gained/kJ invested]). The chick growth rates at Pan de Azúcar (mass: 40.0 ± 18.4 g d–1 vs. 63.0 ± 23.0 g d–1; bill length: 0.32 ± 0.07 mm d–1 vs. 0.47 ± 0.12 mm d–1), as well as chick survival (40% vs. 100%) and reproductive success (0.33 vs. 0.81 fledglings nest–1), were lower compared to Puñihuil. A GFE threshold of 1.08 was calculated for chick survival at Pan de Azúcar. We conclude that due to physiological and morphological constraints the penguins at Pan de Azúcar could not increase diving effort to compensate for lower prey availability, but took extended foraging trips instead. Therefore, foraging efficiency and meal delivery rate decreased, resulting in less chick growth and lower reproductive success.