Identifizierung und Charakterisierung von Nahrungsgebieten von Seevögln in Auftriebsgebieten: Bedeutung von biologischen und hydrographischen Parametern für den Nahrungserwerb auf See

This thesis investigates the foraging behaviour of three seabird species, the African penguin (Spheniscus demersus) and the Cape gannet (Morus capensis) in the Benguela upwelling system as well as the Peruvian booby (Sula variegata) in the Humboldt Current. Biological and hydrographic parameters were considered when evaluating the characteristics of foraging areas and the behaviour of the species studied. Foraging areas used by the birds as well as the birds' diving behaviour were assessed by using data logger technologies and results obtained through these methods were combined with diet sampling and observations from ship-based surveys. Seabirds in both upwelling regions are confronted with changes in environmental conditions and the birds' reaction to alterations in hydrography as well as food limitations have been evaluated. African penguins in Namibia were studied at three breeding colonies which differ in numbers of breeding pairs as well as in oceanographic parameters due to distinct bathymetry. At Mercury Island, studies were performed in two consecutive years allowing for inter-annual comparisons, especially taking into account differences in food availability due to sulphide eruptions in 2005. African penguins at Mercury Island showed similar foraging behaviour as conspecifics in South Africa (Wilson 1985 ; Wilson et al. 1988; Ryan et al. 2004; Petersen et al. 2006), foraging within a range of maximum 32 km from the colony (median of 22 and 21 km in 2005 and 2006) and diving to maximum depths of about 75 m. Considering only dives deeper than 10 m depth (defined as foraging dives), most dives reached depths of 25 – 35 m. In both years, birds frequented the same area north-west of the island in order to forage. In 2005, sulphide eruptions caused hypoxic conditions in the entire water column, dissolved oxygen concentrations being as low as 2 ml/l in surface waters and below 0.2 ml/l at the sea bottom. Birds were almost exclusively feeding on pelagic goby (Sufflogobius bibarbatus), a demersal fish species able to tolerate low-oxygen conditions (Currie et al. submitted). In 2006, birds were feeding on anchovy (Engraulis capensis) and sardine (Sardinops sagax) in addition to pelagic goby. The higher percentage of pelagic prey was also reflected in the diving behaviour, birds diving to lower depths during foraging dives (median 35 vs. 27 m in 2005 and 2006, respectively). Studies at Halifax and Possession Islands included only small numbers of birds, but differences in foraging behaviour as well as in diet composition were observed and may be related to varying bathymetry around the islands. Food limitation seems to affect African penguins at all breeding colonies along the Namibian coast, not only during sulphide eruptions, and has to be considered as a key factor for the population decline observed in Namibia. In order to monitor environmental conditions, the use of seabirds as indicators has been proposed (Wilson 1992; Montevecchi 1993; Wilson et al. 1994; Weimerskirch et al 1995; Wilson et al. 2002; Fedak 2004). In this study, the use of data loggers enabled fine-scale temperature measurements during the diving in penguins. These in situ measurements of water temperature were included in an oceanographic model, showing small scale variations in spatial as well as temporal extensions. Cold upwelling cells were identified along the coast and the impact of local wind patterns on processes of temperature distribution became apparent. Relatively small numbers of logger-equipped birds would be needed for synoptical oceanographic monitoring around penguin colonies and would lead to a better understanding of oceanographic processes influencing seabird foraging. The ability to cover larger distances within single foraging trips and its implication for foraging in areas of reduced food abundance was studied in Cape gannets in the Benguela system. Lower prey availability during the breeding season 2005 due to sulphide eruptions led to diving activity over larger areas and birds were feeding on smaller prey individuals. Birds' foraging took place mainly along the coast but several birds covered large areas offshore. They foraged over water masses with different temperature regimes and presumably encountered a different prey composition over the shelf break. Birds reached distances of almost 200 km away from the colony and about 80 km off the coast. Site fidelity presented by several birds feeding at similar sites in consecutive trips indicates predictable prey but, as in the case of African penguins, numbers of gannet breeding pairs show declining trends in Namibia which is presumably also due to food limitation. Peruvian boobies breeding in northern central Chile were observed foraging almost exclusively in coastal waters. Food availability seemed to be favourable within large bays due to localised upwelling at protruding points and birds left for short foraging trips, which indicates high food abundance. A high variability in prey organisms obtained by the birds was observed between several years studied. Numbers of breeding pairs at Isla Pájaros varied strongly between years and breeding ceased completely during an El Niño year in 2002. Nonetheless, seabirds in the Humboldt upwelling system show fast population recoveries after strong El Niño events which indicates their adaption to this frequently occurring phenomenon. The importance of predictable food abundance in upwelling systems for seabird foraging is pointed out in this study. All species studied showed similar foraging behaviours, commuting between colonies and predictable feeding sites, foraging mostly over coastal upwelled waters. Additionally, site fidelity underlined the predictability of prey patches within the upwelling systems. Birds were able to switch to different prey items and to adapt foraging and diving behaviour to varying environmental conditions. Nonetheless, it may be doubted whether birds will successfully cope with alterations in the ecosystems that could lead to further reduced prey abundance in the future.

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