Factors influencing the foraging behaviour of African Penguins (Spheniscus demersus) provisioning chicks at Robben Island, South Africa

Doctoral Thesis


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University of Cape Town

Urgent and effective conservation is needed to halt the declines of endangered African Penguins. A purse-seine fisheries closure zone was in place out to 20 km around Robben Island for three years. It provided an opportunity to investigate penguin foraging behaviour in relation to estimated local prey abundance and other factors without the confounding variable of local fishing. Penguins provisioning chicks were equipped with GPS temperature depth devices for a foraging trip. Dive data (N = 75) and GPS tracks (N = 78) were collected from 78 penguins. Of those, 14 penguins were at-sea within two days of a fine-scale hydro-acoustic pelagic fish survey. Diet sampling, breeding success and chick body condition monitoring took place in the colony. Nests were followed to outcome. Morphological indicators were developed for sex determination and body condition. Intrinsic factors, brood mass, prey abundance, wave height and direction were explored in respect to foraging behaviour. Kernel density analysis identified foraging areas, confirming consistent use of the closure area. Annual differences in foraging effort were explained by variation in local prey abundance. The time the penguins spent diving and the distance travelled from the colony were negatively related to local prey abundance. There was greater variation in foraging distance when prey abundance was lower. Foraging areas and dive behaviour were similar for the sexes. The survival of chicks in the foraging study did not differ from chicks monitored in the colony for breeding success (control chicks). The trips of penguins with nesting success (N = 44) were on average 4.5 km closer to the colony than the penguins that had nesting failure (N = 21). Chick body condition in the colony was positively related to the Anchovy (Engraulis encrasicolus) mass percentage in the diet and the local pelagic fish abundance; condition was predicted to vary by 245% over the range of local fish abundance observed during the study period (0.5 to 187 thousand tonnes). Identifying ways to avoid depletion of prey resources around penguin colonies is important for conservation of the species.