Abstract
There is a growing desire to integrate the food requirements of predators living in marine ecosystems impacted by humans into sustainable fisheries management. We used non-invasive video-recording, photography and focal observations to build time-energy budget models and to directly estimate the fish mass delivered to chicks by adult greater crested terns Thalasseus bergii breeding in the Benguela ecosystem. Mean modelled adult daily food intake increased from 140.9 g·d−1 of anchovy Engraulis capensis during incubation to 171.7 g·d−1 and 189.2 g·d−1 when provisioning small and large chicks, respectively. Modelled prey intake expected to be returned to chicks was 58.3 g·d−1 (95% credible intervals: 44.9–75.8 g·d−1) over the entire growth period. Based on our observations, chicks were fed 19.9 g·d−1 (17.2–23.0 g·d−1) to 45.1 g·d−1 (34.6–58.7 g·d−1) of anchovy during early and late provisioning, respectively. Greater crested terns have lower energetic requirements at the individual (range: 15–34%) and population level (range: 1–7%) than the other Benguela endemic seabirds that feed on forage fish. These modest requirements – based on a small body size and low flight costs – coupled with foraging plasticity have allowed greater crested terns to cope with changing prey availability, unlike the other seabirds species using the same exploited prey base.
Highlights
The balance between energy expenditure and food consumption determines many aspects of animal ecology, including the role of species within ecosystems and the mechanisms that drive population dynamics[1]
The decreased access to prey is considered to be the key driver of ongoing declines of three endemic seabird species: African penguins Spheniscus demersus, Cape cormorants Phalacrocorax capensis and Cape gannets Morus capensis[28,29,30,31]
Based on the duration and cost of activities performed by breeding adults, we modelled the daily energy expenditure (DEE) and daily food intake (DFI) of adults during different breeding stages
Summary
The balance between energy expenditure and food consumption determines many aspects of animal ecology, including the role of species within ecosystems and the mechanisms that drive population dynamics[1]. In the North Sea, for example, competition with the industrial fishery for lesser sandeel Ammodytes marinus is partly responsible for the low breeding success and population decline of black-legged kittiwakes Rissa tridactyla and several other seabird populations[25,26] Fluctuations in this key prey appeared to affect disproportionately small, surface-feeding species with high foraging costs, leading to the suggestion that such species – including terns – are sensitive indicators of deterioration in the state of marine ecosystems[27]. Estimating energy budgets for the Benguela’s breeding seabirds may help us to understand why numbers of greater crested terns are increasing while the region’s threatened and endemic seabirds that rely on the same resource are decreasing This information will improve our knowledge of food partitioning within the Benguela ecosystem food-web, provide a baseline against which to assess the impact of future environmental change, and assist the development of conservation planning
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