Cape gannets in contrasting environments: ehaviour, demographics and indicators of environmental change

• Main Author: Ryklief, Rabiah
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: Nelson Mandela Metropolitan University 2017
• Subjects: Endemic birds > South Africa Marine pollution > South Africa; Bird Island
• Online Access: http://hdl.handle.net/10948/21167; http://vital.seals.ac.za:8080/vital/access/manager/Repository/vital:29452

Marine communities represent biological networks, where physical and chemical changes in the marine environment can influence the physiology and behaviour of marine organisms. Stochastic marine conditions can alter key biological interactions and thus, influence the mechanisms that underpin population demography and phenotypic plasticity. Marine top predators, such as seabirds, are often controlled by bottom-up processes and their ability to cope with variable ocean conditions lies in their physiological and behavioural response to such perturbations. Seabirds are central-place foragers during the breeding season making them particularly vulnerable to environmental fluctuations as they face constrains associated with reproductive effort in addition to self-maintenance. The Cape gannet Morus capensis is a medium-sized pelagic seabird that is endemic to southern Africa. The Cape gannet has experienced considerable change in population distribution and abundance over the past 50 years. These changes have been linked to a combination of fishing pressure and climate-mediated regime shifts of their preferred prey, i.e. sardine Sardinops sagax and anchovy ngraulis encrasicolus. Historically, Malgas Island was the largest Cape gannet colony in South Africa. However, it is now in a state of population decline, despite its occurrence within one of the most productive upwelling systems in the world, the Benguela Current. At present, Bird Island is the largest Cape gannet colony and has recently stabilised, despite regular immigration of gannets from the west coast. Furthermore, Bird Island lies within the Agulhas bioregion and is subject to low levels of productivity from the nutrient-poor Agulhas current. Malgas and Bird islands are the two most populous of the six extant Cape gannet colonies. Birds at these colonies provided a natural experiment due to the contrasting population trajectories and environmental conditions at their colonies. The foraging ecology and behaviour of Cape gannets were studied during the breeding season to assess the mechanistic links connecting climate and fisheries to population demography. Cape gannets displayed considerable inter-annual variation in foraging effort and distribution across a five-year period. This highlights the importance of long-term datasets to contextualise the degree of natural variation within a species, as well as considerations for conservation management. For the first time, oceanographic variables such as sea surface temperature and chlorophyll-a, indices that reflect marine productivity, were used to predict Cape gannet foraging distribution. Furthermore, Cape gannets at Bird Island had an energy budget deficit in a year when foraging effort was much higher in comparison to what was observed at Malgas Island. This in turn was linked to the first identification of nutritional stress in a Sulid species. Reasons for this include a combination of higher intra-specific competition in conjunction with a lower energetic content of referential prey species at Bird Island. Foraging effort showed more variability at Malgas Island in comparison to Bird Island, likely due to Malgas Island gannets often taking two short foraging trips a day, whereas Bird Island gannets often spent the night at sea. Some degree of phenotypic plasticity was evidenced by substantial variations across breeding seasons and ocean basins in foraging effort and behaviour.