The ecosystem of the Cape Gannet, a protected bird species, has gone haywire. As a result of overfishing, the birds are no longer able to find enough food to rear their young. Pelicans, kelp gulls and seals are becoming increasing threats – the lack of fish means that these predators are attacking Cape Gannet chicks more often. This has been revealed by research conducted by biologist Ralf Mullers. He will be awarded a PhD by the University of Groningen on 4 May 2009.
The Cape Gannet (Morus capensis) is a member of the same family as the pelican. The birds can grow to almost a metre and have a wingspan of nearly two metres. There are only six breeding colonies in the world – three in Namibia and three in South Africa. Since the 1960s, the number of breeding pairs in the colonies in Namibia has been decreasing due to overfishing of sardines and anchovies. In the last ten years, the breeding colonies on the west coast of South Africa have also been getting smaller. This is partly because the schools of anchovies and sardines have moved to the south and east coasts of South Africa.
For his research, Mullers spent four six-month sessions on the uninhabited islands of Ichaboe (Namibia) and Malgas (South Africa). Here he studied chick development, mapped the parents’ behaviour and researched the links between these variables. No fewer than 646 adult gannets were fitted with a GPS logger. This meant that the position of the birds could be precisely followed from minute to minute and it became clear whether they were flying, bobbing up and down on the waves or diving. Never before has the feeding behaviour of the Cape Gannet been so minutely detailed. It turns out that a Cape Gannet flies about 450 kilometres a day in search of food.
The decline in the colony on Malgas is due to the dangers the chicks are exposed to, Mullers discovered. Among other things, pelicans are a particular threat. These birds originally only ate fish, but due to the lack of fish they’ve become accustomed to eating other birds. They’ve also learnt to eat slaughterhouse waste, present in large amounts at neighbouring pig farms. Mullers: ‘Pelicans are originally protected birds too. Now one protected bird species needs to be protected against another one. On Malgas you can see entire colonies of gannets being destroyed by pelicans – they can even swallow chicks weighing almost two kilos.’
On Ichaboe (Namibia) chicks have a greater chance of survival. Although their parents have to fly further for food than the Malgas parents, they return with better quality food. Whereas the Malgas parents mainly bring fish waste back to their chicks, those on Ichaboe bring back more mackerel and pike to replace the anchovies and sardines. There are also fewer predators like pelicans, kelp gulls and seals on Ichaboe. It’s not yet completely clear why the colonies on this island are also declining in size.
Mullers’ research has revealed that parents do not risk their own lives to save those of their young. If there is less food available, they do not make longer foraging flights to ensure their young have enough to eat. Mullers: ‘From an evolutionary point of view that’s very sensible. A gannet can live to be twenty-five. If its chicks don’t survive one year, it has many more chances to produce descendents. It’s not going to risk those chances. It would be interesting to investigate whether the birds will adjust their behaviour if there is too little food for several years in a row.’
If extinction of the gannet is to be prevented, its foraging ranges must be protected. In other words, there must be a limit on the number of sardines and anchovies caught in the bird’s foraging range. Mullers: ‘There are naturally major economic interests involved. It would be great if my research could contribute to the debate on this matter.’
Ralf Mullers (Maastricht, 1975) studied biology at the University of Groningen. His PhD research was conducted under the auspices of the BCN research school and partly financed by the WOTRO institute (Academic Research in the Tropics) of NWO. Mullers’ supervisors at the Faculty Mathematics and Natural Sciences were Prof. S. Daan and Prof. J.M. Tinbergen. His thesis is entitled The commuting parent. Energetic constraints in a long distance forager, the Cape gannet. One of the ‘statements’ accompanying Mullers’ thesis reads ‘Living for years in a seabird colony leads to alcoholism, madness or a thesis’.
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