Predator or prey? Cape Gannets on Bird Island

by CapeNature

Every year tourists at Bird Island stand in awe at the sight of the thousands of Cape gannets (Morus capensis) at the breeding colony between September and April.

Lambert's Bay Bird Island

Lambert’s Bay Bird Island is home to over 20 000 Cape Gannets

But the situation was once very different. Zanri Schoeman, MSc student at Nelson Mandela University takes up the story. “The 2005/06 season was devastating for the Cape gannets as Cape fur seals (Arctocephalus pusillus) cut a swathe of destruction through the colony and caused the gannets to abandon their breeding attempt. It took the colony many years to recover to the state that it was prior to this incident.

Zanri Schoeman conducting research on Bird Island

The purpose of Zanri’s study was to identify the various drivers of Cape fur seal predation rate on Cape gannet fledglings (at sea) and Kelp gull (Larus dominicanus) predation rate on Cape gannet eggs (in the colony).

Kelp gull stealing a gannet egg

Cape gannets are endemic to the waters of southern Africa and they are classified globally as endangered. Predation on the gannets is therefore a concern as the Cape gannet’s breeding range currently extends to only on six islands globally.

Seals preying on gannets

To answer the research questions, Zanri used CapeNature’s data, conducted her own observations, and obtained hydro-acoustic survey data from the Department of Agriculture, Forestry and Fisheries (DAFF). One of the study’s main findings suggested that, in the years with increased fish biomass, both a decrease in Cape fur seal predation rate and an increase in Cape gannet breeding success followed.

A pair of breeding gannets

The study emphasized the importance of fish conservation in helping to conserve this seabird species. The research will also help to improve management decisions in terms of:

1) minimizing Kelp gull predation on Cape gannet eggs as it was identified where and when it takes place in the colony

2) minimizing Cape fur seal predation on Cape gannet fledglings as it was identified where and when it takes place in the water surrounding the island

The study thus ultimately promotes the conservation of this top predator in the ecosystem. To further the investigation on how to improve the management of predation on this species Zanri will start her PhD in 2020. “I look forward to spending more time to investigate where and how this endangered species needs our help to turn their odds against extinction”, Zanri added.

We look forward to to seeing the outcome of your further research Zanri.

Published in Care for Nature

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