The Cape Gannet: the perfectly adapted plunge diver

 

The Cape Gannet’s body is perfectly adapted to plunge dive for its prey. From a height of approximately 30m Cape Gannets plunge into the ocean at speeds reaching speeds of 100km/h and easily capture surprised fish near the surface. Gannets can hit the water at such speed due to air sacs in their face and chest which act like an airbag, cushioning the impact with the water. Their eyes are positioned quite far forward giving them binocular vision, which allows them to judge distances accurately. They also have no external nostrils, so during a deep dive water will not be forced in.

 

In South Africa, the primary threat to gannets is a shortage of their preferred prey, anchovy and sardine, due to the compounding effects of a changing climate and competition for their prey with fisheries. Many birds have taken to scavenging offal from fishing boats to compensate for the lack of their preferred prey, but this ‘junk food’ is far less nutritious than the birds’ sardine and anchovy staples. To be able to survive on this ‘junk food’ diet the birds need to forage a lot harder and consume more fish to meet their energy requirements. The increase in energy and time spent foraging and reduces it from other aspects, such as caring and feeding young, preening or resting. Altering their energy balance in this way can have severe consequences for breeding birds as self-preservation is prioritised over caring for their young, and chick starvation is a common cause of breeding failure. Chicks fed on a diet of junk food also grow more slowly and have a reduced chance of surviving to breeding age.

 

While gannets can to an extent compensate for a lack of preferred prey by foraging on fishing discards, this behaviour introduces the additional risk of entanglement and bycatch in the gear behind fishing vessels. Thankfully, interventions by BirdLife South Africa’s Albatross Task Force, such as the introduction of bird-scaring lines that deter birds from entering the dangerous areas behind boats, have reduced the risk of seabird bycatch by up to 95%. However, because we do not know where they birds go outside of the breeding season, we do not know if they are protected or how abundant their prey may be where they are spending this crucial life history stage.

For the first time, I aim to quantify the non-breeding distribution of Cape Gannets. We will deploy trackers on birds nearing the end of their breeding attempt at colonies along coast of South Africa. We will present these data to the relevant governmental and managerial structures as marine Important Bird Areas as areas in need of greater protection. Links between the foraging behavior of the birds (e.g. foraging 'hotspot' and residency time and links to future breeding success) and anthropogenic and natural drivers (e.g. bycatch, climate and prey removal by fisheries) will be presented to relevant governmental structures to be incorporated in South Africa's Ecosystem-based Approach to Fisheries management practices.