Dammit, gannet! How they outwit scientists when sardines run out
The boffins thought an easy option would be to harvest waste from fishing vessels. The birds had other ideas
If you’re a Cape gannet and the sardines you love are in short supply, an easy option would be to harvest waste from fishing vessels.
But ornithologists who attached tiny GPS trackers to dozens of birds on Malgas Island, off Saldanha Bay, were surprised when the gannets sank their theory.
Over 13 days in October and November of 2017, sardine-starved gannets avoided fishing boats and changed their diet to a slightly less nutritious alternative, Atlantic saury.
Reporting his team’s finding in the journal PLOS One, lead author David Grémillet from the Percy Fitzpatrick Institute of African Ornithology at the University of Cape Town said it was a “strong indication” that gannets foraged in an “ecologically perturbed marine environment”.
He added: “[It] has important implications for our understanding of seabird resilience and conservation.”
The 8.3ha Malgas Island, 800m from the mainland in the northern part of the entrance to Saldanha Bay, hosts one of only six Cape gannet colonies. It is part of the West Coast National Park but off limits to humans other than selected researchers.
Grémillet’s team went ashore when the island’s 19,000 breeding pairs – down from more than 50,000 in the late 1990s – were raising chicks as young as two weeks.
They attached GPS trackers to adult birds with waterproof tape, and after retrieving them compared the data they had gathered with movements of fishing vessels. They also analysed food regurgitated by the birds for their chicks.
Their hypothesis was that “faced with the scarcity of their natural prey, adult birds would approach vessels generating fishery waste”. But the gannets had a better idea.
“[They] seldom encountered vessels and even more rarely attended them for prolonged periods,” said Grémillet.
“Surprisingly, they exclusively caught saury [which] occur irregularly and in small schools [and] are not exploited commercially in South Africa.”
Saury grow to between 35cm and 50cm long and often betray their presence by leaping out of the water. Their energy value to gannets places them between sardines and hake, which constitute most of the waste discarded by fishing vessels in the area.
“Diet calorific value strongly constrains gannet reproductive performance,” said Grémillet, adding this could explain the birds’ quest for more nutritious fish.
Their dietary flexibility allowed them to maintain their physical condition and chick growth rates. However, “prey switching is a strong indicator of reduced availability of sardines, which should be naturally abundant” in a region where the Benguela current upwells.
“A June 2017 sardine biomass survey found the second lowest measured since May 2008, amounting to just over 23,000 tons and substantially lower than the long-term average of around 143,000 tons.”