(Katrin Ludynia, Richard Sherley, February 22, 2017)
When young African penguins leave their nests for the first time they do so alone, without any guidance from their parents. They need to use their instinct to follow cues in their environment to find food and stay alive in their first months at sea. Hard as that may have been in the past, today climate change and high fishing pressure have made it even more difficult.
For penguins in South Africa and Namibia, abundant supplies of their favoured prey, such as sardine and anchovy, are no longer where the penguins expect to find them. This causes the young birds to fall into what is known as an ecological trap. This is when they follow the usual cues to feeding grounds only to find that the sources of food in these places is no longer available. This can be due to changes in stocks of particular foods due to over-fishing or underlying environmental change
African penguins are listed as endangered by the International Union for Conservation of Nature, as numbers along their entire range in South Africa and Namibia have dramatically decreased in the last century and trends currently don’t show any sign of reversing. In the last 50 years, the population has dropped by 80% and there are only about 23 000 breeding pairs in the wild.
In our new study, we followed 54 juvenile penguins – penguins who have lost their down feathers and are now waterproof and ready to go to sea – on their initial journey along the southern African coast using satellite transmitters.
Dwindling fish stocks
The birds moved to areas of the ocean where sea temperatures are low and productivity – in the form of the phytoplankton microscopic food that is the base of many aquatic food webs – is high. To do so, they travelled large distances to areas such as St. Helena Bay along the West Coast of South Africa and Swakopmund in central Namibia. Both are historically known for their high fish abundance.
But large fish stocks no longer exist in these areas. This is because of the combined effects of the changing climate and fishing pressure. Since the lower levels of the ecosystem have not been affected in the same way, the signals that the penguins would have always used to locate their prey are still intact.
For example the phytoplankton is still there and is still preyed upon by zooplankton, microscopic animals drifting in the ocean. But today, the fish that would normally co-occur with their planktonic prey are scarce or absent.
Juvenile penguins are “tricked” into selecting the now poor habitat and fall into this large-scale ecological trap. This previously unnoticed ecosystem-wide phenomena explains the low survival chances of this endangered species, especially during its first year at sea. It contributes to the dramatic decline of the penguin population.
Modelling exercises in the current study showed that with sufficient food in these areas, the African penguin population on the West Coast of South Africa would be twice the size it is now. There would be around 5,000 pairs at Dassen and Robben Islands, instead of only around 2,500. Only juvenile penguins from the Eastern Cape colonies, located in Algoa Bay, foraged in an area which provides sufficient food, the Agulhas Bank.