Cape Town - The African penguin population in the Western Cape decreased by more than 50 percent between 2011 and 2013 – a result of their main food source shifting from the West Coast to the south and east.
Researchers started picking up a south and east shift in sardine, anchovy and rock lobster populations in the late 1990s.
In two papers published recently, in Frontiers in Ecology and Evolution and in The Journal of African Ornithology, lead author Rob Crawford said there was growing evidence that an altered environment, possibly driven by climate change, was driving these movements of marine populations.
However, it was difficult to say exactly why these species had moved, and to disentangle the influence of the environment from the effect of humans, particularly the effect of fisheries.
The researchers reviewed information on the distribution of numbers of certain seabirds that bred in South Africa.
They found that there were winners and losers among the bird populations in response to the environmental changes.
Four seabirds feed mainly on anchovy and sardine: the African penguin, the Cape gannet, Cape cormorant and the swift tern. These birds’ main food source are also the fish species that provide the mainstay of the purse-seine fishing industry.
The movement of the anchovy and sardines to the south and east resulted in a “mismatch” between the spots where some of these seabirds had their breeding colonies and where their food sources had moved to.
The populations of five bird species that feed mainly on anchovy, sardine or rock lobster, all decreased markedly. Three of these – the African penguin, Cape cormorant and bank cormorant – had particularly large decreases.
All of these birds show what researchers call “behavioural inertia”: their foraging ranges are restricted when they are breeding.
However, the Cape gannet and the swift tern showed increases in population. Both these species are able to range over wide areas in search of food, and move between breeding colonies.
Swift terns rapidly shifted their breeding from the northern West Coast in the mid-2000s to the southern Western Cape.
The researchers said because many fishing plants were located on the north-west coast, it was likely that there was increased competition between seabirds and fisheries for prey on the West Coast after these fish populations started to move south and east.
Cape Times