As the climate changes and fisheries transform the oceans, the world's African penguins are in trouble, according to researchers reporting in Current Biology on February 9. Young penguins aren't able to take all the changes into account and are finding themselves "trapped" in parts of the sea that can no longer support them even as better options are available.
"Our results show that juvenile African penguins are stuck foraging for food in the wrong places due to fishing and climate change," says Richard Sherley (@rbsherley) of the University of Exeter and University of Cape Town. "When the young of this endangered species leave the colony for the first time, they travel long distances, searching the ocean for certain signs that should mean they have found an area with lots of plankton and plenty of the fish that feed on it. But rapid shifts caused by climate change and fishing mean these signs can now lead them to places where these fish, the penguins' main prey, are scarce with impacts on their survival--a so-called 'ecological trap.'
"Protecting the penguins--and other species--from falling into similar ecological traps will require better action to account of the needs of predators in managing fisheries and concerted action to tackle climate change."
Sherley and colleagues, including Stephen Votier, also at the University of Exeter, and scientists from the Namibian and South African governments, made the discovery after using satellites to track the dispersal of newly fledged African penguins from eight sites across their breeding range. They wanted to find out whether the penguins were being trapped in what's known as the Benguela Current Large Marine Ecosystem (BCLME). The BCLME is one of four major eastern boundary upwelling ecosystems of the world, Sherley explains. This portion of the ocean stretches from near the Angola-Benguela front in southern Angola in the north to Cape Point in South Africa's Western Cape.
It has historically also been one of the most productive ocean areas in the world, rife with anchovies and sardines, which make good food for penguins and for people. In recent decades, overfishing in Namibia, heavy localized fishing, and subsequent environmental change have reduced the number of sardines and changed the areas that the fish use. In addition, fish and plankton are no longer reliably found together as they were in the past. The problem is that no one told the penguins.
"The penguins still move to where the plankton are abundant, but the fish are no longer there," Sherley says. "In particular, sardines in Namibia have been replaced in the ecosystem by lower-energy fish and jellyfish."
The researchers developed models to show that the penguins travel over thousands of kilometers to find areas where sea surface temperatures are low and chlorophyll concentrations are high, a sign that should mean plenty of plankton and the fish that go with them. The researchers don't yet know for sure, but they suspect that the penguins are responding to substances given off by phytoplankton when they are under stress, as occurs when they are being grazed on by predators.
"These were once reliable cues for prey-rich waters, but climate change and industrial fishing have depleted forage fish stocks in this system," Sherley says.
Young penguins that find themselves in the degraded Benguela ecosystem today often fail to survive. Their breeding numbers are about 50% lower than they would be if they found their way to other waters, where the human impact has been less severe, the new study shows.
Sherley and Votier say it might be possible to protect the penguins by translocating chicks to a place where it's not possible to get trapped. They say there is some evidence that fledglings from the colonies in South Africa's Eastern Cape generally don't get caught in the trap, at least not yet. There are other options, too, such as building spatial fishing closures in key areas where the penguins feed or otherwise increasing the number of sardines in the area.
Sherley says the South African government is working to implement spatially explicit catch limits and management practices in their sardine fishery, which will almost surely help. The researchers are helping to inform their decisions with the penguins in mind. Meanwhile, their work to understand how fishing influences the interactions between seabirds and their prey at different spatial scales and at different phases of the birds' lifecycle and how to protect them continues.
This work was supported by the Natural Environment Research Council, Bristol Zoological Society, Earthwatch Institute, Leiden Conservation Foundation, the Southern African Foundation for the Conservation of Coastal Birds, and the authors' institutions.
Current Biology, Sherley et al.: "Metapopulation Tracking Juvenile Penguins Reveals an Ecosystem-wide Ecological Trap" http://www.cell.com/current-biology/fulltext/S0960-9822(16)31536-6
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