麻豆传媒

OVER-FISHING and changes in climate could leave marine animals struggling on a 鈥渏unk food鈥� diet of low-calorie prey, putting dwindling populations under even more pressure.

That is the conclusion of a team lead by Henrik 脰sterblom of Stockholm University in Sweden, who explored the controversial 鈥渏unk food hypothesis鈥� in 47 marine species.

Andrew Trites of the University of British Columbia in Vancouver, Canada, came up with the hypothesis when studying the collapse of Steller sea lions off Alaska. Trites believes changes in atmospheric and ocean circulation have favoured one of the sea lions鈥� prey, the low-fat pollock, at the expense of the higher-fat herring, meaning the sea lions just can鈥檛 catch enough calories to survive.

鈥淐hanges in ocean circulation favoured the pollock instead of the higher-fat herring so sea lions don鈥檛 get enough calories鈥�

Trite鈥檚 idea has been hotly debated, with several studies blaming the sea lions鈥� plight on increased predation by killer whales and humans. But the new analysis suggests that regardless of the situation with the sea lions, the junk food hypothesis should not be thrown out with the trash.

鈥淔or a long time I have felt that my laboratory has been a lone voice pointing out that quality of prey is just as important as quantity of prey,鈥� says Trites. 鈥淎ll fish were not created equal.鈥�

For example, 脰sterblom and colleagues noticed in the 1990s that common guillemot chicks in the Baltic Sea were lighter than they had previously been. This was surprising since the population of the guillemot鈥檚 favourite fish, the sprat, was actually on the rise because its predator, cod, was being heavily fished. So if quantity was all that mattered, the chicks should have been fattening up.

They found that although there were more sprats around, each one weighed less because they were all competing for the same amount of zooplankton, and each one was getting less. As a result, the guillemot chicks were getting more fish but fewer calories. Then in 2000, when the commercial sprat fishery increased, the chicks began to beef up again. The Swedish team realised they had witnessed the junk food hypothesis in action.

They found more evidence of the hypothesis hidden in the scientific literature. A 2002 study of pigeon guillemots showed that chicks fed a high-energy diet had a greater chance of survival. Western gulls fed a calorie-dense diet are also more successful breeders (Oikos, ).

Ultimately, whether quantity or quality matters most may depend on how the predators get their food. The common guillemot can only carry one fish at a time, so high-quality food is a must. Quality is probably also key for smaller, younger animals that eat less, say the authors.

This sentiment rings true for Michael Fogerty of the NOAA National Marine Fisheries Service. 鈥淭here have been concerns on the east coast of the US about the quality of prey for the bluefin tuna,鈥� he says. They prefer herring but Fogerty and his colleagues have noticed that while there are more herring, the fish are lighter. This is linked to a decrease in the weight of the tuna, which are already experiencing a dramatic collapse.

Endangered species 鈥� Learn more about the conservation battle in our comprehensive special report.