Biologists have determined that marine animals might be losing weight because of feasting on "junk food" in the oceans, which is a result of human activities like overfishing and changes in the climate.
According to a report in New Scientist, as predatory fish such as cod have been removed from the sea in large numbers, fish lower down in the food chain, such as sprat, have increased in numbers.
But individually, the sprats weigh less, and these leaner fish, according to biologists, are effectively junk food.
"They are poor sources of energy for predatory birds and mammals, and as a result these animals are also losing weight," said Henrik Osterblom of the University of Stockholm in Sweden.
In the 1990s, Osterblom and colleagues noticed that 20-day-old common guillemot chicks in the Baltic Sea were lighter than they had previously been.
"We were very surprised because at the same time cod fisheries were increasing," said colleague Olof Olsson of Mistra, a Swedish environmental research foundation.
Less cod meant more sprat, and since guillemot feed sprat to their young, if quantity was all that mattered, the chicks should have been fattening up.
Osterblom's team found, however, that although there were more sprat, on average individual fish weighed less than before.
The researchers worked out that the sprat were competing for the same amount of zooplankton, and each one was getting less of it. As a result, the guillemot chicks were getting more fish, but fewer calories.
When, between 2000 and 2004, commercial sprat fishing in the Baltic Sea increased and the wild sprat stock dropped, guillemot chicks became heavier again. The Swedish team realised they had witnessed the junk-food hypothesis in action.
According to Michael Fogerty of the NOAA National Marine Fisheries Service, there have been concerns on the East Coast of the US about the quality of prey for the bluefin tuna also.
Herring play a large part in the diet of tuna, but Fogerty and his colleagues have noticed that the herring too have increased in number but decreased in weight. This has resulted in a corresponding decrease in the weight of the tuna.
Further evidence lay in the scientific literature: a meta-analysis of 47 studies of the impact of food quantity and quality on marine mammals suggested food quality is at least as important to the predator as quantity.
According to Andrew Trites, head of the marine mammal research unit at the University of British Columbia in Vancouver, Canada, "I am pleased to see Osterblom's group bring a larger body of literature together to evaluate the merits of the junk-food hypothesis."