There are around a billion seabirds around the world, including Atlantic puffins like this one carrying a beakful of sandlaces. (Science/AAAS)

Scientists say human fishing boats must leave a third of sardines, anchovies and other small fish in the ocean to save seabirds from declining.

A new study has found that one third of maximum fish populations is consistently the threshold around the world needed to keep populations of puffins, penguins, gannets, albatrosses and other seabirds stable.

The new findings published Thursday in Science could be "used as a guide to limit the amount of fish taken from the sea in order to maintain seabird populations in the long term," said Philippe Cury, a researcher at the French Research Institute for Development and the University of British Columbia fisheries centre who led the project.

There are about a billion seabirds in the world, said Michelle Paleczny, a University of British Columbia Master's student in zoology who took part in the international working group that did the calculation based on data from seabird colonies around the world.

Seabird population numbers have been falling for around 50 years and the availability of the fish they rely on the as food is thought to play a role.

"We know seabirds eat fish and they eat the same things that humans take out of the ocean," Paleczny said.

Up until now, fisheries managers have never taken that into account in setting fishing quotas.

"The reason in a lot of cases is because they don't have the data," she said. "They don't know how much these top predators need to eat."

To fill that gap, the researchers gathered data about the breeding success of 14 species of seabirds, including murres, gannets, puffins and terns, over 15 to 47 years and compared it to fish abundance near their breeding colonies around the world.

Paleczny said seabirds are long-lived species adapted to the fluctuations in the populations of the fish they eat. They will raise young only years when food is available.

"Every year, they'll lay an egg," she said. "If there's not enough food, they will abandon the egg or chick at a certain point."

The data showed that provided fish populations were greater than one-third of the maximum ever recorded, the number of chicks successfully raised by the birds wasn't affected by changes in their food supply.

But if the proportion of fish remaining declined below one third, the birds produced fewer chicks.

"We were amazed by the consistency of the relationship around the globe," Cury said in a statement.

The fish that seabirds compete with humans for are smaller species that have long been billed as more sustainable dinner choices than larger predators such as cod and tuna.

But Paleczny said the new findings shouldn't be taken as a cue to switch away from eating sardines and anchovies. Most small fish harvested aren't eaten directly by humans, but processed into fish meal and fish oil to feed to livestock and farmed fish, she said. That means populations of small fish — and seabirds — may be negatively impacted even more if people choose other sources of animal protein.

Paleczy acknowledged that the average person doesn't have a great concern for seabirds. But she said they are important indicators about the health of the ocean and fish populations.

She added that they include many magnificent species, such as albatrosses, boobies and terns: "They're not all seagulls."