Birds Eat Birds as Fish Stocks Fall, Study Says

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Owing to conservation measures implemented over the last century—coupled with the rise of the commercial fishing industry and thus a steady supply of discarded fish waste—great skua populations are today robust and healthy.

"The circumstantial evidence suggests that quite a lot of seabirds have done well out of discards from fishing boats," Votier said.

Overfishing of cod and herring had also allowed populations of small fish that swim in schools, called shoaling fish, to swell. These fish also serve as easy prey to seabirds.

But cod and herring quotas have been reduced in recent years to allow those overfished stocks to recover. Cod and herring increases have not helped the sandeels, which cod and herring prey on. Sandeels have failed to recover despite the closure of the North Sea sandeel fishery.

In addition, with the reduction in discarded fish, Votier and his colleagues began to wonder about the impact on the seabirds.

Bird Eat Bird

By using long-term data on the diet of a great skua colony in the North Sea, Votier and his colleagues were able to show a direct link between the availability of fish waste and its consumption by the predatory birds.

In years when a lot of fish waste is around—the availability fluctuates from year to year, depending on the success of the commercial fisheries—great skuas eat a diet rich in fish waste. But "in years when discards are not readily available, they increasingly feed on other birds," Votier said.

A hypothetical calculation was performed by the researchers to show how many birds are consumed by skua when just 5 percent of their diet shifts from fish to their feathered fellows. It revealed a potential problem: Thousands of birds would be eaten.

For example, when applied to the well-studied North Sea colony, the hypothetical calculation showed that a 5 percent increase of bird in the skua diet results in nearly 33 percent of the adult black-legged kittiwake population being consumed.

Since seabirds like the kittiwakes, puffins, and guillemots have no readily available alternate food source to the dwindling discards and sandeels—i.e., they can't eat other birds—increased predation by great skuas may cause them to experience marked declines before the skuas do themselves, Votier and his colleagues conclude.

"The results suggest the utmost should be done to conserve sandeel stocks," Votier said. "It seems evident that if you have a fishery for sandeels, it might not help them a lot."

Conservation of sandeel, Votier added, is preferable to increasing fishing quotas as a means to ensure a steady supply of discards for the seabirds to consume, which serves as an artificial resource.

Dunn, with the Royal Society for the Protection of Birds, said that there are so many changes going on in the North Sea that he doubts a ban on fish discards would ever result in a reversion to its natural, pristine state. Nevertheless, he supports the call for a ban.

"My view, like the authors of the paper, is that mass discarding has disrupted and shifted the ecosystem and is not something to be welcomed, not least because it has in many ways been a symptom of unsustainable fishing practices," he said.

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