As the footballer Eric Cantona noted, when the seagulls follow the trawler, it is because they think sardines will be thrown in the sea - but British scientists have discovered that that isn't so true any more.
The reason: dwindling fish stocks - of cod, whiting and haddock - mean that fewer small fish are thrown back into the sea.
And so the bigger bird predators are instead attacking and feeding on smaller seabirds.
Researchers from a number of British universities have discovered that the schemes set up to protect the falling fish stocks in the North Sea are cutting the number of fish thrown back from trawlers.
That has meant that birds such as the great skua are going after black-legged kittiwakes, the northern fulmar, Atlantic puffin, common guillemot and the European storm-petrel in increasing numbers - which could have a dangerous effect on seabird populations, the researchers warn.
"Although it would not be appropriate to maintain current rates of discarding [of fish from trawlers] for the sake of seabirds, further drastic cuts in white fish catches in the North Sea will exacerbate the problem of great skua predation in the short term," the team notes in a paper in the science journal Nature published today.
Discarded fish are a surprisingly large source of food for seabirds: current estimates suggest there are 25 to 30 million tonnes of undersized fish discarded from trawlers every year.
The great skua, which lies at the top of the bird "food web" in the North Sea, has grown rapidly in numbers in the past century, partly because it is so large that it can fly the long distances required to follow fishing trawlers, and compete with other birds for the fish thrown back.
The team, led by Stephen Votier of the University of Glasgow, found that where there are more discarded fish, the skuas attack other birds less.
The cutback - to meet wider European needs to conserve fish stocks - of discarded fish could have a dramatic effect on the populations of other birds, the team estimates. If the number of skuas increases by 5 per cent, they might eat nearly one-third of the adult population of black-legged kittiwakes, according to a calculation that looks at the skua's energy needs.
But there is one possibility that could relieve the pressure on the smaller birds: the sandeel, a small fish which plays an important part in the diet of all of the seabirds and also of the cod and haddock.
Where sandeels are plentiful, the skuas tend to feed on those rather than birds - suggesting that there might be some hope to restore the balance of the North Sea's ecology.
Last month EU ministers continued a four-year ban on sandeel fishing off the north-east coast of Scotland, specifically to safeguard populations of puffins and kittiwakes. However stocks were found to be seriously depleted in 2003: Danish ships, which had a catch limit of nearly one million tonnes of North Sea sandeels, only managed to catch one-third of the allowance.
However, the findings of the latest study are likely to lead to calls for a wider ban across the North Sea on sandeel fishing - so future generations of skuas will have something to follow.Reuse content