Is seabird's population rise down to cod's decline?

The remarkable breeding success of Britain's most prolific seabird may be a sinister warning, rather than the sign of a healthy ecosystem, a world expert on the common guillemot believes. Their steadily increasing numbers may be caused by a decline in the marine environment, Professor Tim Birkhead says.

The remarkable breeding success of Britain's most prolific seabird may be a sinister warning, rather than the sign of a healthy ecosystem, a world expert on the common guillemot believes. Their steadily increasing numbers may be caused by a decline in the marine environment, Professor Tim Birkhead says.

Overfishing of British waters could have caused a "regime shift" in fish stocks, and the sprats on which the guillemot lives may have become more plentiful, because the larger fish which preyed on them have themselves been heavily fished.

In 1992, the huge cod fishery on Canada's Grand Banks, off Newfoundland, collapsed from overfishing, and despite a halt to fishing, has never recovered. Smaller fish and marine creatures which prey on cod eggs and larvae - and which were eaten by adult cod - are now believed to have increased and established a new dominance which has stabilised the Grand Banks cod population at a much lower level.

Guillemots, cliff-dwelling chocolate-brown seabirds, began increasing in numbers on Britain's coasts in 1980 after a 30-year decline. That may be linked to oil spills, because guillemots are more vulnerable to oiling than other seabirds, and are always the major casualty in any oiling incident on northern European coasts.

But in the past 25 years the bird's population has risen from 650,000 to 1.9 million, making it the most common British seabird. Professor Birkhead, professor of behavioural ecology at the University of Sheffield, and his colleague Dr Ben Hatchwell run the longest and most detailed survey of a guillemot colony, on the island of Skomer off Pembrokeshire, in south Wales.

Guillemots have been counted on Skomer every year since 1961. But what makes the Sheffield-run survey unique is that for 20 years, the histories of 8,000 individual birds identified by coloured rings have been followed.

In many bird species, huge numbers of young do not survive until the following summer to breed. Professor Birkhead and his colleagues believe that the survival rate of the juvenile guillemots is 50 per cent, astonishingly high, because something happened in the marine environment to favour guillemots.

It is probably an increase in the supply of sprats, the small, highly nutritious members of the herring family on which Skomer guillemots feed almost exclusively. But the sprats may have increased because larger fish which fed on them - such as cod - have greatly declined.

The researchers hope funding from the Countryside Council for Wales and the Natural Environment Research Council will allow the study to continue, to find if global warming is affecting the changes.