On the Case
Morten Frederiksen is tracing the precipitous declines of northern Scotland’s black-legged kittiwakes and the fish stocks that are their food. He hopes to finger the culprit, and help reverse the trend—before it’s too late.
|Morten Frederiksen holding a common murre.
|Centre for Ecology & Hydrology
Audubon: Can you give me an idea what's been happening with the seabird population in northern Scotland over the past several years? How dire is it?
Frederiksen: The three last breeding seasons (2004-2006) have been among the worst recorded in Scotland, with conditions particularly bad in the east in 2004 and 2006, and in the west in 2005 and 2006. The breeding problems have affected most seabird species. Most populations are still relatively stable in size, but black-legged kittiwakes have declined by more than 50 percent since 1990 along the east coast, and by more than 85 percent in Shetland.
Q: Why is there a food source problem? Is this problem temporary?
A: I wish I knew . . . The main food of most breeding seabirds in Scotland is sand lance, a small schooling pelagic fish. Sand lance recruitment has been very poor in the North Sea since 2002, and this is likely to be the direct cause of the breeding problems among seabirds. No one really understands why sand lance are faring so badly, although increasingly warm winters and a lack of suitable food at the right time of year have been implicated. So we don't know whether the problem is temporary or not.
Q: What do you mean by “sand lance recruitment?”
A: In fish biology, “recruitment” means the number of young fish joining the stock. It’s a combination of reproduction (the number of eggs spawned and larvae hatched) and survival over the first months of life.
Q: I know your winters have been warmer recently. What about the water temperature in the North Sea? Has warmer water temperature pushed the sand lance north?
A: Water temperatures in the North Sea have increased all year, with the largest increase in summer. However, studies suggest that sand lance are more sensitive to increasing winter temperatures, probably because they spawn in winter. Sand lance distribution is not known in great detail, so we can't say for sure whether they have shifted north, although many other fish have. In fact, winter temperatures in the North Sea show a reverse gradient, so that it is warmest in the north, where waters are deeper and where the major inflow of Atlantic water happens. So it’s entirely possible that warming in the North Sea could push sand lance south.
Q: Are we looking at a crash in the seabird populations? What would a crash mean?
A: For the moment, the only species where “crash” is a reasonable term is the black-legged kittiwake. If other seabird populations decline dramatically, such as the ever-popular Atlantic puffin, this could have implications for eco-tourism in Scotland.
|The black-legged kittiwake is a seabird common in Scotland.
Q: Is it possible that the seabirds could able to adapt to eating other food?
A: There’s little evidence to suggest that they can. In the last few years, the snake pipefish has increased dramatically in abundance around Scotland, and many seabirds have started to bring these long, bony fish to feed their chicks. However, they have a low nutritional value and are very difficult to swallow, and chicks often reject them or occasionally choke on them. So for the moment, we’re not optimistic about seabirds being able to switch to other prey.
Q: Is there a scenario whereby the birds would abandon their habitat and move in search of food?
A: Once they’ve reached the age when they start to breed—often at four to six years—most seabirds are extremely site-faithful, so are unlikely to move even though conditions deteriorate. However, some species regularly skip breeding in particularly poor years, and this could become more common if food continues to be scarce. It’s likely that birds are better able to track changes in the distribution of food outside the breeding season, but too little is known about the precise distribution of both food and birds during winter to answer this with confidence.
Q: Is there anything people can do to help this situation?
A: There’s not much people can do directly to help seabirds suffering from a lack of food. Closing industrial fisheries for sand lance close to colonies has benefited black-legged kittiwakes. We can also try to reduce or eliminate other threats so that birds can take full advantage of any increase in food availability. Potential problems include by-catch in fishing gear, oil pollution, nest predation by introduced mammals like rats and mink, and human disturbance in sensitive locations. In the long term, reducing the rate and extent of global warming is likely to be beneficial for marine ecosystems and for the seabirds that depend on them.
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Click here for a review of a recent documentary on the Bermuda petrel.
Feature story link to "Seeking Higher Ground"