The Swedish island of Stora Karlsö in the Baltic Sea became a nature preserve with regulated hunting back in 1880, and members of the Swedish bourgeoisie frequented the island in the early 1900s. By the 1920s, the private company that owns the island began offering daily tours. Nowadays, Stora Karlsö still receives thousands of visitors a year, and that means its seabird colonies have been photographed again and again for almost a century. 

In the absence of long-term scientific records, researchers studying amateur photographs have been able to track the rise and fall of the island’s common guillemots (Uria aalge), a fish-eating seabird that nests in breeding colonies. After suffering severe declines, their numbers are currently at a historic high, according to findings published in Current Biology this week. 

Stora Karlsö hosts two-thirds of the entire Baltic Sea population of common guillemots. These large, black-and-white auks were heavily depleted in the early 1900s because of intense hunting and egg collecting. But while those practices ceased, the birds then faced oil spills, chemicals, and tangles of fishing gear.

Jonas Hentati-Sundberg and Olof Olsson of Stockholm University collected 113 photographs spanning 37 years between 1918 and 2005 from national and regional archives, commercial stock photo agencies, and requests to the public in magazines, internet forums, and a local radio station. They complemented their collection with a photo documentation of the colony from 2006 to 2015. Using these photos, the duo counted the number of breeding birds each year based on 65 subareas. 

As expected, photos from the early 20th century showed the lowest numbers of breeding guillemots. Then, the use of pesticide DDT and environmental contaminant PCB may have caused the dip they observed in the mid-1960s and mid-1980s. Also during that time, Atlantic salmon fisheries using driftnets were expanding rapidly, and adult seabirds sometimes ended up as bycatch. 

The population has been growing since the 1980s, thanks to a combination of hunting regulations, fewer oil spills, decreasing environmental contaminants, and the decline (and recent prohibition) of driftnet fishing. Now, while many common guillemot populations are decreasing worldwide, the population on Stora Karlsö is more than five times bigger than it was in the early 1900s. They’re currently increasing at an unprecedented rate of about 5 percent a year. 

^{Image in the text: breeding Common guillemots on the island of Stora Karlsö in 1960. Gösta Håkansson/Gotland museum collection}