What the Troubling Plunge in Seagull Numbers Means for the Survival of Killer Whales
Are seagulls the canaries in the coal mine of the fragile Pacific Northwest?
A new study published in the journal The Condor: Ornithological Applications reported that the number of glaucous-winged gulls, the most common seagull species found in the region, has plummeted by nearly 60 percent since 1986. The drop coincides with declines in other marine species, including fish-eating killer whale populations.
One reason why the endangered marine mammals are in trouble: fewer fish to eat.
“Gulls are an indicator of our coastal marine ecosystems,” the study’s lead author, Louise Blight of the University of British Columbia, said in a statement. “If they are experiencing a population decline, the gulls may be telling us that there have been some fairly profound changes to local marine ecosystems.”
Fish and shellfish once comprised nearly all of the gulls’ diet, but over time the birds turned to more land-based food, such as worms and garbage. “The things they prefer to eat are less available," Blight said.
“They bring Pacific herring and sand lance back to their nests,” Blight added in an email. “Sand lance aren’t well studied but there are trend estimates for herring out there.” Those trends have shown substantial declines, much like the decline of chinook salmon, the preferred prey of Resident killer whales.
The researchers looked at 100 years of seagull population data in the Canadian portion of the Salish Sea, which stretches from the north end of Canada’s Strait of Georgia to the south end of Washington’s Puget Sound. They found that the number of birds increased from 1910 through 1986 but then experienced a severe drop.
“We estimated the 2010 Georgia Basin population at about 5,600 nesting pairs, about 7,400 fewer pairs (-57 percent) than were breeding during the regional census conducted in 1986,” the authors wrote.
The study offered various hypotheses for the significant increase in population numbers from 1910 to 1986 and the dramatic decline since then.
In the early 20th century, humans collected gull eggs and hunted the birds, a practice that was largely halted by a U.S.-Canadian treaty in 1916. After people stopped killing gulls, the birds’ other main predators, bald eagles, suffered population declines through hunting and, later, exposure to DDT and PCBs.
Bald eagle populations have rebounded in recent decades. Could that explain the decline in seagull numbers?
Probably not, the researchers concluded.
“Gulls do not appear frequently in the diet of nesting eagles in the Georgia Basin,” they wrote. “Mobbing and aggression by gulls may reduce the susceptibility of colonies to eagle attack.”
That leaves the “food limitation” hypothesis. Feather samples collected since 1860 have shown a gradual reduction of fish in the gulls’ diet, the study said. Fish food shortages may also increase “egg cannibalism” in some colonies.
“Declines in egg and clutch [brood] size and nesting success have all been attributed to reductions in forage fish in other Larus gulls with similar life histories,” the authors wrote, “and these demographic vital rates have decreased over time in our study area.”
That’s why the seagull problem may be related to the threats faced by Salish Sea orcas. As the Native American saying goes: No fish, no blackfish.
“We need to be restoring ecosystems along the coast,” Blight said, “and that includes restoring fish populations."