Study tracks shift in nesting grounds as oceans warm
FRISCO — Scientist think climate change may be a key reason that thousands of seabirds are leaving their nesting grounds on an island in the Gulf of California and moving north.
In a new study, researchers from the University of California at Riverside looked at Isla Rasa, where more than 95 percent of the world’s population of elegant terns and Heerman’s gulls have traditionally nested.
In the past 20 years, the seabirds have abandoned the island and moved to other nesting grounds in Southern California including the San Diego Saltworks, Bolsa Chica Ecological Reserve, and Los Angeles Harbor.
The first big shift was observed during the strong 1998 E; Niño, when warmer ocean waters in the eastern Pacific resulted in a big downturn in oceanic productivity from Chile to California. The birds again abandoned the island in 2003, and then more frequently, in 2009, 2010, 2014, and 2015.
The biologists used nest counts to show that the birds have started moving into Southern California.
“Whenever the terns perceive the conditions in the Gulf as inadequate to ensure successful reproduction, they move to alternative nesting grounds in Southern California including the San Diego Saltworks, Bolsa Chica Ecological Reserve, and Los Angeles Harbor,” said said study leader Enriqueta Velarde, a researcher at Universidad Veracruzana in Jalapa.
The study showed that the move is related to the fact that, during the last 15 years ,the Gulf of California has been getting abnormally warm during some seasons.
“When the Gulf waters get unusually warm, the sea becomes capped by a layer of warm surface water and the upwelling of cold, nutrient-rich waters fails to reach the surface, said Exequiel Ezcurra, a longtime collaborator of Velarde and a professor of ecology at the University of California, Riverside, “Productivity declines and, with it, the availability of small pelagic fish, on which the seabirds feed, also falls.”
The collapse in food for the seabirds that results from warming waters is compounded by the reduction in sardine populations caused by intensive fishing in Mexico.
Overall, the scientists concluded that ocean warming and overfishing are driving the ecological collapse of the Gulf of California’s productive Midriff region.
The study was published last week in the AAAS journal Science Advances.