Sea-level warming forces seabirds to abandon nesting grounds

Warming oceanographic conditions and fishing pressure are driving nesting seabirds away from their ancestral breeding grounds in the Gulf of Mexico into California harbours, a study has found.

Isla Rasa, in the Gulf of California, is renowned for its massive aggregations of nesting seabirds. However, during last couple of decades, the seabirds have arrived at the island in April, as they usually do, but leave soon after - without nesting.

"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 Enriqueta Velarde, project leader and researcher at the Universidad Veracruzana in Jalapa, Mexico.

Velarde and her team analysed the nesting process of Elegant Terns (Thalasseus elegans), a model species to monitor ocean dynamics and found that the species expanded from the Gulf of California, in Mexico, into Southern California during the last two decades.

The reason for the migration is to be found in global warming.

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. Productivity declines and, with it, the availability of small pelagic fish, on which the seabirds feed, also falls," explained Exequiel Ezcurra, professor of ecology at the University of California, Riverside.

The collapse in food for the seabirds that results from warming waters, is compounded by the reduction in sardine populations brought by intensive fishing in Mexico.

Confronted with lack of food, the seabirds take off towards the coasts of Southern California.

The study appeared in the journal Science Advances.