Scientists have used satellite images to determine that an area of the ocean marked by low biological activity has expanded to reach the Hawaiian Archipelago.

Jeffrey Polovina, of the National Marine Fisheries Service, said the area is low on chlorophyll for plant and animal growth.

"What's happening is large portions of the area are becoming less productive," he said.

That means there may be fewer larger fish, such as tunas and marlins, while populations of smaller fish, such as mahimahi and skipjack, may grow, he said.

Polovina and two colleagues, Evan Howell and Melanie Abecassis, analyzed nine years of data provided by the Sea-viewing Wide Field-of-view Sensor on the orbiting SeaStar spacecraft. Launched in 1997, the craft maps the oceans in color, with surface chlorophyll in green reflecting the amount of plant life supporting the base of the food chain.

The scientists said, in a paper published in Geophysical Research Letters, that from 1998 to 2007, the least-productive ocean waters of the Pacific and Atlantic oceans expanded at average annual rates of 0.8 percent to 4.3 percent, with total expansion of about 2.5 million square miles, or about 15 percent.

An area in the North Pacific "has expanded to the northeast, reaching portions of the Hawaiian Archipelago and well into the eastern Pacific," they said.

The rapid expansion of low surface chlorophyll suggests global warming as the likely cause, Polovina said.

"If it is due to an increase in greenhouse gases, it really does represent something that is going to be a long, permanent change," he said.