When the Bering Sea warms, there are telltale signs. One is a bloom of phytoplankton that turns the water's normally gray surface to a lovely turquoise.

"It does feel like you're in the Caribbean," said Janet Duffy-Anderson, a research biologist with the National Oceanic and Atmospheric Administration's Alaska Fisheries Center.

Though it is pretty, that bloom means ugly conditions for much of the sea life in the Bering Sea, the source of about half of the commercially harvested seafood in the United States.

The phytoplankton creating the turquoise bloom is coccolithophoe, a tiny marine plant that thrives in warm, nutrient-poor conditions, "so they are a harbinger of problems when they are in the Bering Sea," Duffy-Anderson said.

This is the second consecutive year the Bering Sea has been unusually warm -- and turquoise -- and scientists from NOAA, the University of Washington and the U.S. Fish and Wildlife Service are on a monthlong cruise aboard the research vessel Oscar Dyson to try to understand what is happening in these waters.

The warmth of the past two years comes courtesy of "the blob," an unusually large and stationary mass of warm Pacific water, along with other forces. With a powerful El Nino system building, warm conditions are expected to continue.

What happens next as the blob and El Nino clash, on top of a warm phase of the cyclical Pacific Decadal Oscillation, remains to be determined, NOAA experts say.

"We've never seen quite this arrangement before," said Phyllis Stabeno, an oceanographer who works at NOAA's Pacific Marine Environmental Laboratory.

Last year, the scientists aboard the research cruise monitored and measured the changing water and weather conditions, Stabeno and Duffy-Anderson said. This year, they are trying to learn what the current conditions mean for Alaska pollock and other sea life -- "to catch these changes as they are affecting the biology of the organisms in the water," Duffy-Anderson said.

The harvest of Alaska pollock is the largest of all U.S. seafood catches. Over 1 million metric tons of the whitefish is pulled from the U.S. zone of the Bering Sea each year (along with about another 100,000 metric tons from the Gulf of Alaska) to supply the nation and the world with fast-food fish patties, fish sticks, imitation crab and the malleable fish paste known as surimi. A similar amount is harvested from the Bering Sea's Russian waters.

But successive warm years in the Bering Sea bode ill for pollock and the pollock business.

In summers when waters are cold, the tiny copepods that pollock eat tend to be bigger and carry more fat, helping the juvenile fish survive the cold winter. "They go into their first winter kind of beefed-up a little bit," Duffy-Anderson said. In warm years, the copepods are plentiful but much lower in lipids. The young fish are able to eat a lot and grow fast -- until winter, when they seem to run out of energy and have difficulty surviving, Duffy-Anderson and Stabeno said.

"Long, skinny pollock are not good. You want to be short and fat," Stabeno said.

In the past, she said, the Bering Sea changed from year to year, so a single warm year did not necessarily have significant effect on pollock stocks or conditions for other sea life. But lately, the area has been going from warm to cold and back to warm in several-year stanzas, a pattern more common in the Gulf, she said.

The Bering had a series of warm years starting in about 2001. In 2007, the cold conditions came back, but impacts on the pollock stocks lagged. Stock declines caused federal fishery managers to impose big cuts in the allowable pollock catch for the 2008 and 2009 seasons. Pollock stocks increased in subsequent years, as did catch quotas.

Whether two consecutive warm years will affect catch quotas for 2016 is yet to be determined. The North Pacific Fishery Management Council, the regional group that oversees commercial fishing in federal waters off Alaska, is scheduled to set next year's quotas at its December meeting.

The southern Bering Sea research cruise is scheduled to run through Oct. 6.