Marine ecologists have been grappling with a puzzler. They had expected that, as climate change warms the oceans, most species would migrate toward the poles, fleeing the ever hotter waters near the equator and tracking the zone of their preferred water temperature as it shifts. But some studies revealed that some species seemed to migrate in the “wrong” direction.

Now, however, researchers have apparently solved the riddle: For the past four decades, marine species found along North America’s coasts mostly have followed cooler water, but that doesn’t always mean moving poleward.

“This is really quite a neat study,” says Trevor Branch, a fisheries scientist at the University of Washington who was not involved in the work. “It is likely to be the highest-profile fisheries paper this year and an instant classic.”

Scientists had long assumed warming oceans would generally drive species’ geographical ranges toward higher latitudes. But some studies have found just the opposite, says Malin Pinsky, a marine ecologist at Rutgers University in New Jersey. Off the shores of California, he notes, some species have been moving south, not north. Other researchers have seen the same trend in the Gulf of Mexico.

“Scientists were asking themselves, ‘Why aren’t certain species doing what we expect?’ ” Pinsky notes.

In a new analysis, Pinsky and his colleagues show there’s more to the story: Many of those species were moving to cooler waters, it’s just that the studies weren’t detailed enough to reveal that.

The team looked at data gathered during coastal surveys between 1968 and 2011 – a census of more than 60,000 trawler hauls from coastal regions that together cover more than 3.3 million square kilometers, an area almost twice the size of Alaska.

The tally includes more than 128 million organisms representing 360 species or groups of closely related species. “There’s no better data set than this for North America,” Branch says.

From water temperatures measured during the surveys, the researchers calculated which way and how fast lines of constant temperature (similar to temperature contours on a weather map) had been moving. All of a sudden, some of the odd-looking results made sense, Pinsky says.

Along the California coast, the “climate velocity” pointed south, so sea creatures had to head that way – not toward the pole – to stay ahead of warming waters. And along the Gulf Coast, species had been moving south to reach deeper – and therefore cooler – waters.

Overall, more than 70 percent of the species that shifted latitude or depth did so in the direction predicted by climate velocity, the researchers reported Thursday in Science.

The new research “increases confidence in the science linking oceanography, fish physiology, and (species) movements,” says Daniel Pauly, a fisheries scientist at the University of British Columbia.

The findings also show that over time, as climate change continues, most marine organisms will gradually move away from what’s thought of as their traditional territories.

The new study will likely help scientists anticipate the future movements of marine species – information that may help people better manage fisheries or pick better sites for areas intended to protect marine species or ecosystems.

“There’s no point in putting a marine preserve in one place, and then have species move out of it” a few decades later due to climate change, Branch says.

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