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Two Takes on Climate Change in the Ocean


This animation shows changes to the distribution of silver hake from 1968--2007. 
Like many other marine species, silver hake are shifting their range poleward in response to climate change. This shift allows them to stay within their preferred temperature envelope as ocean temperatures rise. Credit: adapted from maps 
by Janet Nye/NOAA.

Kevin Friedland, a fisheries oceanographer with NOAA, is the lead author
of the paper that discusses Atlantic cod. In this video, Friedland shows that environmental change is a complex process, and that the ocean is not warming uniformly.

More Information

The Oceanography Branch at NOAA’s Northeast Fisheries Science Center conducts plankton surveys and collects other key ecosystem indicators along the U.S Northeast continental shelf.

Those and other data are synthesized into an Ecosystem Advisory that is published twice annually in a form that is useful to decision-makers.

In 2012, sea surface temperatures along the U.S. Northeast Continental Shelf reached their highest level in 150 years.

The Science Behind Changing Ocean Temperatures and Atlantic Cod

 

A pair of recent studies show how marine ecosystems are changing as ocean temperatures rise, and that these changes are happening more quickly than expected.

Are you a big-picture person, or do you focus on the details? Either way, there's been plenty of news for you lately on climate change in the ocean, and both perspectives lead to the same conclusion. The ocean is changing, and more rapidly than expected.

NOAA Fisheries scientists recently worked on two eye-opening investigations into how marine ecosystems are responding as ocean temperatures rise. The first, a meta-analysis of earlier studies, found that marine species are shifting their distribution in response to climate change much faster than expected. The second study zooms in for a closer look at a specific case: the Gulf of Maine, where the food web is shifting in ways that may make it harder for once-great stocks of Atlantic cod to recover from overfishing.

Together these two studies offer a compound view onto what’s happening in the ocean, and they highlight the challenges ahead. As the climate changes, it is more important than ever that we fish sustainably, monitor ocean conditions over time, and consider how our actions affect not just individual species but entire ecosystems as well.

Species are Shifting Toward the Poles

The meta-analysis, published last month in Nature Climate Change, marked the first time scientists set out to compile all previous studies on how marine life is responding to climate change. One common response is range shift—species are moving toward the poles in order to stay cool as things heat up. Ranges are shifting both on land and at sea. The surprising result of this study was that ocean species are marching forward an average of ten times faster than species on land.

"That’s happening across the board," said Frank Schwing, a NOAA Oceanographer who was among the authors of the study. "For all taxa we looked at and no matter where the organisms were in the food web, we consistently found positive results."

In addition to range shifts, the timing of seasonal events such as migrations and spring blooms is also changing. But as species shift in space and time, they’re not all moving together, and this can disrupt the food web.

Many species time their reproductive and migratory cycles around their prey—think of whales migrating to their Arctic feeding grounds in summer, or of young salmon hitting the ocean just as seasonal upwelling brings nutrients to the surface. “When these patterns are disrupted,” said Schwing, “we see mass strandings, starvation, and poor reproductive success.” Events like this have always occurred periodically, such as during El Niño years when the Pacific is unusually warm. This study suggests that such events may occur more frequently in the future.

Changes Can Have a Big Economic Impact

Of course, it’s not only salmon and migrating whales that expect their prey to be available at certain places and times. Fishermen do as well, and that brings us to a second piece of research, this one looking at a specific example of food web disruption, and one that involves what used to be one of the most valuable fisheries in the world.

Atlantic cod were once so abundant that colonial empires were built on their salted flesh. But centuries of overfishing have finally reached their foreseeable conclusion, and today there isn’t enough cod to support even the small fishing fleet that remains.

To allow the stocks to rebuild, fishery managers in the United States and Canada have lowered catch limits on cod. But cod stocks have yet to respond. One possible explanation, according to a recent paper in Progress in Oceanography, starts with higher sea surface temperatures along the U.S. Northeast continental shelf.

“Last year the Northeast Shelf was the warmest ever recorded,” said Kevin Friedland, the lead author of the study. Along with increased temperatures there has been a shift in the dominant species of zooplankton—the tiny creatures near the base of the food web—in important cod fishing grounds such as the Gulf of Maine and Georges Bank.

In a few areas, higher temperatures have had the paradoxical effect of cooling bottom ocean waters, as increased melting of Arctic ice sends fresher, cooler water south. Zooplankton species have changed in these areas as well.

Cod prey on zooplankton during their larval stage, and they appear to depend on specific zooplankton species, which are usually abundant and small enough for the larval fish to eat. “The survival of fish larvae is often dependant on the quantity and quality of food they encounter while drifting in the water column. If oceanographic conditions do not favor key zooplankton species, the young cod can suffer,” Friedland said.

Friedland and his co-authors found that in areas where the zooplankton has shifted, cod appear to have lower reproductive success than those in other areas. These results suggest that warming-driven changes to the food web may be making it harder for Gulf of Maine and Georges Bank cod to recover.

What Now?

There are other theories about what might be holding Atlantic cod back, and surely a combination of factors is at work. But one thing is certain: small, overfished stocks are more readily impacted if environmental conditions turn against them. And if conditions become favorable again, they may be unable to capitalize on the situation through improved survival or reproduction. In a warming ocean, it is more important than ever to prevent overfishing and to allow overfished stocks to rebuild.

These studies also highlight the importance of monitoring ocean ecosystems over time. Scientists knew that zooplankton had shifted in the Gulf of Maine because they’ve been collecting plankton samples there, along with other core ecosystem indicators, every other month since the 1970s.

Monitoring programs may seem unexciting, and they’re often the first to be cut when budgets are tight. But we can only understand changes in the ocean by holding up today’s events against the light of a long-term dataset. Without that, we cannot hope to manage the oceans wisely.

Finally, these studies show that while species respond individually to climate change, it’s the relationships between those species that determine how the effects play out. Ecosystem approaches, such as managing multiple species as a complex rather than individually, will help us avoid putting pressure on parts of the food web that are already stressed.

It will also help us spot new opportunities when they arise. As species shift toward the poles, warmer water species will come in to take their place, and where people now fish for lobster they might one day be fishing for crab. Ocean ecosystems are reconfiguring rapidly, and this will bring both losses and opportunities. Navigating these fast-changing waters is the central challenge in marine science today.