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ON THE surface of the sea conditions can be atrocious, with freezing gales and enormous swells. Slip beneath the waves and descend into the blue, however, and you enter another world, a more tranquil place where the violence of the waves above turns into an ever-gentler to-and-fro, and the temperature never falls much below freezing.

We tend to assume that life in the oceans is insulated from the vagaries of the weather. But it is turning out that what goes on above the water’s surface is intimately linked to the survival of the creatures below it, even far down in the depths of the ocean.

Bumper catches

A single storm hitting at just the right moment, or a particularly icy winter, can lead to bumper catches of fish or traps full of crabs a few years later. Longer term climatic changes can also lead to huge swings in fish numbers. The biggest fisheries disaster so far, the collapse of the Grand Banks cod fishery off Newfoundland, Canada, might have been linked to such factors.

“A single storm can lead to bumper catches a few years later”

With many other fisheries teetering on the brink, these new insights could prove vital. If we can predict how fish numbers will vary naturally months or even years in advance, managers can reduce quotas before stocks are decimated, or increase them when conditions are favourable.

It all began in 1989. Richard Beamish was attending a meeting in the Soviet Union when a Russian scientist slipped him a book containing salmon catch records, something Beamish is sure he did not have permission to do. Thanks to that book, Beamish, now at the Pacific Biological Station in British Columbia, Canada, was able to study the total numbers of salmon caught in the North Pacific over the last century.

Striking link

He was surprised to discover that, despite their very different management approaches, the numbers caught by the various countries in the region had risen and fallen in synchrony. When he compared these trends to climatic indicators, he found a striking link: the fortunes of the salmon were closely tied to the storminess of winters.

Since Beamish’s work was in 1993, more and more researchers have started studying how climatic factors affect marine life, and they are discovering a mind-boggling number of connections. In many cases, though, it comes down to the survival of the young. “It’s all about making it through that first summer,” says Nicholas Bond of the University of Washington in Seattle.

Take the walleye pollock in the Bering Sea off Alaska, the basis of the largest single fishery in the world. These are the unnamed white fish you get in supermarkets and fast food restaurants in the US. When Bond looked at the years in which high numbers of young pollocks survived, he they were characterised by summer storms.

Predictable

Storms matter because they bring up deep water rich in the nutrients essential for the growth of phytoplankton. “These events essentially fertilise the oceans,” says Bond. During calm summers, phytoplankton growth slows early as the nutrients brought up by winter storms are used up. Summer storms refresh the surface waters, allowing phytoplankton to continue thriving – and young pollock to grow fat.

While predicting individual storms is not possible, says Bond, predicting their effects is. Three years after a stormy summer, when pollock that hatched in the good year become large enough to be caught, catches will rise. Recent falls in pollock numbers could be partly due to a series of calm, warm summers, he says, although 2006 was a good year for young pollock.

A similar connection has been uncovered by Earl Dawe at the Northwest Atlantic Fisheries Center in St John’s, Newfoundland. His team studied the snow crab catch in the Labrador Sea, between Canada and Greenland. The crabs are caught when they are 8 to 10 years old and, as Dawe told last year, there is a strong link between the size of the catch and the amount of ice cover 8 to 10 years earlier.

Crabs like it cold

This clearly points to a connection with the larval stage. “Although snow crab live on the bottom, the larvae immediately jet to the surface,” says Dawe. It is not clear exactly how ice cover helps the larvae survive, but it could be simply to do with temperature: lab studies show the crabs thrive in colder conditions.

Understanding how warm or cold years will affect crab numbers a decade hence could prove very useful for managers in setting catch quotas, says Dawe: “You’ve got a lot of lead time.”

Some climatic phenomena, such as the El Niño-Southern Oscillation in the Pacific, have an even more dramatic impact. The effect of El Niño events on fish has probably been observed for millennia, if not understood. Normally, during non-El Niño years, easterly trade winds blow warm surface water across the equatorial Pacific, causing it to “pile up” in the west. This produces a strong upwelling off the coast of Peru, as cold, deep water rich in nutrients rises to replace the departing warm water. Fish, especially anchovies, thrive in these productive waters: around 10 per cent of the world’s total fish catch comes from Peru.

Drastic changes

But not during an El Niño year. This occurs when the trade winds weaken, and the warm water spills back across the Pacific, reducing the upwelling off Peru. The resulting fall in phytoplankton, along with higher water temperatures, and more predators migrating into their waters, decimates anchovy populations. It’s hard not to notice such a drastic change.

Tuna are also affected by El Niño. Tropical species like skipjack and yellowfin tuna spawn in the western Pacific, whose productivity rises during El Niño conditions. A few months after the extremely strong El Niño in 1997/1998 there were so many skipjack tuna – especially juveniles just a few months old – that the canneries couldn’t keep up with the catch and the price dropped by 60 per cent. On the other hand, albacore tuna, which prefer the cooler waters in the east, decline significantly during an El Niño.

El Niño can be predicted to some extent by comparing atmospheric pressures on each side of the Pacific Ocean. Based on this, at CLS, a satellite-imaging company based in Ramonville, France, and colleagues have developed that predicts changes in abundance of tuna in the central Pacific of an El Niño. The model predicts the movements of existing fish as well as the survival of young fish, so it helps reveal whether changes in catches in a particular area are due to actual changes in fish numbers or just their movement. “That should be used by managers, but for the moment it’s not,” Lehodey says.

Subtle processes

Other researchers are teasing out more subtle, longer-term processes. The is a measure of air pressure differences between Iceland and the Azores that see-saws every few years. When the NAO is positive, the westerly winds across the North Atlantic become stronger and shift northward, bringing warmer, rainier conditions to Europe and the North Sea, but cooler, icier conditions to the Labrador Sea and Newfoundland.

The NAO shifted into a more overall positive regime in the late 1970s, which could explain why the snow crabs in the Labrador Sea have been thriving: on top of the year-to-year change associated with icier winters, the crab fishery has increased steadily since the late 1970s.

However, a positive NAO is bad news for cod. Those in the North Sea exist at the warm end of their temperature tolerance, so as things heat up the survival of larvae drops. On the other side of the Atlantic, cod in the Labrador Sea are living at the cold end of their tolerance, so they too suffer as temperatures drop in that region. In both cases, it seems a shift in the type of plankton available for juvenile cod is also partly to blame. While no one doubts that overfishing caused the collapse of the Newfoundland cod fishery, it might not be a coincidence that it happened when the NAO peaked in the early 1990s.

Barrier

Further south, on the north-east coast of the US, the positive NAO has brought rain and increased warmth. The result is warmer, less-salty surface water, which forms a barrier that prevents deeper, nutrient-rich water from rising to the surface.

That has led to a drop in productivity in regions such as the Gulf of Maine and Georges Bank, says Michael Fogarty at the Northeast Fisheries Science Center in Woods Hole, Massachusetts. “If temperatures continue to increase we’d expect that a lot of these areas will begin to become uninhabitable for cod,” he says. There might be a return to conditions more like those of the 1940s, when catches were a great deal lower.

Huge shift

In the Pacific too, slow ups and downs in temperature have a big impact on fish. A measure called the index, based on sea surface temperatures, has become the most common indicator of the overall climate regime in the Pacific Ocean. When sea surface temperatures are warmer in the eastern Pacific and colder in the central Pacific, the PDO index is positive.

The PDO was mostly negative from 1950 to 1977, after which there was a strong shift to a mostly positive regime. This shift reverberated through many ocean and terrestrial ecosystems. “It’s quite remarkable how significant that change was,” says Beamish. “We have not seen something of that magnitude since.”

Walleye pollock went from being a minor player in the Bering Sea to dominating the ecosystem after 1978. Alaskan and Canadian salmon flourished, while salmon off the west coast of the US declined. The North Pacific halibut fishery was completely depleted by the mid 1970s but by the early 1980s it was thriving. Crab and other crustaceans suffered severe declines in the Gulf of Alaska and the Bering Sea.

Back in time

The reason? The north Pacific is warmer and stormier when the PDO is positive, good news for fish but bad news for crabs. At the same time, winds weaken off the west coast of the US, reducing the upwelling that occurs in that region and so reducing productivity.

After discovering these relationships between decades-long fluctuations in fish and climate all over the world, scientists are now probing farther back in time to see if similar relationships exist on the order of centuries and millennia.

Historical records seldom go back very far. Instead, Bruce Finney’s team at the University of Alaska in Fairbanks devised a way to work out how abundant sockeye salmon were using sediment cores from the freshwater lakes where they spawn. Adult salmon die after they spawn and their bodies decompose on the bottom of the lake, leaving behind a record in the form of nitrogen isotopes in the sediment.

Dramatic change

Using the method, Finney reconstructed salmon numbers over the past 2000 years (), and he now has unpublished work looking back 14,000 years. And what he’s found is that in the past salmon have generally thrived when the north Pacific has been colder.

That’s surprising because over the past century, Alaskan salmon have thrived when it is warm. “There can be a productive ocean in the Alaska area under both cold and warm conditions,” says Finney. In previous centuries, the north Pacific grew stormier without pulling up warmer air from the south, he thinks. That is very different from what has happened in the 20th century.

All this makes it very hard to answer the big question: what’s going to happen as climate change accelerates? The only thing the experts agree on is that there will be change, and that it will be dramatic. The populations of some common species could crash, while others that are scarce today boom.

Findings like Finney’s suggest that the intricate connections between ocean life and climate are already being reshaped as the planet warms. “Just as we’re beginning to realise the importance of climate, we’re changing it,” Beamish says.