For Tim McClanahan, a zoologist studying fisheries, what happened in Kenya during the spring of 1998 was a wake-up call.
Between March and July of that year, a rare climatological double whammy sent ocean temperatures spiking 1 to 2 degrees Celsius above the normal range for spring and summer. An unusually intense El Niño weather pattern coincided with the warm phase of another cyclical area weather event.
This turned out to be a slow-motion disaster. Half the corals in the region bleached and died that year. Some had a 90 percent loss. "The bleaching and mortality event took about six months to fully unfold, but many of the reefs have not recovered even today -- 14 years after the event," said McClanahan, an employee of the Wildlife Conservation Society. He has spent more than 20 years working along Kenya's southeastern coast.
It took four years before scientists could definitively show dramatic declines in three commonly caught species of food fish. The lag and the devastating results got McClanahan thinking about climate change's potential to damage the economies of communities that traditionally rely on fish to eat and fish to sell.
He's not alone in pondering the fate of the world's fisheries in a changing climate, and how the fortunes of fish will affect the lives and livelihoods of more than 1.5 billion people who depend on seafood for at least a fifth of the animal protein they consume.
"This is an area that is pretty seriously underresearched, I think," said Edward Allison, a senior fellow at the University of East Anglia's School of International Development. "The rest of agriculture sometimes forgets fisheries, and the fisheries sector has been a little slower than others to realize the potential seriousness of climate change impacts."
Already, there is evidence that as the ocean warms, many commercial fish stocks are moving poleward in search of cooler waters. Rising ocean temperatures have triggered coral bleaching events that have caused widespread damage to the world's reefs, which serve as a habitat for many species.
A case of 'double jeopardy' for Africa and Asia
Researchers are also concerned about the effects that shifting ocean chemistry will have on marine ecosystems. As the world's carbon dioxide output has risen, oceans have absorbed more and more of the heat-trapping gas, leaving seawater 30 percent more acidic than it was before the Industrial Revolution began.
Eventually, ocean acidification could scramble ocean ecosystems by making it harder for sea creatures like oysters, coral and plankton to grow the hard, chalky shells that protect them from predators.
But experts say the consequences of those changes for fisheries are uncertain, though many believe that climate change will ultimately separate fish species, fisheries and the human communities that depend on them into winners and losers.
A crop of recent studies is just beginning to figure out who those winners and losers might be.
When researchers at the Malaysia-based WorldFish Center tried to rank countries by the vulnerability of their fisheries to climate change, Gambia topped the list -- and all but two of the top 10 nations were African, hailing from a continent where fish accounts for half the animal protein consumed each day and often provides significant income.
For many countries in Africa, climate change amounts to "double jeopardy," threatening food supplied by land and sea, said Allison, who led the WorldFish Center analysis.
Countries like Sierra Leone, Niger and the Democratic Republic of the Congo already face "extremely alarming" levels of hunger, according to the International Food Policy Research Institute. With climate change expected to decrease yields of staple crops like rain-fed maize and irrigated rice up to 20 percent by 2050, loss of fisheries catch as well could prove devastating.
Research suggests that many countries in Southeast Asia face similar risks. Altogether, 400 million people in Africa and Southeast Asia rely on fish and other marine foods like seaweed to provide half their essential protein and minerals.
Rashid Sumaila, a fisheries economist at the University of British Columbia, said "rough estimates" at modeling the effect of climate change on the world's fisheries suggest catches in the tropics could decline 40 percent by 2055 due to a panoply of factors including warming waters and ocean acidification.
Existing problems like overfishing complicate the picture, making it more difficult to project the effects of climate change.
Like losing 10M bulls every year
"If you have a fishery that is already badly managed, so stocks are not in good shape, and you add another stressor like climate change heating it -- well, then it just goes," said Sumaila.
Eighty-four percent of the world's fish stocks are fully exploited, overexploited or depleted, according to the U.N. Food and Agriculture Organization (FAO).
That overfishing takes an astounding toll on the world economy, ecosystems and food security in areas that rely on fisheries as a cheap and reliable source of food.
When the World Bank recently tried to tally the economic cost of overfishing, poor management and other inefficiencies, it arrived at a princely sum: $50 billion per year, a number that represents both the increased cost of chasing after scarce fish and the price of maintaining an oversupply of fishing vessels.
For the world's poor, many of whom depend on fish as a cheap, reliable source of protein, there is another shocking number: 10 million metric tons. That's the weight of catch lost each year due to overfishing, according to researchers at the University of British Columbia.
"Recently, I tried to convert that estimate to the equivalent in mature bulls," Sumaila said. "They weigh on average 1 ton each. So we are talking about losing 10 million extra bulls every year out of the ocean because of overfishing. When I turned the fish into bulls, that shocked me."
It's also enough food to save at least 20 million malnourished people -- assuming they were fed only fish, he says.
Meanwhile, the global demand for fish is rising. Production of fish and fish products grew from 140 million metric tons in 2007 to 145 million metric tons in 2009, a historic high according to the FAO. Much of that growth has been fueled by aquaculture, which is increasing at a rate of almost 7 percent per year.
Sebastian Troeng, senior vice president for marine conservation at Conservation International, says farming fish is in some ways a more sustainable source of protein than livestock. Producing 1 kilogram of beef (about 2.2 pounds) requires 61.1 kilograms of grain, while producing the equivalent amount of fish protein requires just 13.5 kilograms of grain.
"You can make more with less," Troeng said.
But there are drawbacks to farmed fish that include, in some cases, nutritional trade-offs, said Allison. Farmed fish may not contain all the nutrients their wild cousins do. A fish that is fed grain may not contain as many omega-3 fatty acids as do wild fish or farmed fish that are fed fish meal or smaller fish.
Keeping a reef from 'cement and cockroaches'
In Kenya, McClanahan is hoping that improving the management of the area's fisheries will help gird them against future climate change.
"If you get better management in place, you can buffer these environmental impacts more than if you don't have good management," he said. "If you have already knocked a system back to cement and cockroaches, it doesn't take a lot to make it worse."
Where McClanahan works, along Kenya's southern coast, fishing is small-scale -- as he calls it, "artisanal." Fishermen use sailboats and canoes instead of motorboats. They haul their catch in using nets, lines, spear guns and traps made from local materials. Many eat most of what they catch and rely on more than one job to make ends meet.
Part of McClanahan's job involves working with communities to adopt more sustainable fishing practices, like traps with small slits that allow juvenile fish to escape -- protecting the next-generation catch.
With climate change in the back of his mind, he's also working to identify reefs that may prove hardier than most in the face of rising temperatures because they have adapted to a wide range of water temperatures, or because they are located in areas fed by tidal, rather than wind-driven, currents.
"Some reefs are pretty much doomed by climate change, in my opinion. Others will probably struggle but get by," he said. "We talk to communities and say, 'This area has a high potential to survive climate change. This is a reef you might want to protect.'"
Some communities are more receptive than others. The tiny village of Mkwiro, perched on the border of Kenya and Tanzania, is one that has embraced conservation measures. The Kenyan government has established a national marine reserve nearby that allows fishing within its boundaries and has created a local tourism economy.
But there are already hints of a changing climate.
"Bleaching used to be an oddity," McClanahan said. "Now it's become a fairly regular thing. It's not regular every year, but it occurs somewhere every year, more frequently than it used to. The Indian Ocean Dipole, a cyclical warming event, used to be a 10- to 12-year cycle at the beginning of the last century. Now it's two to four years. Winters are less extreme."
Studies suggest that efforts to create more sustainable fisheries and reduce existing stresses such as overfishing can only go so far in the face of a changing climate.
Marine-protected areas, a tool embraced by governments and conservationists, have been shown to increase the number and size of fish and keep corals thriving. But recent research has found those benefits can be overwhelmed by the effects of rising temperatures and changing weather patterns.
"Nobody wants to talk about mitigating CO2 emissions," said Sumaila. "It is easy, politically, to talk about adaptation. But ultimately, if we don't deal with the pumping of CO2, it's going to be tough to adapt -- even for the strong countries."