Global Warming a Hoax & the Earth is Flat (© David Horsey)

It's a Friday night in 2050. It's been a long week at work and even if you could be bothered to cook, there's nothing in the fridge. So what fast food will you pick up on your way home? How about some squid and chips? Perhaps an algae burger? And don't forget the crunchy fried jellyfish rings on the side.

One thing's for sure, unless something changes soon, familiar favorites such as cod and haddock will be off the menu. In fact, if we're not careful, an assortment of exotic alternatives will be all the ocean has left to offer us.

This may seem an extreme vision of the future, but marine biologists are alarmed by the imbalances that are appearing in marine ecosystems. The ocean is changing fast -- too fast, it seems, for us to reliably predict the combined effects of overfishing, pollution and climate change. What is clear is that the changes, by and large, are not good news for our bellies.

"We are entering a time of great uncertainty," says Boris Worm, of Dalhousie University, Halifax, Nova Scotia, Canada, and the Census of Marine Life project. "If we continue as we have been, in 50 years there may not be much left to take from the ocean."

Worm and an international team of ecologists have taken a comprehensive look at the state of the world's fisheries. Their results, published in the journal Science in 2006, make grim reading. In short, catches of wild fish are plummeting and the researchers predict that without steps to protect biodiversity, all current commercial fish and seafood species will collapse by 2050.

If we do empty the oceans of fish, it will leave a gaping hole in our diet. Fish provide around 20 percent of our intake of animal proteins, according to a 2007 estimate of the United Nations' Food and Agriculture Organization (FAO). That means each of us wolfs down an average of 16.4 kilograms of fish per year. National figures vary widely, from virtually none in some landlocked nations like Afghanistan, to about 20 kilograms per person per year in the UK and United States. and a whopping 180 kilograms in the Maldives.

This demand is increasing rapidly, as a result of the rising global population and increasing prosperity in the developing world. Maintaining catches at current levels is becoming difficult, let alone increasing them. According to the FAO, more than 75 percent of the world's fish stocks are either fully exploited, over-exploited, or recovering from past depletion.

Overfishing is not only affecting those whose diets depend on fish, of course. It's also creating huge gaps in marine ecosystems that are quickly exploited by opportunistic species. The shrimp and crab fisheries off the coast of Nova Scotia and Newfoundland are the direct result of the removal of large cod and haddock stocks through fishing.

While replacing one tasty marine food may not seem like much of a hardship, not all of the replacements for fish will be as delicious. In recent years, the fishing industry has shifted its focus down the food chain, taking larger numbers of small, plankton-eating fish like sardine and anchovy. This could be a dangerous strategy. Small fish are not only crucial to the survival of larger predatory fish such as hake, as well as birds and marine mammals, they also help maintain balance in the species below them in the food chain.

"If you remove small fish there is every possibility that other species in the food chain, like jellyfish, will have a good time of it," says Tom Anderson, a marine ecologist at the National Oceanography Center, Southampton, UK.

This is already happening in one of the world's most productive fisheries, the Benguela current off the coast of Namibia in southern Africa. When Christopher Lynam of the University of St. Andrews in the UK and his colleagues surveyed the area in 2003, they found that the ecosystem, which once supported large populations of sardines and anchovies, had been taken over by two species of jellyfish. The study estimated the biomass of jellyfish in the region at 12.2 million tons, more than three times that of mackerel, hake, sardine and anchovies combined (Current Biology).

The reasons for these changes are complex. Shifts in climate, currents and sea temperature will have played a part, but a major factor is the collapse of the once abundant sardine and anchovy fisheries. In the late 1970s, the total fish catch was around 17 million tons per year. Now it is closer to 1 million tons. And since jellyfish eat fish eggs and larvae, as well as compete with young fish for food, the shift to a jellyfish-dominated ecosystem rather than a fish-dominated one may be irreversible, say the team.

Blooms of jellyfish have also appeared in the overfished waters of the Black Sea, Alaska, the Mediterranean and the Gulf of Mexico. In the Sea of Japan, overfishing of sardines and anchovies, plus blooms of phytoplankton caused by nutrient-rich coastal run-off, have led to a jellyfish problem of epic proportions: autumn blooms of the giant jellyfish Nemopilema nomurai , which can grow to more than 2 meters in diameter. In 2003 alone, this jellyfish cost the Japanese fishing industry over $100 million, "clogging and bursting nets, causing high mortality of the catch due to venom, increasing the risk of capsizing trawlers and giving fishermen painful stings," says Masato Kawahara, a marine ecologist at Hiroshima University in Japan.

Removing fish from an ecosystem may also have other consequences. In the Benguela current, the crash in phytoplankton-eating fish has also been linked to more frequent phytoplankton blooms (Ecology Letters). That can spell bad news: when the blooms die off, bacteria gobble them up, along with most of the oxygen in the water.

Even overfishing large predatory fish could encourage these blooms. Zooplankton-eating fish thrive once their predators are gone, leading to a decline in their own prey. With fewer zooplankton to feed on phytoplankton, the latter can bloom unchecked.

The collapse of cod, haddock, hake, pollock, plaice and flounder fisheries off Nova Scotia have all coincided with an increase in phytoplankton. So has a reduction in the number of salmon in the north Pacific. And last year, Michele Casini of the Swedish Board of Fisheries in Lysekil, and colleagues, found a strong link between the collapse of cod stocks in the Baltic Sea in the early 1980s and phytoplankton blooms.

Squid, too, are increasingly thriving throughout the oceans. While changes in water temperatures may play a part, the main reason is the removal of their predators.

"Almost everything eats squid in the ocean -- tuna, marlin and swordfish hardly eat anything else - so if you remove the squid's predators, how can it not have an impact?" says George Jackson of the University of Tasmania, Hobart, Australia. And as squid grow quickly and live for less than a year, their numbers can rise rapidly if the conditions are right. "They're the weeds of the sea," he says.

The best-documented example is the Gulf of Thailand, which has been heavily overfished in recent decades. Here the Indo-Pacific squid Sepioteuthis lessoniana has moved in to fill the gaps in the ecosystem, forcing the fishing industry to adapt. "You see fishermen walking down the beach in Thailand with baskets of squid," says Jackson.

Off the U.S. coast, the Humboldt squid, Dosidicus gigas, has begun to expand its territory north from the east Pacific equatorial waters to the seas off central California. This has happened before, during El Nino years, when the water warmed enough for them to spread their range. The last time this happened, in 1997-98, predation and competition from tuna and billfish sent most of them back south when the waters cooled. In the past five years, though, they have stayed put despite cooler seas, and seem to be thriving. Now they even threaten the Californian Pacific hake fishery.

If the outlook for today's fisheries is as bleak as some suggest, we can expect to see growing numbers of gelatinous, rubbery and slimy creatures swimming or drifting through the oceans. So what will that leave us to eat with our fries?

Shrimp and crab aside, squid are likely to be the most palatable bet since they are already well established on menus worldwide. Larger species like the Humboldt squid are also commercially fished in Mexico, Peru and northern Chile. They yield a decent-sized steak and, as long as they are tenderized with lemon juice and not overcooked, they need not be tough or rubbery.

Nutritionally, squid are high in protein -- about 16 percent -- low in fat and a good source of zinc, vitamins B2 , B3 and B12 , as well as some trace elements such as phosphorus, copper and selenium. On the downside, they are very high in cholesterol.

JELLYFISH CRUNCH

Compared to jellyfish, though, squid are positively nutritious and delicious. A common ingredient in Asian cuisine, jellyfish have been eaten for more than 1,000 years in China, where they're often added to salads (Hydrobiologia). In Japan, they're served as sushi and in Thailand they're turned into a kind of crunchy noodle. For those with a Western palate, though, the taste and texture may take some getting used to.

"I wouldn't describe it as a sensation that would sweep the globe," says Kevin Raskoff of Monterey Peninsula College in California. "It's reminiscent of slightly tough strips of cucumber."

Jellyfish are low in fat and high in copper, iron and selenium but they are only about 5 percent protein. Furthermore, they're typically prepared by being dried and salted so, unless a new approach is taken, jellyfish products could end up too high in sodium to become dietary staples.

Each year, around a quarter of a million tons of jellyfish are landed worldwide. As well as China and Japan, there are small fisheries in Australia, India and the U.S. Cost remains a challenge: once caught, jellyfish have to go through a multistage treatment to reduce water content, decrease pH and firm the flesh -- a process that can take up to 40 days. This, together with its unpredictable population and low nutritional value, makes further commercialization a challenge.

So while inventive locals in Obama, Japan, coped with a bloom of giant jellyfish in 2006 by turning them into sushi, soup and even novelty cookies, it is unlikely that they will replace fish as a major food source for the world.

"If it's a question of could we eat jellyfish, then yes we could, but the nutritional value is quite low," say Raskoff. "I'd be concerned if they were the last things left on the menu."

This leaves plankton as a possible fish replacement. The idea is not as odd as it first seems. After all, the Aztecs are said to have eaten a kind of "cake" made from the dried froth of blue-green algae, probably spirulina, that grew on the surface of Lake Texcoco. According to the accounts of Spanish conquistadores, it was highly nutritious and tasted like cheese. It is still eaten in a number of countries, including several in central Africa, where it is harvested from Lake Chad.

Marine phytoplankton, too, are packed with omega-3 fatty acids and trace minerals. Since their blooms tend to consist of mostly one species they could, in theory, be scooped up and turned into food. Even so, converting fishing trawlers to fish for algae would be unlikely to work in practice, says Peter Franks, a plankton ecologist at Scripps Institution of Oceanography, La Jolla, California. "Plankton blooms are dense -- with up to a million cells in a litre of water. But a million cells would hardly make a phytoplankton cracker," he says.

Besides, adds Franks, the practicalities of predicting where a bloom will occur and ensuring that the catch is not contaminated by the handful of species that are toxic would make it financially unfeasible. "I would not want to be running a business based on filtering plankton from the ocean as human food. You would be better off growing spinach," he says.

Even squid, which look like a good option on paper, could be a risky bet as a major fishery, warns Jackson. "Squid are very much boom and bust," he says, a characteristic that makes fisheries tricky to manage. "If there is a lot of food, they grow fast and reproduce early. But if conditions change, the population crashes." He adds that "if you put pressure on squid populations, you'll just create the same problem as we have with fish."

Worm, though, dismisses talk of converting the global fishing fleet to seek out alternatives to fish. He points out that our fisheries are not quite finished, and says there are several instances where creating "no take" zones next to fisheries has allowed them to recover, making them more profitable than others where no such measures have been taken.

And, he says, we are beginning to learn from our mistakes. Proposals to fish for krill off Nova Scotia have not yet been acted on because krill is such an important food source for other organisms, says Worm. "I think people are drawing a line in the sand. Doing this to a predator is one thing, but taking away the foundation of the ecosystem? That's too risky."

Worm is so confident we can turn the situation around that he is willing to bet on the oceans of the future having more fish than we have today. "In 2048, I'll be 80 years old and I hope I'll be able to host a fish supper to celebrate," he says. It is certainly not impossible, but it will take a seismic shift in the way we manage the oceans. Without some serious changes, Worm's seafood supper could be nutritious, it might even be delicious, but it probably won't be fish.

Caroline Williams is a life sciences editor for New Scientist and based in Boston, Massachusetts.