Seafood may be a slimy dish

It's a Thursday 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?

One thing's for sure: unless something changes soon, familiar favourites such as cod, haddock, hake and plaice 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. The ocean is changing fast - too fast, it seems, for us to reliably predict the combined effects of overfishing, pollution and climate change. "We are entering a time of great uncertainty," says Boris Worm, of Dalhousie University in Halifax, 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."

Dr Worm and an international team of ecologists have taken a comprehensive look at the state of the world's fisheries. Their results 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 per cent of our intake of animal proteins, according to a 2007 estimate of the UN's Food and Agriculture Organization, FAO. That means each of us wolfs down an average of 16.4 kilograms of fish per year.

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. "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 Centre in Southampton, England.

This is already happening in one of the world's most productive fisheries, the Benguela current off the coast of Namibia in southern Africa. The ecosystem, which once supported large populations of sardines and anchovies, has been taken over by two species of jellyfish. The study estimated the biomass of jellyfish in the region at 12.2 million tonnes - more than three times that of mackerel, hake, sardine and anchovies combined.

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 runoff, have led to a jellyfish problem of epic proportions: autumn blooms of the giant jellyfish Nemopilema nomurai, which can grow to more than two metres in diameter.

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. That can spell bad news: when the blooms die off, bacteria gobble them up, along with most of the oxygen in the water. Squid, too, are increasingly thriving throughout the oceans. "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 in Hobart, Australia. "They're the weeds of the sea."

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. Off the US 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. 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 tenderised with lemon juice and not overcooked, they need not be tough or rubbery.

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 are often added to salads). In Japan they are served as sushi and in Thailand they are 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. "If it's a question of could we eat jellyfish then yes, we could, but the nutritional value is quite low. 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. 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 in 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." Besides, adds Dr. 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. "You would be better off growing spinach," he says.

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