The humble Antarctic krill (Euphausia superba) bears a heavy burden. It may be just a small, shrimp-like crustacean, but its sheer abundance makes it one of the largest protein sources on Earth, eagerly sought by fish, penguins, whales — and man.

Ecologists are now warning that the rapid growth in krill fishing is adding to the pressure of environmental changes threatening the creatures, and are calling for better monitoring and precautionary management of krill fisheries.

The global fish-farming industry is increasingly relying on krill-based fish feed, and enzymes and chemicals derived from krill are included in a number of dietary and medical products. Last year, for example, Aker Biomarine, an Oslo-based company specializing in harvesting and processing Antarctic krill in the Southern Ocean, upgraded its krill harvesting vessel, the Saga Sea, to boost its catch. In the first half of 2010 it produced 8,600 tonnes of krill meal for the aquaculture market, up from 6,200 tonnes during the whole 2009 catch season. The total krill catch this season is expected to be 150,000–180,000 tonnes, exceeding last year's total by about 40%.

In May, Aker Biomarine's krill fishing was certified by the Marine Stewardship Council (MSC), a London-based organization that aims to promote sustainable fishing practices by allowing catch from MSC-certified fisheries to be labelled as such (see 'Grabbing a bite'). The Pew Environment Group, an environmental advocacy group based in Washington DC, has objected to this, arguing that fishing for fishmeal should not be eligible for MSC certification. The row will come to a head at the October meeting of the Convention for the Conservation of Antarctic Marine Living Resources (CCAMLR), an international body responsible for managing the fisheries in the Southern Ocean.

One issue for debate will be the CCAMLR's annual catch limit of 3.47 million tonnes in the Atlantic-bordering section of the Southern Ocean, the main fishing ground for Antarctic krill. "Current krill management fails to take account of the subtleties of the ecosystem," says Volker Siegel, a marine biologist at the Institute for Sea Fisheries in Hamburg, Germany, and member of the European Union's CCAMLR delegation. Siegel says that rather than setting an ocean-wide limit, krill fisheries should be regulated on a smaller scale, because much of the fishing is limited to a few sites.

Another worry is the number of fishing vessels being deployed in the Southern Ocean. Norway is now operating three ships, for example, and China is expected to rapidly increase its krill fishing after sending its first vessel this year. "If China starts fishing in a big way, catch will expand rapidly, outstripping our ability to orderly manage it," says Steve Nicol, a marine ecologist with the Australian Antarctic Division in Kingston, Tasmania, who advises the Australian government on krill fisheries.

Researchers suspect that Antarctic krill are also feeling the effect of climate change. Krill larvae feed on algae living on the bottom of sea ice, which is rapidly dwindling around the Antarctic Peninsula. According to one estimate, the number of krill in the Southern Ocean may have dropped by 80% since the 1970s. But "there is no definite answer as to how the krill responds to warming", says Nicol. It is also unclear whether krill stocks are transient or fixed to given areas, and how many live deeper than 200 metres, below the most heavily fished and studied region of the ocean.

Mandatory scientific observers on board all krill fishing vessels — as is common practice in all other Antarctic fisheries — could help to answer these questions, Nicol says. Scientists are welcome on board the Saga Sea, but Japanese and North Korean ship owners are resistant to the idea. While there is still so much uncertainty about krill populations, "we must not allow the fishery to expand too quickly", says Nicol. "We don't want to get in a situation where we have to tell people to get their boats out because we allowed them to catch too much, as has happened in other fisheries."