Scientists estimate that about 500,000 tons of fish, or one third of total catches, are thrown away every year in the North Sea alone, Andersen said.

Many species are threatened by extinction after decades of over-fishing despite cuts in quotas.

Huge amounts of fish are thrown out, either because they are too small or because a trawler has filled its quota. Most of the fish which are thrown back die from exhaustion, he said.

The camera takes a digital photograph of the catch which is then divided into a grid, allowing a computer to measure the shape and color of each fish in the grid. It needs one tenth of a second and identifies 98 percent of fish correctly.

By linking the camera to a sorting device aboard trawlers, fishermen can swiftly sort out unwanted fish, limiting the amount of discarded fish to around 25 percent, Andersen said.

The onboard version is being tested by Norway's Institute of Marine Research and Norwegian marine electronics maker Scantrol.

Andersen, a project manager at Danish engineering consultancy group COWI, said engineers were also working on an underwater model of the device, which could be ready within two years. It would make it possible to sort out unwanted fish even before they are taken aboard.

However, the technology that will handle the sorting underwater needs more development before it can operate, mainly because of vast differences in the size and shape of fish.