When a farmer sends his livestock to the slaughterhouse, he knows just how many animals go into each truck and thus how many he will be paid for. Things are not so simple for fish farmers, few of whom slaughter their fish themselves. The fish are usually picked up by well-boats that take them to specialised slaughterhouses, where they are vacuumed or netted out of sea-cages. These are full of fish from a particular year-class whose individual sizes vary. The quantities and size distributions involved have therefore tended to involve a certain amount of guesswork. But SINTEF's fish-counting project is about to change all that.

"With the help of a video camera, lights, mirrors and a computer, the fish are photographed from two different angles as they pass through a transparent section of the pipe that sucks them out of the sea-cage and into the boat. On the basis of these images, their volume and thus their weight can be calculated", says project manager Jens T. Thielemann of SINTEF Electronics and Cybernetics. Information about the size, weight and numbers involved enables the slaugherhouse to prepare for its part of the job, while the farmer knows exactly how many fish he has delivered.

"Our biggest problem was that of separating fish that entered the counter at almost the same time, but this has been solved with the help of image analysis and high-speed electronics. We can count smolt at a rate of 50 per second, twice as fast as the image refresh rate on a TV screen. The error rate, i.e. the difference between our measurements and control weighings, is only 3%".

The system was tested on silhouettes cut from paper.

"A lot of salmon profiles were fixed on to a tape and run past the camera. Later, we obtained videos of salmon together with the slaughterhouse reports of the actual numbers and weights of the fish that they showed. In fact, we never really got our fingers wet", says Thielemann.

The fish counter was developed in cooperation with Brdr. Wingan A/S in Sundlandet, and is already in use on board eight well-boats in Norway and abroad. In an hour it can process up to 100 tonnes of salmon weighing from 2 to 10 kg each, or 180 000 smolt with weights of 20 g to 5 kg. For shorter periods of time, the counter can double these rates.

For further information please contact Jens T Thielemann.