News

Dr. sea urchins

Sea urchins are one of the world’s best paid seafood products, but few people farm sea urchins in Norway. A new doctoral degree offers good advice to those wanting to go in for sea urchins.

"The sea urchin has grazed down the kelp forest and has been a pest and a nuisance along large parts of the coast. Concurrently, it is gourmet food in Japan and France. My motivation has been to prepare for a new industry," says Senior Scientist Sten Siikavuopio at Nofima, who has engaged in research on sea urchins for nearly 15 years.

In the autumn, he presented his thesis for the degree of dr. philos. at the University of Tromsø.

The work comprises a total of 10 papers about the green sea urchin.

This is the sea urchin species that is most abundant in Norwegian waters and the one with the greatest commercial value.

Roe

The roe is the part of the sea urchin that is consumed.

Over a period of many years, Siikavuopio has carried out research on which factors provide the greatest growth and also the best roe quality, including what it likes to eat, water temperatures, water quality and stocking density in the tanks.

"The optimal water temperature for adult sea urchins lies between 10 and 12 degrees in summer and around eight degrees in winter," he says, adding: "If the temperature rises above 12 degrees, it results in reduced gonad growth."

The experiments show that the sea urchins make strict demands concerning water quality and have significantly lower tolerance levels to carbon dioxide, nitrite and ammonia than, for example, salmon.

High levels mean reduced gonad growth and increased mortality. The sea urchin still eats the same amount, which provides poor utilisation of the feed.

The sea urchins, which have low oxygen consumption, are also extremely sensitive to oxygen content in the water. Consequently, the water needs to be replaced regularly.

"Behaviour of solidarity"

Sea urchins lack a heart and brain and their movement patterns are random, particularly when they are searching for food.

This is important when it comes to the shape of the tanks. The sea urchins need vertical surfaces to attach themselves to.

These surfaces must not be too high, to avoid traffic jams and collisions between the sea urchins during feeding.

"Too many collisions lead to the sea urchins getting injured," says Siikavuopio.

"This results in reduced gonad growth. Our results are not unambiguous, but to be on the safe side you should not exceed a rate of 6 kg sea urchins per square metre."

It is also important to spread the food in the tanks to minimise the number of collisions.

"We have observed what I call ‘behaviour of solidarity' in the tanks. A short time after the feeding, the sea urchins had divided the feed amongst themselves. All of the sea urchins got food and that surprised us. This is something we cannot explain," says the Senior Scientist.

What remains in order to achieve a viable industry?

"The technology is ready and a new feed is also commercially available" says Siikavuopio.

"A couple of companies are doing sea urchin farming today and achieving good prices. We now need to upscale this to a larger industry in Norway."

Several of the research results concerning sea urchins are outlined in a report "Sea urchin farming - advice for farmers", which was published by Nofima earlier this year.