Science - December 2, 2013

Against the stream

Rob Ramaker

Fat, stressed and unhealthy, many farmed fish are hopelessly unfit. That is bad not only for the fish, but also for the farmer. The solution is simple: fish must swim.

‘They are real athletes,’ says Arjan Palstra, a DLO researcher at Imares, bent over a tank in which yellowtail kingfish are moving about with ease. This farmed fish is popular in Southeast Asia. Viewed from above, the fish tank, part of the research facility in Zeeland’s Yerseke, looks like an athletics track, with the fish restricted to one of the long sides. The fish ‘in the outside lane’ are struggling against a strong current while those in the inner lane are gently swimming around in calm water. After a couple of weeks Palstra will study the differences between the groups. He is looking to prove an elegant idea: that a swimming fish is healthier, more muscular and perhaps even tastier. Intensive swimming is not exactly standard behaviour for the average farmed fish. ‘Most farmed fish are crowded together and don’t move about much. This has negative effects,’ says Palstra. As an example, he mentions the situation in the salmon industry, which accounts for some 50 percent of European aquaculture. ‘You see a lot of obese fish there. They have heart abnormalities, their organs are surrounded by fat and they bite the dust at the slightest sign of stress.’ When young farmed salmon, for example, are transferred from fresh water to salt water, some 15 percent don’t survive. That may not seem like many, but annually we are talking tens of millions of salmon. That there is so much room for improvement illustrates that fish farming, also called aquaculture, is still in its infancy. People have always kept fish in ponds, but largescale aquaculture is only 30 or 40 years old. Closed systems for keeping fish have only been seen in the Netherlands since the mid-1980s. Compare that with the experience of cattle farming, which goes back thousands of years in the region. So it is logical that it can be done much better, thinks Palstra. ‘That is also recognized in the industry.’ While aquaculture in the Netherlands is limited mostly to mussel farming, elsewhere in the world (mainly China) explosive growth is occurring. This development, also called the blue revolution, makes the need for improvement urgent. Forty percent of all fish used by humans already comes from fish farms. For fish destined for human consumption that figure exceeds 50 percent. Owing to declining fish populations in seas and oceans, with catch restrictions as a consequence, that percentage will only continue to rise.

Free-range fish

Superficially at least, Palstra’s message seems of little interest to fish farmers. After all, more room to move means higher costs per fish. But the researcher disputes that. One of the most striking results of his research is that fish with more room to move grow faster. In 2010 Palstra demonstrated this using zebrafish. Eighty fish swam six hours a day, five days a week for four weeks against the current in a swimming tunnel. Afterwards it turned out that these swimmers were a good 5 percent longer and no less than 41 percent heavier than the resting fish. Initially, that seems counterintuitive. After all, the fish were spending more energy on moving. However, swimming causes energy to be used more efficiently, and ensures that muscle building – and thus fillet building – has priority. With the yellowtails on his athletics track Palstra is particularly keen to research whether swimming makes the fillet more appetizing. In other words, is the fillet of a swimming fish tastier? During the present experiment he is investigating this using the pH, colour and the energy content of the fillet, but later he also wants to work with taste teams. For farmed salmon in particular, a firmer fillet would be a good step forward, says Palstra: ‘When you now cut a salmon fillet, the flesh sometimes tears. And that means huge losses in terms of price and quality.’ But the fish also stand to benefit from their hard work. Swimming has a positive effect on the wellbeing of farmed fish. Active swimmers are less sensitive to stress, for example. What’s more, their immune systems function better so they get sick less often. Finally, energetic swimmers are less aggressive, so there are fewer casualties caused by fish biting each other ‘It will make consumers happy too,’ adds Palstra, ‘because in fact they are getting a free-range fish.’

Forty percent of all our fish already comes from fish farms

Robot fish

The greatest challenge is to translate all these ideas into practice. There are various options for doing this. Farmers could copy Palstra’s ‘athletics track’, creating a current in their tanks with the aid of pumps or engines. In larger tanks, however, that’s impractical. This has prompted the company OptoSwim to propose a more economical solution. By placing a ring of lights that flash in sequence round the edge of the tank, the illusion is created of a moving background. Fish respond to this by following the lights. A couple of Spanish engineers have an even more exciting proposal. They outline how a robot fish can be built to lure a school of other fish to swim after it. Ahead of these practical problems, however, there are scientific challenges to be tackled. More evidence must be accumulated to confirm the positive effects. And results gained with one fish can’t be applied just like that to another species. One species is a sprinter, while the other is a long distance swimmer. What’s more, the optimum swimming speed hasn’t been used by every researcher, so that experiments have delivered conflicting results. And in any case the research has always been very small-scale and fragmented features across various fields of specialization. All this is why Palstra is busy setting up a network of fish researchers who are interested in swimming behaviour. In 2010 he organized a workshop on the subject. Recently he secured a European COST-action, which is intended not for research but to develop his specialist area. Researchers may use the funding to travel, organize meetings and to train others. This puts the recipients in a better position to score in the next round of European grant allocations. Until then Palstra has to be patient. He peers intently at his yellowtails. His student Daan Mes, who feeds the fish every day, has been throwing them grains of food at regular intervals. ‘So,’ say Palstra, with a hint of impatience, ‘can you see a difference already?’