Young turbot grow a lot faster if more water runs through their ponds. The growth rate of the fish can be increased by up to thirty percent in this way. Little is known yet about the underlying mechanism.
Faster growth does not automatically mean bigger profits. But the principle does work. The research shows that quite incontrovertibly. Young turbot grow faster if their water is changed more often than is currently usual on fish farms. It’s a straight correlation: the more frequently the water is changed, the faster the fish grow. The increase only levels out when the water is changed five times as often as usual.
Why the fish grow faster in fresher water is not entirely clear. It certainly doesn’t have anything to do with the available oxygen. There was enough of that anyway, says Schram. It is now being investigated whether the amount of carbon dioxide is the reason. The more often the water is changed, the lower the level of CO2. ‘The CO2 level is the best explanation we have found so far’, Schram confirms. But it is not altogether satisfactory, he thinks. Other explanations cannot be ruled out. For example, the water quality in the ponds is probably not the same everywhere. ‘Turbot lie still at the bottom of the pond for the best part of the day, piled up on top of each other. The water quality between the fish is probably not the same as the quality where we take measurements higher up. That is an interesting aspect, which I would like to look into.’
According to Schram, the Netherlands only has two turbot farms. But elsewhere in Europe, there are many more. A French fish farmer has been applying Schram’s knowledge for a couple of months now. ‘They’ve switched there from changing the water once an hour to twice an hour. And that’s increasing the profits. But it is too soon for firm conclusions.’