Student - June 4, 2009

TURBOT GROWS FASTER IN FRESH WATER

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.

It seems too good to be true: just change the water more often, and the fish farmer will be raking it in. But it’s not quite as simple as that, explains researcher Edward Schram of Wageningen Imares. ‘Thirty percent is quite something, that’s true. But we should add that this applies to young fish. Whether it applies to adult fish we don’t yet know. And you also have to consider the cost of pumping all that extra water. The energy that uses has to be paid for too.’

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.’