Fish growing quicker when fed in the dark

How reliant are fish on seeing what they eat? Unlike other fry, cod fry grows quicker when it is fed in the dark, new research findings from Nofima show.

It is normal for fish farmers to feed fry in light conditions, as studies of a host of species, including salmon, has shown fry grow best when they receive a lot of light.

Studies have also shown cod larva is reliant on light to eat and grow. However, no studies have been carried out on whether the same effect could be shown on cod fry.

Given this, Senior Scientist Sten Siikavuopio at Nofima commenced trials with three groups of juvenile cod to study how much the juvenile cod eat and grow when fed in light and dark conditions respectively.

Large growth in short time

All the juvenile cod received 12 hours of light conditions and 12 hours of dark conditions a day for a six-month period. The first group was fed when it was both light and dark, the second group only during light hours and the third group only during dark hours. The water temperature was 12 °C.

At the start of the trial, the fry weighed five grams. At the conclusion of the trial, they weighed around 90 grams, with no significant weight difference between the three groups. However, the scientists observed a tendency that the juvenile cod fed in the dark were a little larger than the other cod.

Opening new possibilities

"This shows that the light conditions for juvenile cod are not of great significance," says Siikavuopio, adding that the findings open the way for more flexible methods for feeding juvenile cod.

The findings also indicate that the need to see food changes in the transition from cod larvae to juvenile cod. The scientists still do not have knowledge about how the cod eat in the dark. Do they eat as the feed is falling or do they find it on the tank bottom?

"This would be interesting to find out, especially with respect to whether this knowledge can be transferred to cod in cages, where the fish are reliant on eating the feed as it is falling," says Siikavuopio.