Free fatty acids in feed kills cod larvae, a new NIFES study finds.
NIFES scientists wished to find out whether predigested (i.e. hydrolysed) fat in fish feed would improve metabolic utilisation of the fat, leading to more rapid weight gain and development in the early life stages of cultivated cod. The results of the study, which has recently been published in the British Journal of Nutrition, showed just the opposite trend.
“We found a strong correlation between the levels of free fatty acids and mortality in cod larvae. In fact, even small amounts are lethal,” says NIFES research scientist Øystein Sæle.
Free fatty acids are the chemical form taken by fat after its original molecular structure has been broken down in the course of digestion. Fat can be digested artificially by means of hydrolysis before it is added to feed.
“These findings mean that we need to be extremely careful to ensure that the content of free fatty acids in feed for farmed fish is kept as low as possible. There is no safety margin,” says Sæle.
Foto: Sam Penglase/NIFES
Until now, it has been widely assumed that free fatty acids make feed toxic to fish only when the fats become rancid, but the NIFES study contradicts this theory. The scientists tested relevant oxidative stress markers, but found that they had no effect on the outcome.
In the trials, groups of cod larvae were given feeds that incorporated six different levels of hydrolised fat. After only a few days, the larvae began to suffer health problems, and mortality rose in line with the amount of free fatty acids in the feed.
“We believed that the larvae would be able to deal with the lowest levels without difficulty, but they could not. Quite the reverse, in fact; we found that a short period on even small amounts of free fatty acids had fatal effects on their digestive system. Finally, parts of the cell walls in the gut simply dissolved,” says Sæle.
Now Sæle and the NIFES research team hope to carry out model studies using zebrafish, which would provide them with a more general understanding of how free fatty acids affect organisms. In the long run, knowledge of this sort could also be of relevance to human medicine.
“Not much research has been done on this field. One early study of piglets suggests that the physiology of the porcine digestive system is very similar to that of fish, but it is too early say anything very definite at this point,” says Sæle.
Contact: Øystein Sæle
Tel: +47 991 20 835