Higher survival rate among cod larvae fed dry feed with partially digested proteins
Farmed cod and halibut larvae require live feed the first days after hatching before they can eat dry feed. This is a vulnerable period for the larvae. Studies carried out at the National Institute of Nutrition and Seafood Research (NIFES) show a higher survival rate among cod larvae when they are fed partially digested proteins.
In a feeding trial carried out by NIFES, cod larvae were given dry feed with varying levels of partially digested proteins i.e. hydrolysed protein from pollack and squid 41 days after hatching. Halibut larvae were fed a corresponding feed 63 days after the first feeding of live feed. In this period of time it is common that both cod and halibut are fed dry feed.
"When we increased the content of hydrolysed proteins from zero to 400 grams per kilo of dry feed, the survival rate for the cod increased from around 7 per cent to 18 per cent. The cod had then reached the age of 82 days, says researcher Kristin Hamre at NIFES".
"The opposite result was obtained with the halibut. When the content of hydrolysed proteins in the feed was increased from zero to 450 grams per kilo of dry feed, the survival rate decreased from 57 per cent to 22 percent", said Hamre. "This may be because the halibut eat more slowly than cod, and that nutrients may have leaked from the pellets into the water before the halibut had time to eat them".
Leakage of nutrients from dry feed?
It is important that the fish larvae ingest, digest and absorb the nutrients they need. This is a challenge, especially in the first period before the digestive system is fully developed. Nutrients can leak out from the pellets when they are in the water and before they are eaten by the fish. In order to examine the amount of the nutrient loss, the leakage from the feed in the study described above was compared using a heat-coagulaed feed, and two other feeds, one agglomerated and one protein encapsulated, with different particle sizes (less than 0.3 mm, 0.3-0.6 mm and 0.6-1.0 mm). The leakage was examined in feed containing intact protein and in feed with hydrolysed protein.
"The results show that there is leakage of nutrients from the dry feed, especially in the first two minutes after the feed is placed in the water. The leakage increases with decreasing particle size and decreasing molecular weight of the nutrients", said Hamre.
"The agglomerated feed showed the largest leakage of nutrients, followed by the heat-coagulated and the protein encapsulated feeds. Subsequent experiments have demonstrated extensive leakage of nutrients from the protein encapsulated feed during production".
"The hydrolysed feed showed a higher degree of leakage than the feed containing intact protein", he said.
Based on the scientific articles:
Kvåle, A., Mangor-Jensen, A., Harboe, T. and Hamre, K. (2009) Effects of protein hydrolysates in weaning diets to Atlantic cod (Gadus morhua L.) and Atlantic halibut (Hippoglossus hippoglossus L.). Aquaculture Nutrition 15, 218–227.
Kvåle, A., Yúfera, M., Nygård, E., Aursland, K., Harboe, T. and Hamre, K. (2006) Leaching properties of three different micropaticulate diets and preference for the diets in cod (Gadus morhua L.) larvae. Aquaculture, 251, 402-415.
The development of the cod and Atlantic halibut larvae in fish farms
Cod larvae are 4 millimeters and quite immature at hatching. Its mouth is small and the digestive system is not fully developed. The first two to three days after hatching, the larvae are supplied with nutrients from a yolk sac underneath the stomach. Later, it is fed with live rotifers. 25-30 days post hatching the larvae can eat dry feed. Work is in progress to reduce the time the larvae is dependent on rotifers by developing satisfactory dry feed and feeding regimes.
Atlantic halibut larvae are 12 millimeters and quite immature at hatching. In this life stage, until they reach 45 days of age, the larvae depend on nutrient supply from the yolk sac. Larvae from 45 to around 100 days are not capable of eating dry feed, and are therefore usually fed the crustacean Artemia. Work is in progress to reduce the time the Atlantic halibut larvae is dependent on live feed. Improved dry feed already enables some fish farms to wean their Altantic halibut larvae earlier than by 100 days of age (after hatching).