Fat provides the majority of energy in fish feed for salmonids, and feed for large salmon can contain 30-40 percent fat.
Nofima has a unique respiration laboratory at Sunndalsøra, where it is possible to measure the fish's metabolism of energy and nutrients from the feed. This is achieved through continual measurement of the gasses in the water, so that the fish's consumption of oxygen can be measured along with the production of CO2, ammonia and urea. Scientists also measure the energy and nutrients in the feed, excrement and in the growth of the fish. Collectively, these measurements provide detailed information about the fish's energy and nutrition metabolism, which enables us to find the fat, protein and carbohydrate levels in the feed which are best utilised by the fish and how the various feedstuffs are utilised.
Proteins
In terms of the amount and value, protein is the most important nutrient in fish feed and comprises amino acids. The fish needs the individual amino acids. The largest proportion of protein the fish consumes is used to make muscle protein as the fish grows, but amino acids are also building stones for other proteins as enzymes, transport proteins and connective tissue protein. They are also precursors for bioactive molecules such as hormones, neurotransmitters and components of DNA. Knowledge about the fish's requirements for each individual amino acid is still limited, and Nofima is working to chart this.
The fish can oxidize protein to give energy, but as this is generally an expensive energy source we want to limit the amount that is oxidized. In addition, the waste product forms ammonia when the amino acids are broken down, and this should be kept to a minimum in the water environment. As a result, the feed is usually designed to give enough protein to fulfil needs for musculature and the like, but not so much that it may be used as an energy source.
Fishmeal, which was previously the main ingredient in salmon feed, is now a limited and expensive resource. To reduce the proportion of fishmeal in the feed, a lot of work is now taking place to find alternative sources of protein. Nofima has several projects in which such alternatives are being tested. In addition to fishmeal, sources of protein in fish feed include animal by-products, various soya products, corn gluten and wheat gluten.
Many of Nofima's nutrition scientists are part of the Aquaculture Protein Centre (APC), a Norwegian Centre of Excellence comprising scientists from the Norwegian University of Life Sciences (UMB), the Norwegian School of Veterinary Science (NVH) and Nofima, which is funded by the Research Council of Norway. The centre works purposefully to increase the utilisation of protein resources based on knowledge about the nutritional requirements of farmed fish.
Fat
Fat is the second most important nutrient. As well as being an important energy source, fatty acids have several physiological functions. Nofima has found that salmonids in general have a requirement for the essential omega-3 fatty acids, which comprise about one percent of their diet. These fatty acids may be supplied via marine feed ingredients such as fishmeal and fish oil.
In fishmeal-based feed, the requirement for essential omega-3 fatty acids may be covered without fish oil. Nofima has used vegetable oils in experiments without experiencing reduced growth. However it has influenced several health parameters. High levels of vegetable oil will influence the fish's fatty acid profile, which can in turn affect the sensory quality of fish.
Some common sources of fat in fish feed are fish oil, rape seed oil, sunflower oil and soya bean oil. Krill oil and oil derived from the marine algae Calanus finmarchicus has potential, but is not currently used to any extent commercially. Fishmeal, which is primarily used in feed as a source of protein, contains about 10 percent fat and as such also contributes as a source of fat.
Nofima has built up a significant team of scientists working specifically on fat. Large-scale research programmes provide an opportunity to study the salmon's utilisation of fat in feed and look at the lipid metabolism in fish down to the finest detail. Optimal utilisation of the feed's fat is an important factor in resource-efficient feeding, and one of the goals is that the knowledge we are producing contributes to the fish storing less fat as intestine fat and instead oxidises the fat.
Nofima has proven that fat in salmon is not pure energy storage, but that it has several important physiological functions. Through experiments with cell cultures and experiments looking at gene regulation, we have found that fatty tissue produces hormone-like substances or signal molecules. Various fatty acids in the fat tissue influence the formation of these substances, which further influences the energy metabolism and the immune system.
Nofima is also studying the significance of fatty acids on quality. To chart whether the different fatty acids in the feed influence the post-slaughter muscle breakdown, Nofima shall utilise molecular biological methods to study the mechanisms behind muscle break down.
Carbohydrates
Carbohydrates are not a necessary part of the fish's diet, but are added to the feed mainly as a binding agent and to provide a good texture during processing. Small amounts of gelatinised starch can have a positive influence on the fish's growth, but in general feed utilisation is reduced by increasing the starch level in the feed above a certain level.
Wheat is often used as a source of carbohydrate/binding agent in fish feed.
Further description
