Fish, like humans, are influenced by taste. Research suggests that fish taste is more simplified than human taste; instead of responding to sweetness, fish taste receptors have a positive response to amino acids, allowing the fish to anticipate feed characteristics. Taste buds occur on the tongue, but also throughout the gut, and are responsible for identification of positive and negative taste responses, but also triggering hormonal signals for digestion and growth. This ‘nutrient sensing’ is the other pathway for hunger signalling, and we have found that stronger stimulation of gastric taste also promotes an elevated hunger signal when the stomach is empty. Based on this knowledge, Skretting ARC has found that changing the amino acid composition in feed can make it more appealing and thus achieve better feed intake. The 'hunger hormone' - ghrelin - is released from taste receptors in the stomach and acts on the hypothalamus in the brain, triggering hunger and feeding behaviour to prepare the body for food intake. Fish can anticipate feeding time, in much the same way as people can get hungry at the same time each day. Research has shown that it is possible to exploit this trait and achieve better digestion when fish are fed at the right time, when their bodies are primed to receive feed into the system.

Skretting ARC has conducted numerous trials to determine the taste that fish prefer. Results have shown that raw materials can be made more appetising by changing the structure of the proteins. Proteins are long chains of amino acids, and when proteins are processed in a particular way, they can be made more or less active in stimulation of the taste receptors. The final product is determined by the processing conditions and the specific raw materials used. The figures below show a significant increase in feed intake when using three palatability enhancers (shown in red) when compared with the control feed. A significant increase was also observed when compared to the krill meal positive control. These results formed the basis of the new Prime & Express feeds.

The vertebrate stomach possesses specialised cells that produce ‘the hunger hormone’ - ghrelin. When the stomach is filled with food, the stretching of the stomach walls stops the secretion of ghrelin, generating the feeling of satiety. As food enters the stomach, the process of digestion begins, breaking feed down into simple nutrients that can be absorbed. Efficient digestion is critical to achieve optimal nutrient distribution to growing tissue and thus promote growth. To investigate whether feed anticipation behaviour can be exploited to facilitate better digestion, Skretting ARC measured ghrelin levels over time when feeding different diets. Higher levels of ghrelin mean that the fish is preparing for food intake, including digestive and growth processes. Feeding fish at regular intervals can maximise the digestive capacity.

Skretting ARC analysis of feed transit in the stomach has shown that the time needed for feed to move from the stomach to the intestine can be significantly reduced. The figure below shows that in fish eating the same amount of feed, the fish fed Express have both eaten more feed than those eating the control feed (Premium) and the transit of the feed through the stomach and into the intestine is faster. Effectively, the fish are eating more and digesting faster – meaning faster and increased growth.

Prime & Express are optimised to support better transport of feed nutrients into the depths of the pyloric caeca. The effects have been documented by Skretting ARC by inserting radio-dense glass spheres into the feed, allowing x-ray visualisation of feed location over time.

X-ray showing better feed penetration in the pyloric caeca in fish fed Express compared with a standard diet.

The resulting images clearly show increased penetration of the feed in the pyloric caeca in the fish fed Express when compared with the control fish. The increased mobilisation of fat through the digestive system was further quantified by measuring the vacuolised areas of the pyloric caeca. Nutrients need to be mobilised through intestinal cells into the blood to allow further nutrients to be absorbed. When feeding Prime & Express, the vacuolised area was significantly reduced, indicating that fat is much more readily absorbed without being held up in the intestinal cells.

Histological assessment of vacuolised area in pyloric caeca. The top images show a section with a high level of vacuolisation (Control feed), with quantification to the right. The bottom images show a section with a low level of vacuolisation (Express), with quantification to the right.

The salmon intestine is a complex, multifunctional organ and maintains the balance between efficient absorption of nutrients and protection. To achieve this balance, the intestine has a selective and complex barrier that works together with innate and adaptive immune responses.

The enterocytes are intestinal cells in the epithelial layer. Intestinal enterocytes function as a primary site of nutrient processing and absorption, but also act as a very efficient physical barrier against pathogens or resident microbiota, while at the same time allowing passage of nutrients to the systemic circulation.

The intestine is home to different kinds of immune cells, and the structure and cell activity of the posterior intestine suggests that immune responses may be initiated in this region. With such important functions, it is important that the intestine is part of the investigation into new feed development, and Skretting ARC has completed advanced histological investigation to eliminate the possibility of any ill effects in fish fed Prime and Express.

The liver regulates nutrient distribution around the body, and is responsible for converting a lot of nutrients into the specific interchangeable forms that the body needs. The liver is also the major site of amino acid metabolism. Most proteins are completely digested into free amino acids in the intestine, which are then absorbed by the intestine and can be interconverted or metabolised in the liver. Proteins are essential for plentiful structural and metabolic tasks in every type of cell, including muscles, bones, organs, tendons and ligaments. There 20 primary amino acids. Plants and microorganisms can usually synthesise all amino acids, however fish and other animals are dependent on a dietary supply for some of them. Amino acids that cannot be synthesised by a fish on its own are referred to as essential amino acids (EAA) in contrast to the nonessential amino acids (NEAA). The respective NEAA requirements of a fish are covered by synthesising the required amount using the amino group of other feed amino acid. If a diet is inadequate in any essential amino acid, protein synthesis cannot proceed beyond the rate at which that amino acid is available. This is called a limiting amino acid. This can be described further using the "barrel theory". Each colour of the barrel represents an essential amino acid as a proportion of the daily requirement. If the colour corresponding to one amino acid is short, then the levels of all amino acids available in the body fall to that level, like water in a barrel with one short stave. Skretting ARC has undertaken many investigations to determine the correct balance of amino acids to optimise salmon growth, and Prime & Express are formulated to contain the right balance of amino acids.

Bones are live tissue, and the most important cells are those that are responsible for mineralisation - the process of deposition of minerals into the bones. This is what gives the bones their strength. Minerals are essential, but cannot be synthesised by the fish. A high proportion is supplied from the feed intake. While minerals are essential for bone strength, they are also important for other metabolic processes. During the smoltification process, the digestibility of minerals can be reduced. Prime has been specifically formulated to meet the requirements of rapidly growing post transfer fish, providing all the essential components to ensure maximum bone strength.

Skretting ARC trials have shown a reduction in certain minerals during and after transfer to seawater. Post seawater transfer salmon experience a rapid growth phase where mineral requirements may be higher due to rapid growth. At the same time there is an increase in muscle mass that will increase the forces transmitted to the bones. Previous Skretting ARC studies have shown that insufficient dietary minerals will stop bone mineralisation.

Muscle tissue is made of muscle fibres and can grow in size in two ways: by adding more fibres to the existing ones or by increasing the size of existing fibres. The first type of growth is known as hyperplastic growth while the latter is known as hypertrophic growth. Both types of growth start from proliferating myoblasts whose destiny can either be to fuse with other myoblasts and form a new fibre or “sacrifice” themselves by donating their nucleus and cellular content to an existing fibre in order to increase its size. In salmon, it has been shown that hyperplastic growth is especially important in earlier life stages, characterised by a marked increase in fibre number, and fades out towards harvest when hypertrophy takes over and growth is driven mainly by the growth of existing fibres. It is important to ensure that younger fish produce “good” growth. That is, a growth that underlies substantial recruitment of new fibres that can provide plenty of potential to later hypertrophic growth and deposition of adipose tissue to produce the final fillet.