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Trout Digestion in Cold Water

Feed can deliver enhanced fat digestibility and performance, even in cold water.

In cold water, trout become less active and their metabolism slows down. Digestion slows down too and fat digestion, especially, becomes less efficient. This is in line with the trout’s natural lifestyle whereby very little food is available during the winter period.

Water Temperature Small

It's all about growth

The digestive tract is not accustomed to handle a lot of food. The enzymatic processes depend on the water temperature and take place at a low rate when the water is cold. However, in a trout farm growth is desirable during the whole year, including the colder months. Feeds that do well in summer may fail to give good results in winter. In intensive trout farms and farms with a restricted water availability, more pollution can therefore sometimes be detected during the winter time. This happens especially when the water temperature sinks to 8 °C and below.

Parr And Smolt

Keeping a good condition

The trout adapts its physiology to the cold period and one important aspect is that its requirement for essential fatty acids changes, compared to higher water temperatures. This adaptation is necessary for the fish to keep a good condition and stay healthy. Migrating salmonids go through a similar process when they smoltify. Initially, at the onset of smoltification, the fish will move into brackish water, changing to a marine diet of shrimp and bait fish.

Their camouflage changes to a silvery skin with a dark back and white belly. This helps them to blend in with the surroundings. The marine diet, rich in omega-3 fatty acids, assures a high membrane fluidity in the cold sea water so that they can move swiftly. This physiological adaptation assures a good digestion of food during very cold conditions.

At Alltech Coppens, we aimed to develop a trout feed concept that allowed optimal digestion, health, and performance during very cold periods. After extensive research at the Alltech Coppens Aqua Centre we have managed to considerably improve fat digestibility in very cold water and match the trout’s changed requirement for essential fatty acids.

Fat digestion is determined by four major factors:

  • The water temperature
  • The melting point of the fat in the feed
  • The fatty acid profile of the feed
  • The feeding level

The melting point of the oil in the feed determines whether the fatty acids can be emulsified well at a given water temperature. When the oil becomes more solid in cold water emulsification is compromised and digestion reduced. The key is to have the right melting point at a water temperature at 8°C and lower. Our R&D efforts have provided crucial data to get the melting point right in winter conditions!

Trout Digestion in Cold Water

Maintaining functionality

In order to function well in cold water, trout require more of the long-chain omega-3 fatty acids (LC-PUFA) as these are structural lipids that are built into the cell membranes. By incorporating more LC-PUFA in the cell membranes, as the water turns colder a high membrane fluidity and functionality is maintained. This is essential for the trout’s physiology and promotes good fat digestion. The latter, of course, is key to getting a good performance in the cold months.

“The trout adapts its physiology to the cold period”

The feed rate is also decisive for fat digestion. Since the fish’s metabolism slows down in cold water, it takes a long time for trout to digest a meal. This can take up to a few days. A high feed rate overloads the digestive tract and the trout is less capable of digesting it efficiently. This leads to a poorer FCR and more excrements, which may be visible to the farmer in the form of a higher degree of pollution. Our research has shown that a slightly lower feed rate helps to increase fat digestion and this is in line with the physiological adaptation the trout goes through in winter. In this respect, less is more. Apart from a slightly lower feed rate, it is also important to feed rather slowly in the cold period so that the slower trout have sufficient time to take up the pellets and none are wasted.

Alltech Coppens Crystal

Crucial points

Making sure that the feed has a high attraction and is palatable to the trout is also important so that the fish immediately react when feeding commences. For this purpose, the R&D team at Alltech Coppens has done feed trials to investigate the palatability of the different ingredients so that the right combinations can be made, in addition to meeting the nutritional requirements. These crucial points have all been addressed in Alltech Coppens’ trout cold water concept. This concept meets the requirements of the trout and the challenges of a trout farmer in winter. The trout feeds that excel in cold conditions are the highly digestible Supreme-22, Supreme Astax, Ultra, and Crystal (Astax).