EE201700039A - Fish feed, a process for its preparation, and a method for feeding fish - Google Patents

Abstract

The present invention relates to fish feed, a process for the preparation thereof, the use of said feed for feeding fish and a method for feeding fish. The fish feed according to the invention provides higher berry production and, at the same time, better fish growth and less gutting loss compared to feeds known in the art.

Description

The present invention relates to a fish feed, a method of preparing the same, the use of said feed for feeding fish, and a method of feeding fish. The fish feed according to the invention provides higher berry production, better fish growth, and lower gutting loss compared to feeds known from the prior art.

Background of the invention

In Finland, Sweden and parts of Denmark, rainbow trout production is economically highly dependent on berry production. The Norwegian salmon industry, on the other hand, does not focus on berry production because the quality of the fish meat is considered to be lower when the fish reach sexual maturity. Therefore, the development of salmon and trout feed has focused on breeding fish that do not reach sexual maturity.

The amount of berry in fish varies and depends, for example, on how close to spawning the fish are killed. In Finnish conditions, the normal GSI (gonadosomatic index, maturity coefficient, berry mass as a percentage of live fish) for rainbow trout in December-January is 10-14%.

The nutrients required for growth, reproduction and other physiological functions of fish are similar to those of other animals, i.e. proteins, minerals, vitamins and lipids. The components used in commercial fish feed can therefore be classified as protein (amino acid) sources, lipid sources, carbohydrate and vitamin and mineral sources.

Fish feeds are available in different formulations and grain sizes, designed for different types of fish at different growth stages. Fish larvae need a diet rich in vitamins and proteins to grow. As the fish grows, the energy level of the feed increases to allow for efficient growth. Since high-quality protein is more expensive than fat, a high energy level in the feed is usually ensured by increasing the fat content. A conventional extruded feed, which is used for trout weighing more than 350-500 g in their final growth stage, contains about 36% protein and 35% fat by weight.

Johnson et al. (North American Journal of Agriculture, 73, 409-417, 2011) studied the effects of dietary lipids on egg composition and embryo and larval quality when coco salmon broodstock were fed diets with two different fatty acid profiles during vitellogenesis (yolk formation). Dietary lipids, such as docosahexanoic acid (DHA), incorporated into egg lipids but did not affect embryo survival or larval survival and growth.

Patent application WO 2010087715 A1 discloses an extruded fish feed containing a higher amount of the amino acid arginine. Patent application WO 9612415 A1 discloses a fish feed containing urea and TMAO (trimethylamine oxide) to give the fish meat a stronger color. Patent application 9945796 A1 discloses a fish feed containing a high concentration of lysine, and document EP 1250850 A2 discloses a fish feed containing added carotenoids and polyphenols.

A review article by Peng et al. (Aquaculture, 251 (2006) 141-152) discusses the role of nucleotides and their metabolites in fish nutrition. According to this article, exogenous nucleotides have shown promise as dietary supplements in enhancing fish immunity and disease resistance. It has now been found that it is possible to significantly increase egg production for human consumption and/or fish farming by feeding fish during vitellogenesis, i.e. the period when the fish develop and produce eggs in their bodies, with a diet containing a higher amount of protein than in the regular diet. Even higher egg production was obtained when nucleotides and/or nucleosides were added to a high-protein diet. Egg production and fish meat productivity also increase when fish are fed a high-protein diet only during part of vitellogenesis, i.e. during early (endogenous) vitellogenesis, or alternatively during late (exogenous) vitellogenesis, but then the increase in fish growth and egg production is smaller than that observed when high-protein diet is provided throughout the vitellogenesis period.

Summary of the invention

The present invention provides a method of feeding fish raised for human consumption, the method comprising providing the fish with a feed composition during vitellogenesis, i.e. the period when the fish develops and produces eggs in its body, the feed containing carbohydrates and a high amount, preferably 38-48%, of protein and 25-34% of fat.

A further object of the invention is to use a fish feed containing a high amount of protein to increase the egg production and/or growth of fish by feeding said fish feed to the fish during vitellogenesis, i.e. the period when the fish develops and produces eggs in its body. More specifically, the fish feed used according to the invention contains carbohydrates and 38-48% protein and 25-34% fat.

The invention also relates to a fish feed comprising carbohydrates and 38-48%, preferably 39-45%, even more preferably 41-42% protein and 25-34%, preferably 28-33% and more preferably 30-33% fat with added nucleotides and/or nucleosides. The feed preferably also comprises added vitamins and amino acids. A further object of the invention is a method for preparing a fish feed comprising the steps of mixing the feed raw materials, except fat, extruding the resulting mixture and then vacuum coating the resulting extrudate to obtain pellets of suitable size, wherein the feed comprises carbohydrates and 38-48%, preferably 39-45% protein and 25-34%, preferably 28-33% fat, wherein the majority of the fat is added during the vacuum coating step of the pellets.

Detailed description of the invention

The feed according to the invention is particularly suitable for fish used for berry production, such as rainbow trout, salmon, sea trout and whitefish, especially rainbow trout and whitefish, during the period when the fish begins to develop and produce berry in its body, i.e. during vitellogenesis. The feed according to the invention contains carbohydrates and 38-48%, preferably 39-45%, more preferably 40-44% and even more preferably 41-42% protein, while the fat content is 25-34%, preferably 28-33% and more preferably 30-33%. The protein content can be selected depending on the specific species of fish fed. For rainbow trout, the most preferred amount of protein is 41-42%. For whitefish, the preferred amount of protein used according to the invention is 44-45%, while the fat content is 25-26%. An important source of protein in the feed according to the invention is fishmeal. However, the amount of fishmeal in the feed according to the invention does not have to be as high as in conventional feed for broodstock, which means cost savings for the fish farmer. Other protein sources include, for example, soy, soy protein, wheat, wheat gluten, corn, corn gluten (i.e. corn protein), field bean, lupin, pea protein, rapeseed/canola, potato protein, cottonseed meal, animal protein sources such as blood meal, hemoglobin meal, and other animal proteins such as poultry and insect proteins.

Sources of fat include vegetable oils such as rapeseed oil, palm oil, soybean oil, olive oil, linseed oil and cottonseed oil, fish oil and animal fats. Preferred sources of fat are rapeseed oil and fish oil. By adjusting the ratio of rapeseed oil to fish oil in fish feed during fish growth, it is possible to obtain optimal levels of the healthy fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) in fish muscle.

The feed according to the invention may also contain added nucleotides and/or nucleosides, in particular in free form, to further increase berry productivity. Nucleotides and nucleosides, which are the building blocks of DNA and RNA and ATP, accelerate cell division. Even a small amount of added nucleotides/nucleosides has an effect, but the amount of nucleotides/nucleosides added to the feed should be at least 0.005%, preferably at least 0.01%, more preferably at least 0.05% and even more preferably at least 0.1% and may also be above 0.5%. The upper limit of the added nucleotides is not critical, since nucleotides are not harmful to fish even in large quantities. If the feed contains less than 30% fishmeal, it is particularly advantageous to add nucleotides/nucleosides to the feed. Especially if the feed contains less than 20% fishmeal, some form of nucleotides/nucleosides should be added to the feed to further guarantee an increase in berry production.

The nucleotides/nucleosides added to the feed according to the invention are commercially available and are usually obtained from yeast nucleic acid extracts. The nucleotides are preferably added in free form.

It is known that fish food must contain the essential amino acids arginine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine, because fish cannot synthesize these amino acids from other molecules. Some amino acids, such as cysteine and tyrosine, are non-essential amino acids, because cysteine can be synthesized from methionine and tyrosine from phenylalanine. To guarantee an adequate level of essential amino acids in fish food, for example, methionine and lysine are added to fish feed to ensure sufficient absorption of these amino acids.

It has now been found that the berry production of fish is further improved by adding large amounts of amino acids to the diet. In particular, it has been found that the addition of the amino acids tryptophan, methionine and cysteine/cystine in particular has a positive effect on berry production. Accordingly, the feed according to the invention preferably contains at least one added amino acid from the group of methionine, tryptophan, cysteine, cystine and lysine in amounts greater than those recommended by the NRC (National Research Council) in 2011, or in amounts greater than those recommended by the other amino acids. The feed according to the invention preferably contains at least one added amino acid from the group of tryptophan, methionine and lysine in amounts such that the feed contains said amino acid(s) in amounts greater than those recommended by the NRC in 2011. The preferred added amino acid is methionine. More preferably, at least two amino acids from tryptophan, methionine and lysine are added to the feed according to the invention in amounts such that the feed contains said amino acids in amounts greater than those recommended by the NRC in 2011. Even more preferably, the amino acids tryptophan, methionine and lysine are added to the feed according to the invention in amounts such that the feed contains said amino acids in amounts greater than those recommended by the NRC in 2011. In addition to the amino acids tryptophan, methionine and/or lysine, other amino acids may be added to the feed according to the invention in amounts greater than those recommended by the NRC in 2011.

Based on NRC recommendations (2011), rainbow trout require 0.3% tryptophan, 0.7% methionine, 1.1% methionine + cysteine and 2.4% lysine (in weight percent of the feed calculated on the basis of digestible amino acids). When using the feed and method according to the invention, methionine is added in such an amount that the feed contains digestible methionine of more than 0.7%, preferably more than 1.0%, more preferably more than 1.4% and even more preferably more than 2.1% of the feed weight. Tryptophan is added in such an amount that the feed contains more than 0.3%, preferably more than 0.4%, more preferably more than 0.6% and even more preferably more than 1.0% digestible tryptophan. The amount of digestible lysine in the feed according to the invention is above 2.4%, preferably above 2.8%, more preferably above 3.2% and even more preferably above 7% of the weight of the feed.

The fish feed according to the invention also contains vitamins that are necessary for the growth of fish and the production of eggs. The level of vitamins in the feed is regulated by adding a vitamin premix to the feed, which contains all the vitamins necessary for fish.

Vitamins can include any of the vitamins, such as vitamin C, vitamin E (tocopherol), vitamin B, vitamin A, vitamin D, and vitamin K. Vitamins C and E together provide a strong antioxidant effect. Vitamin E includes various tocopherols (α-, γ-, and δ-tocopherol), which protect polyunsaturated fatty acids, such as eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), from oxidation. In addition to its antioxidant effect, vitamin C reactivates the effect of vitamin E.

Vitamin A refers to a group of compounds that have similar biological activity. The main forms of vitamin A include, for example, retinol, retinoic acid and carotenoids. Carotenoids are precursors of vitamin A, the most important of which is beta-carotene. Carotenoids also include astaxanthin, which is a common additive in fish feed. Astaxanthin gives salmon its red color and also has antioxidant properties. The amounts of vitamins and astaxanthin in fish feed are known to those skilled in the art and can be adjusted accordingly. The amount of vitamins in the feed according to the invention may, for example, be lower than in feed intended for broodstock or fish larvae.

The feed according to the invention may also contain beta-glucan to increase phagocytic activity and thus activate the immune system. If beta-glucan is used, its amount is within the limits known to those skilled in the art of fish feed. Other possible additives in the feed according to the invention are, for example, phytase enzyme and other useful enzymes. Since phosphorus is stored in plant material as inositol hexaphosphate (phytic acid), phytase is required to break down inositol hexaphosphate in the digestive tract and release phosphorus. The natural activity of phytase is almost non-existent in the digestive tract of fish and when plant sources of phosphorus are used in the fish feed, the added phytase allows the release of phosphorus from inositol hexaphosphate. The amount of phytase added may be, for example, about 750 FYT per kilogram of feed. The feed according to the invention preferably also contains lecithin. In fish feed, lecithin is primarily used to emulsify fat, thus improving digestion and feed efficiency, but lecithin also has a nutritional role as a source of phospholipids, namely phosphatidylcholine, phosphatidylinositol and phosphatidylethanolamine. Lecithin also acts as an antioxidant to extend the shelf life of feed.

Fish feed comes in different pellet sizes and densities to meet the needs of the species at different growth stages. The pellet size should generally be about 20-30% of the mouth size of the fish species. In practice, the pellet size for rainbow trout varies from 1.7 mm pellets for small fish larvae to 9 or even 12 mm pellets during the final growth period. For use according to the invention, a pellet size of 7 to 12 mm, preferably 7 to 9 mm, is preferred for rainbow trout. Fish may eat smaller pellets than recommended at a particular growth stage, and therefore the actual pellet size is not critical.

The means of the present invention provide significant advantages. The method of the invention can produce meat with much higher productivity, especially in fish that produce eggs. When the test feed according to the invention and the control feed (a conventional feed with 36% protein and 35% fat) were fed for about 6 months to fish that were expected to reach sexual maturity that year, it was found that the test feed had a better feed conversion rate (FCR) than the control feed. The test feed also gave higher meat productivity and a much higher percentage of eggs, which means a significant increase in the absolute amount of eggs. Reduced gutting losses were also observed, even when the test feed was fed only during the last 4.5 months of vitellogenesis.

In the method according to the invention, the time of starting feeding with the new feed depends on the age and species of the fish involved, as well as the temperature of the water in the farm, light, etc., i.e. factors that have an impact on the timing of egg production in the fish's body. The reproductive cycle of rainbow trout with the first spawning begins approximately 12 months before the actual spawning time. The maturation of the eggs and the formation of the yolk (vitellogenesis) is a hormonally controlled and regulated process, whereby the vitellogenin produced in the liver is stored in the mature eggs. Vitellogenesis includes different phases, starting with previtellogenesis, followed by endogenous and exogenous vitellogenesis. In rainbow trout, under Finnish conditions, previtellogenesis begins in the spring (April-May), followed by endogenous vitellogenesis and finally exogenous vitellogenesis until spawning.

The method according to the invention provides the fish with a feed composition during vitellogenesis. The vitellogenesis period varies in different fish species and also depends on the light and temperature conditions. The preferred period for feeding rainbow trout according to the method of the invention is at least about 3 months, more preferably 3-4 months, even more preferably 5 months, 5-6 months, 6-7 months, 7-8 months, 8-9 months, 9-10 months or more than 10 months. The most preferred period for feeding rainbow trout according to the method of the invention is 6-8 months under Finnish conditions. The berry production and/or meat productivity, especially the berry production of salmonids, is affected even at shorter periods (1-2 months, typically at least 1 month, more typically at least 2 months, most typically at least 3 months) and this is within the scope of the invention.

In one embodiment of the invention, fish are fed according to the invention during early vitellogenesis for 3-4 months. In a more preferred embodiment of the invention, fish are fed according to the invention for 3-4 months during late vitellogenesis. In another embodiment of the invention, fish are fed according to the invention during part of vitellogenesis or any combination of times during vitellogenesis. In a preferred embodiment of the invention, fish are fed according to the invention during the entire vitellogenesis.

The invention also relates to the use of a high protein fish feed to increase egg production for human consumption and/or fish farming by feeding said fish feed to the fish during vitellogenesis, i.e. the period when the fish develops and produces eggs in its body. More specifically, the fish feed used according to the invention contains carbohydrates and 38-48% protein and 25-34% fat. Preferred feed compositions are disclosed in this specification. Both the eggs produced and the fish meat are intended for human consumption.

In this specification, percentage (%) means weight percent unless otherwise specified.

The following experimental section illustrates the invention.

Example 1

The fish tested were rainbow trout, which were about to reach sexual maturity and had an average initial weight of 0.88 kg. The average final weight at slaughter was 2.85 kg. The diets contained the usual components of fish feed with the following differences in the feed composition.

Control feed (%): protein 36.0

fat 35.0

Test feed (%): protein 41.0

fat 31.0

Both feeds had the same food energy.

The fish (1328 pcs) were divided into 8 cages (166 fish per cage). Four cages were fed with control feed (Hercules LP, 7 mm) and four cages with test feed from late May to early September. Due to high water temperatures, some mortality occurred and the fish had to be starved for a longer period in the summer. In September, all fish in the cages were divided into two to allow for a change in the feeding of the subgroups.

The groups were labeled as follows.

AA - received only control feed

AB - received control feed at the beginning of the testing (about 3 months) and at the end of the testing period

(during the exogenous vitellogenesis phase, i.e. approximately 3 months) of the test feed

BB - received the test feed throughout the testing period (approximately 6 months)

BA - received test feed at the beginning of testing (during endogenous vitellogenesis, approximately 3 months) and control feed at the end of the testing period (approximately 3 months)

Otherwise, feeding was the same in each cage. Fish were fed according to the feeding instructions of the commercial feed manufacturer (Raisioagro Oy feeding table for rainbow trout, 2014), which was based on the temperature of the farm water and the average weight of the fish in each cage. Samples were taken and fish were weighed at the beginning of the test, at the beginning of September and at the beginning of December.

In December, 30 fish from each cage were weighed, stunned, bled and their weight after evisceration and sexual maturity were recorded. Evisceration loss was determined as the weight of the viscera as a percentage of the live weight and the weight of the eviscerated fish. Gonads were weighed in December. GSI (Global Salmon Initiative) values were determined as the weight of the gonads as a percentage of the live weight of the fish and the weight of the eviscerated fish.

Results

The mean values of FCR (feed conversion ratio) and SGR (specific growth rate) for each feeding group are presented in Table 1. SGR values illustrate the growth rate of fish (%) per day, based on their live weight. FCR values indicate the amount of feed required to produce 1 kg of fish. The lower the FCR value, the more efficiently the feed is converted into fish growth. Relatively high FCR values in the first test period are likely to result in suboptimal rearing conditions during the hot summer.

The results show that the group that received only the test feed showed a significant increase in growth compared to the group that received only the control feed throughout the experiment. The groups that received the test feed only for part of the test period also showed better growth and growth rate than the control group.

Table 1. Average FCR and SGR in treatment groups

Weight of gutted fish, viscera weight, gonad weight and GSI (%) are presented in Table 2.

As can be seen from Table 2, the weight of gutted fish in the test group (BB) was significantly higher than in the control group (AA) and also higher than in groups AB and BA.

Table 2. Live weight, gutted fish weight, gonad weight, viscera weight, GSI (%) and viscera (%) in sexually mature fish in December

The absolute amount of gonads increased surprisingly much in all three groups tested, and especially in group BB.

The results also show a surprisingly reduced gut loss (i.e., reduction in gut mass) in the group fed the test feed.

Table 2 also shows the growth rate coefficient (GSI) (%) based on the weight of live fish (1) and gutted fish (2). The results show an increase in the growth rate coefficient in fish fed the test diet compared to fish fed only the control diet. As already noted, fish fed the test diet had a higher weight than those fed the control diet. Thus, even though the fish were larger, the GSI values in group BB were unexpectedly even higher than in the control group AA.

Example 2

The test fish were rainbow trout, reared for commercial consumption in a fish farm. One cage was fed the test feed, the other was fed the control feed. Testing began in May. The feed compositions were as in Example 1. The daily feed rates used in the test followed the recommendations given in the feeding tables (Raisioagro, 2014). The fish were killed in December, i.e. during berry production. Approximately 100 fish were sampled from each cage.

FCR (feed conversion ratio) values were calculated for the entire group of fish. The FCR of the control group was 1.37, the FCR of the test group was 1.28. The FCR of the test group was significantly better than that of the control group. Other results are shown in Table 3.

Table 3. Results at the end of testing, calculated for sexually mature fish

The results show that in addition to significantly better FCR values, the test group also had a surprisingly higher berry production (GSI) compared to the control group. The results also show a surprisingly reduced weeding loss in the test group.

Example 3

The test fish were rainbow trout, reared for commercial consumption in a fish farm. The testing involved two cages of fish: one cage was fed the test feed of Example 1 (Test Feed 1) and the other cage was fed the same test feed as Test Feed 1, but with the addition of a commercial nucleotide (Rovimax NX) in an amount of approximately 0.03% (Test Feed 2). According to the manufacturer, the commercial nucleotide had a free nucleotide concentration of over 40% (total nucleotides of over 80%).

Testing began in early July and the fish were killed in February, i.e. during berry production. The daily feed rates used in the test followed the recommendations given in the feeding tables of the manufacturer of the control feed corresponding to Example 1 (Raisioagro, 2014).

Table 4. GSI results at the end of testing, calculated for sexually mature fish, and their live weight at the beginning and end of testing

The results show that even higher berry production is obtained with the feed according to the invention, which, in addition to a high protein content, contains nucleotides and/or nucleosides as additives.

Claims (28)

1. A method of feeding fish raised for human consumption, the method comprising providing a feed composition to the fish during vitellogenesis, the feed composition comprising carbohydrates, 38-48% protein and 25-34% fat by weight of the feed. 2. The method according to claim 1, wherein the feed composition contains 39-45%, preferably 41-42% protein. 3. The method according to claim 1 or 2, wherein the feed composition contains 28-33%, preferably 30-33% fat. 4. The method according to any one of claims 1-3, wherein the feed composition comprises added nucleotides and/or nucleosides. 5. The method according to claim 4, wherein the amount of nucleotides and/or nucleosides added to the feed composition is at least 0.005%, preferably at least 0.01%, more preferably at least 0.05%, even more preferably at least 0.1% and even more preferably at least 0.5% or more by weight of the feed. 6. The method according to any preceding claim, wherein the feed composition comprises added amino acids selected from the group consisting of arginine, cysteine, cystine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine. 7. The method of claim 6, wherein the added amino acids are selected from the group consisting of methionine, cysteine, cystine, lysine and tryptophan. 8. The method of any preceding claim, wherein the feed composition contains amino acids in amounts greater than those recommended by the NRC. 9. A method according to any preceding claim, wherein the feed composition comprises more than 0.7%, preferably more than 1.0%, more preferably more than 1.4% and even more preferably more than 2.1% digestible methionine and/or more than 0.3%, preferably more than 0.6%, more preferably more than 1.0% digestible tryptophan and/or more than 2.4%, preferably more than 2.8%, more preferably more than 3.2% and even more preferably more than 7% digestible lysine by weight of the feed. 10. The method according to any preceding claim, wherein the feed composition further comprises nutrients selected from the group consisting of vitamins, minerals, astaxanthin, phytase enzyme and other beneficial enzymes, beta-glucan and lecithin. 11. A method according to any preceding claim, wherein the method comprises feeding the fish during all or part of vitellogenesis, e.g. during early vitellogenesis, during late vitellogenesis and/or at any combination of times during vitellogenesis, preferably during all vitellogenesis. 12. A method according to any preceding claim, wherein the fish is selected from the group consisting of salmonids such as rainbow trout, salmon, sea trout and whitefish, the preferred fish being rainbow trout. 13. Use of a high protein fish feed to increase egg production and/or fish breeding by feeding said fish feed containing 38-48% protein and 25-34% fat during vitellogenesis. 14. Use according to claim 13, wherein the feed composition contains 39-45%, preferably 41-42% protein and 28-33%, preferably 30-33% fat. 15. Use according to claim 13 or 14, wherein the feed composition further comprises added nucleotides and/or nucleosides. 16. Use according to claim 15, wherein the amount of nucleotides and/or nucleosides added is at least 0.005%, preferably at least 0.01%, more preferably at least 0.05%, even more preferably at least 0.1% and even more preferably at least 0.5% or more by weight of the feed. 17. Use according to claim 15, wherein the feed composition comprises added amino acids, said added amino acids preferably selected from the group consisting of methionine, cysteine, cystine, lysine and tryptophan. 18. Use according to any one of claims 13-17, wherein the feed composition comprises more than 0.7%, preferably more than 1.0%, more preferably more than 1.4% and even more preferably more than 2.1% digestible methionine and/or more than 0.3%, preferably more than 0.6%, more preferably more than 1.0% digestible tryptophan and/or more than 2.4%, preferably more than 2.8%, more preferably more than 3.2% and even more preferably more than 7% digestible lysine. 19. Use according to any one of claims 13-18, wherein the fish are selected from the group consisting of salmonids, such as rainbow trout, salmon, sea trout and whitefish. 20. The use according to claim 19, wherein the fish is rainbow trout. 21. Use according to any one of claims 13-20, wherein the feed composition further comprises nutrients selected from the group consisting of vitamins, minerals, astaxanthin, phytase and other beneficial enzymes, beta-glucan and lecithin. 22. Use according to any one of claims 13-21, wherein the fish are fed with said fish feed during the entire vitellogenesis or during part of vitellogenesis, for example during early vitellogenesis, during late vitellogenesis and/or any combination of times during vitellogenesis, preferably during the entire vitellogenesis. 23. A fish feed comprising carbohydrates, 38-48% protein and 25-34% fat by weight of the feed, wherein the fish feed further comprises added nucleotides and/or nucleosides. 24. A fish feed according to claim 23, further comprising added amino acids in amounts greater than those recommended by the NRC. 25. Fish feed according to claim 23, wherein the amount of added nucleotides is at least 0.005%, preferably at least 0.01%, more preferably at least 0.05%, even more preferably at least 0.1% and even more preferably at least 0.5% or more by weight of the feed. 26. The fish feed of claim 24, wherein the added amino acids are selected from the group consisting of methionine, cysteine, cystine, lysine and tryptophan. 27. Fish feed according to any one of claims 23-26, wherein the feed contains more than 0.7%, preferably more than 1.0%, more preferably more than 1.4% and even more preferably more than 2.1% digestible methionine and/or more than 0.3%, preferably more than 0.6%, even more preferably more than 1.0% digestible tryptophan and/or more than 2.4%, preferably more than 2.8%, more preferably more than 3.2% and even more preferably more than 7% digestible lysine. 28. A method for preparing fish feed, comprising mixing feed raw materials, except for a portion of fat, extruding the resulting mixture and then vacuum coating the resulting extrudate to obtain granules of suitable size, wherein the feed comprises carbohydrates, 38-48%, preferably 39-45% protein, 25-34% fat and added nucleotides and/or nucleosides, with most of the fat being added during the vacuum coating step.