© 2005 International Council for the Exploration of the Sea
Nutrition in cod (Gadus morhua) larvae and juveniles
National Institute of Nutrition and Seafood Research (NIFES) PO Box 2029, Nordnes, 5817 Bergen, Norway
*Correspondence to K. Hamre: Tel: +47 55905130; fax: +47 55905299. e-mail: kristin.hamre{at}nifes.no.
Over the past few years, great progress has been made in culturing cod larvae in indoor hatcheries using rotifers and Artemia or formulated feed as start-feed (intensive systems). However, when compared with natural systems based on copepods grown in seawater lagoons, the growth potential has not been fulfilled, and deformities of larvae and juveniles increase production costs. The deformities, which are seldom seen in natural systems, also constitute an ethical problem. The differences in growth and development of deformities in intensive and natural systems may be dependent, in part, on nutrition, but are caused by environmental conditions and early husbandry practises as well. To identify nutrients that may be deficient or in excess in live feed, we are in the process of screening the nutrient compositions of rotifers and Artemia grown or enriched on different feeds and comparing them with the composition of copepods and published requirements for larger fish. Replacing live food with formulated diets as early as possible is a goal of marine larval aquaculture. It is important that these diets contain protein which is available for the larvae and phospholipids that promote the absorption and transport of fat. The optimum macronutrient composition in diets for cod juveniles has been determined and can be extrapolated, with caution, to the larval stage. A problem in using formulated diets is the extensive leakage of nutrients as a result of the large surface area to volume and the short diffusion distance in the microparticles. Leakage leads to rapid loss of small, water-soluble molecules such as free amino acids, vitamins, and minerals, but extensive leakage of water-soluble protein has also been shown. The demand for protein available to the larvae, which probably will make the protein more water soluble, is therefore in conflict with the need to reduce protein leakage from the feeds. Development of feed production technology to prevent nutrient leakage is essential in order to make formulated diets a good alternative to live feed.
Keywords: cod, deformities, formulated larval feeds, Gadus morhua, juveniles, larvae, live feed, nutrient requirements
Received 13 June 2004; accepted 16 November 2005.
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Introduction |
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 Top Introduction Feeding regimes and growthDeformitiesNutrient composition of live...Development of the digestive...Leakage from formulated feedsLipid in formulated feedsOptimal macronutrient...Summary and conclusionsReferences |
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The first artificial production of cod juveniles was carried out in Norway in 1886, where larvae were stocked in a seawater basin at Flødevigen Research Station and fed natural zooplankton. Experiments were conducted to test the viability of hatched larvae, which were to be released on the Norwegian Skagerak coast, and continued until 1971 (Solemdal et al., 1984). In the 1980s, the experiments were resumed at Austevoll Research Station, Institute of Marine Research, and aimed at establishing a basis for commercial production of cod. The first real production of 70 000 juveniles took place in the lagoon Parisvatnet in 1983. Production in Norway reached a climax in 1989 when approximately 900 000 juveniles were produced in seawater lagoons (Svåsand et al., 2004), i.e. with extensive and semi-extensive production methods.
The extensive production method involves fertilization of the seawater in the lagoon to stimulate algal growth, which, in turn, stimulates the production of natural zooplankton. Eggs or newly hatched larvae are introduced into the lagoon and are allowed to grow to the juvenile stage before they are harvested. The larvae feed mainly on copepods. Sometimes, extra feed, i.e. a formulated diet, is given from the late larval stage until harvest. Before the lagoon is taken into production, the water is treated with Rotenone to eliminate predators. The semi-extensive production method utilizes natural zooplankton produced in a seawater lagoon, but keeps the larvae in enclosures floating in the water or in tanks on land.
In the 1990s, the interest in cod farming diminished owing to abundance in natural stocks, high catches, and low market prices. Juvenile production concentrated mainly on production for the (re)stocking of natural habitats. However, since approximately 2000, a decline in wild stocks and an increase in prices have led to new interest in cod aquaculture, but now the juveniles are produced using intensive methods, based mainly on developments in Japan and the Mediterranean. The intensive method, in brief, involves feeding larvae on the rotifer, Brachionus plicatilis, until approximately 25 days after first-feeding (dpff) and, thereafter, shifting the diet to Artemia or formulated feed. The larvae are held in indoor tanks under well-controlled environmental conditions. The advantage of this method is the substantially lower volume requirements for live feed production, which allow indoor cultures. This, together with light manipulation of the broodstock to spawn at pre-defined times, makes it possible to produce juveniles year-round.
Currently, the capacity for cod juvenile production in Norway has increased to 70 million per year. The actual production is increasing steadily, and reached 8 million in 2004 (Karlsen et al., 2005), but deformities in larvae and juveniles increase production costs and constitute an ethical problem. The deformities may be the result of nutrition, in part, because larvae that are fed copepods in extensive and semi-extensive systems usually develop normally. The high growth rates obtained under extensive and semi-extensive rearing conditions show that cod larvae have a very high growth potential. In intensive culture systems, this potential has not been fulfilled and nutrition may be one of the limiting factors. Development of formulated feeds for larvae will reduce the cost and labour-intensive production of live feeds. However, the larval feeds currently available are sub-optimal in several aspects, and there is an urgent need for more research and development of both the technical quality and availability of nutrients from such feeds.
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Feeding regimes and growth |
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TopIntroductionFeeding regimes and growthDeformitiesNutrient composition of live...Development of the digestive...Leakage from formulated feedsLipid in formulated feedsOptimal macronutrient...Summary and conclusionsReferences |
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Cod hatch from small eggs (1.2–1.5 mm diameter) and are approximately 4–5 mm standard length (SL) at first-feeding. In intensive culture, the larvae are normally offered rotifers from first-feeding until 20–30 days post-hatch (dph), after which they are fed Artemia. Alternatively, larvae can be weaned with a formulated diet, during which time they are offered rotifers together with the dry feed (co-feeding) for varying periods of time. Shields et al. (2003) transferred cod larvae from rotifers to Artemia 5, 15, and 25 dph and found a large increase in feeding incidence, growth, and survival (until 36 dph) with increasing age at transfer. MacQueen Leifson (2003) reported that the growth of cod larvae stagnated after 24 days when the larvae were fed on rotifers only. Both results support the common industry practise of shifting from rotifers to Artemia or a formulated diet 20–30 dph. In extensive and semi-extensive systems, cod larvae prey mainly on copepod nauplii at first-feeding, switching to older copepod stages as they grow.
The growth of cod larvae fed with natural marine zooplankton in extensive and semi-extensive systems can be very high. Finn et al. (2002) reported an average growth rate from 0 to 50 days post first-feeding of 13% per day, while in the period 44–50 dph, the larvae grew at 30% per day. Otterlei et al. (1999) studied the growth in cod larvae held at constant temperature regimes and fed on zooplankton harvested from the sea. They found a growth rate of >25% per day in larvae 15 dpff held at 14°C. For the larvae to fulfil this high growth potential, a well-balanced feed with good availability of all nutrients is required. Generally, growth in intensive systems is lower than in extensive ones. R. N. Finn and T. van der Meeren (unpublished) found an average growth rate of 10% from 0 to 50 dpff in cod larvae fed rotifers and Artemia, which they consider to be in the high range for intensive systems (T. van der Meeren, pers. comm.). MacQueen Leifson (2003) found that the growth rate of cod larvae on a similar feeding regime was 9.5% per day. There was no surge in growth in the intensively reared cod larvae like the one found in the semi-extensive system.
Marine fish larvae that are fed formulated diets before development of the stomach generally show less growth than larvae fed rotifers and Artemia. This is also the case with cod. MacQueen Leifson (2003) observed growth rates of 10.7% and 7.2% per day in cod larvae fed Artemia and a formulated feed from 24 to 36 dph, respectively. In another experiment, Callan et al. (2003) used similar feeding regimes from 22 to 64 dph and obtained growth rates of 7.1% and 5.1% per day, respectively.
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Deformities |
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TopIntroductionFeeding regimes and growthDeformitiesNutrient composition of live...Development of the digestive...Leakage from formulated feedsLipid in formulated feedsOptimal macronutrient...Summary and conclusionsReferences |
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The most common deformity in intensively reared cod larvae is the bend in the body immediately posterior to the head or the "stargazer" syndrome, which can affect a large proportion of juveniles produced by industry. Craniofacial disorders and shortened opercula have also been observed, but the number of fish affected is small. The bend in the "neck" can be observed in larvae around 10 dph, where the notochord and developing vertebrae are bent upward from the skull (G. K. Totland, pers. comm.). One hypothesis is that extension of the swimbladder puts pressure on the notochord, resulting in this bend. Also, a distended intestine resulting from a large intake of food can have a similar effect. However, nutritional imbalance is also one of the many possible explanations for the vertebral deformities found in intensively reared cod juveniles. It is well known that nutrient imbalances can create vertebral deformities in fish. Dedi et al. (1997) fed Artemia enriched with vitamin A to Japanese flounder (Paralichthys olivaceus) larvae and found a dramatic increase in vertebral deformities, where the vertebrae were compressed. The most sensitive period was 20–30 dph when the notochord segmentation took place. In mammals, vitamin A deficiency and excess give similar symptoms (Maden, 1994). Further, excess vitamin D caused increased rates of vertebral deformities in Japanese flounder when fed from 22 dph (Haga et al., 2004). The deformity was especially expressed as winding of the abdominal vertebrae, similar to the bend in the body immediately behind the skull seen in cod.
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Nutrient composition of live feed compared with tentative larval requirements |
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TopIntroductionFeeding regimes and growthDeformitiesNutrient composition of live...Development of the digestive...Leakage from formulated feedsLipid in formulated feedsOptimal macronutrient...Summary and conclusionsReferences |
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Depending on the analytical method used, the protein content of live feed varies substantially in the literature. In rotifers, the variation is 24–61% of dry weight (Table 1). We have recently measured total amino acids, which is the most accurate measure of protein content, and found a relatively stable protein content of 36–41%, with minimal dietary influence (Srivastava et al., in press). For copepods and Artemia, a similar variation is found (Table 1), but measurements of total amino acids have not been performed for these feed organisms yet. The protein requirement of cod juveniles is 41% of dietary dry matter (Hamre and Mangor-Jensen, in press), but fish larvae seem to retain a limited fraction of the dietary protein and compensate with increased feed intake (Øie et al., 1997), indicating that the protein requirement in larvae, in terms of feed fraction, may be lower than in metamorphosed fish.
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The lipid level of rotifers depends on the lipid level of their diet and may vary between 10% and 25% (Olsen, 2004). Also, rotifers are quite predictable when it comes to fatty acid composition, as the dietary fatty acid composition is mirrored in the rotifers. Olsen (2004) gave the following equations for the levels of n-3 fatty acids (EPA, eicosapentaenoic acid, 20:5n-3; DHA, docosahexaenoic acid, 22:6n-3):
- Rotifern-3 = 0.91 Feedn-3 (up to 60% dietary n-3)
- RotiferEPA = 0.88 FeedEPA
- RotiferDHA = 0.72 FeedDHA
- RotiferEPA = 0.88 FeedEPA
The lipid level in enriched Artemia is typically between 20% and 30%. It is difficult to get a good fatty acid profile in Artemia, and the data in the literature show a variation in DHA content of 3–10% of total lipid or fatty acids (Estevez and Kanazawa, 1995; Næss et al., 1995; Tocher et al., 1997; Shields et al., 1999; McEvoy et al., 1998; Hamre et al., 2002). An exception is Evjemo and Olsen (1997), who found 15–20% DHA in Artemia enriched with Super SelcoTM and DHA-SelcoTM. Many strains of Artemia contain EPA, and the catabolism of EPA in Artemia is lower than that of DHA (Evjemo et al., 2001). ARA (arachidonic acid, 20:4n-6) is found in unenriched Artemia nauplii and can be quite high in enriched Artemia, even though the enrichment medium contains little ARA (Hamre et al., 2002). Copepods may vary between 6% and 16% in lipid content; DHA and EPA are typically high (20–40% and 15–20% of fatty acids, respectively), and ARA is low (<1% of fatty acids, van der Meeren, 2003; Hamre et al., 2002).
The optimal fatty acid composition and lipid levels for cod larvae are not known, but lately, interesting reports on the fatty acid requirements in flatfish connected to malpigmentation have appeared. Estevez et al. (1999) varied the DHA/EPA ratio and the EPA/ARA ratio in live feed fed to turbot (Scophthalmus maximus) larvae and found that increasing ARA level, and thus decreasing EPA/ARA ratio, reduced pigmentation success dramatically. Variation in the DHA/EPA ratio had no effect on pigmentation, contrary to expectations. This has been confirmed in studies of turbot and halibut (McEvoy et al., 1998), yellowtail flounder (Limanda ferruginea) (Copeman et al., 2002), and Senegal sole (Solea senegalesis) (Villalta et al., 2005). However, a high level of DHA is required to ensure normal development of neural tissue and vision (Koven, 2003). Assuming that cod have a similar fatty acid requirement, one should aim at enriching the live feed with high levels of DHA and EPA and low levels of ARA. Since copepods contain 6–16% lipid (van der Meeren, 2003), one may argue that the live feeds used in intensive cod culture are too high in lipid. However, cod juveniles fed formulated feed from 0.26 g needed 15% lipid to reduce cannibalism, and up to 25% lipid had no negative effects on the fish (Hamre and Mangor-Jensen, in press).
Few data exist on the micronutrient composition of rotifers and Artemia, but some of our own data (unpublished) and data from van der Meeren (2003) are summarized in Table 1. Thiamine may be quite low, both in rotifers and Artemia, but can be enriched easily in both feed organisms to >20 µg g–1 dry weight (K. Hamre, unpublished). Thiamine is a co-enzyme in carboxylation and decarboxylation reactions, for example in the decarboxylation of pyruvate to acetyl-CoA, and therefore, is important for energy production, especially from carbohydrate as well as from amino acids. Vitamin C levels in copepods are very high and, although the levels in Artemia are somewhat lower, they are so far above the requirements of larger fish that one may assume vitamin C levels are sufficient. In rotifers, vitamin C levels seem to be lower and more variable, and the variation should be investigated further. It is generally assumed that larvae have a higher vitamin C requirement than larger fish (Merchie et al., 1997).
Vitamin A is generally below the detection limit of our analytical methods, both in copepods and Artemia (Moren et al., 2005). However, Atlantic halibut convert both astaxanthin and canthaxanthin to vitamin A at a rate probably corresponding to a requirement in larvae of about 500 µg g–1 dry weight in feed (Moren et al., 2004a). A similar ability to convert carotenoids is probably present in cod, as they also prey on copepods in the wild. Levels of astaxanthin in copepods and canthaxanthin in Artemia are above this "requirement" level (Table 1) (van der Meeren, 2003), so probably, the requirement for vitamin A in larvae is covered with these feed organisms. Rotifers cultured with yeast and oil may contain sufficient amounts of vitamin A, originating from the oil, to cover larval requirements (2.4 µg g–1 dry weight in halibut juveniles; Moren et al., 2004b), but culturing rotifers with RotimacTM or other algal compounds may not give any vitamin A at all, and the level of carotenoids may be far too low to cover the vitamin A requirement (Table 1). Vitamin A is a key factor in regulating embryogenesis in vertebrates (Maden, 1994) and may affect larval development by similar mechanisms. As mentioned earlier, excess vitamin A causes vertebral deformities in Japanese flounder (Dedi et al., 1997), and deficiency of vitamin A often causes symptoms similar to those caused by excess vitamin A (Maden, 1994). Therefore, vitamin A deficiency is one candidate for an explanation of the vertebral deformities found in cod fed on rotifers.
Iodine is necessary for production of thyroid hormone, a key regulator of metamorphosis in fish larvae. Copepods contain 100–700 times more iodine than rotifers and Artemia, and although fish obtain iodine from seawater, Atlantic halibut larvae fed copepods had higher levels of iodine and slightly higher levels of thyroid hormone than larvae fed Artemia (Solbakken et al., 2003). It is possible that marine fish larvae have adapted to the high iodine level in their natural feed and that live feeds used in the intensive larval production should be enriched with iodine.
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Development of the digestive tract and protein digestibility |
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TopIntroductionFeeding regimes and growthDeformitiesNutrient composition of live...Development of the digestive...Leakage from formulated feedsLipid in formulated feedsOptimal macronutrient...Summary and conclusionsReferences |
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In cod larvae at first-feeding, the pancreas and liver have started to develop, and the hindgut shows pinocytotic activity (Kjørsvik et al., 1991). In general, the exocrine pancreas in marine fish larvae is functional and supplies the midgut with digestive enzymes. Brush border membrane with microvilli develops and increases the absorptive surface of the intestine. Brush border membrane enzymes, including the peptidases, are also present and active, and all the protein digestive enzymes, except for pepsin, seem to be present (Kjørsvik et al., 2004), although not in adequate concentrations to promote total protein digestion (Kolkovski, 2001; Kjørsvik et al., 2004). The concentrations of pancreatic and brush border membrane enzymes increase during the larval ontogeny (Cahu and Zambonino Infante, 2001; Kjørsvik et al., 2004).
The stomach in cod starts to develop at about 15 mm SL (Pedersen and Falk-Pedersen, 1992), corresponding to 25–30 dph (Otterlei et al., 1999), but gastric glands are not seen until 20 mm SL (35 dph). The stomach develops gradually and is not completed until the fish is 45 mm SL (0.95 g and 60 dph). The pyloric caeca develop in parallel with the stomach. The development of the stomach in cod occurs after what is considered as metamorphosis (15 mm SL, disappearance of the larval finfold) and is slow compared with other marine fish larvae (Pedersen and Falk-Pedersen, 1992).
The low pH of a functional gastric stomach denatures ingested protein, so that peptide bonds buried in the three-dimensional structure become accessible to proteolysis by pepsin. Pepsin is also secreted into the stomach and is active at low pH. Because pepsin is an endopeptidase, peptides of varying length will be produced by protein digestion in the stomach. Further, the stomach is important for food storage, and it releases the digesta to the intestine at rates controlled by complex neural and hormonal mechanisms. In the intestine, the proteolysis continues by action of pancreatic proteases, such as trypsin and chymotrypsin, and the small peptides formed are broken down into free amino acids by the brush border membrane enzymes. In the absence of a stomach, the first step in protein digestion is omitted. This may not be critical when the larvae feed on live organisms, since more than 50% of the nitrogen in these organisms is found in free amino acids, peptides, and other water soluble proteins (Carvalho et al., 2003), which are probably easily digested. Structural proteins, which are most abundant in fishmeal used in formulated larval diets, probably have very low digestibility and would need a functional stomach to be fully available. This may be one reason why larvae normally grow slower on formulated than on live feeds. It is necessary, therefore, to develop protein processing methods and choose protein sources that will improve the availability of protein in formulated feeds for marine fish larvae.
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Leakage from formulated feeds |
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TopIntroductionFeeding regimes and growthDeformitiesNutrient composition of live...Development of the digestive...Leakage from formulated feedsLipid in formulated feedsOptimal macronutrient...Summary and conclusionsReferences |
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One of the problems of developing more digestible, and thus more water-soluble proteins for formulated larval feeds, is the high leakage rates from these feeds. Measurements of leakage rates from two commercial and two experimental larval feeds showed that 18–42% of the protein leaked from the feed within 2 min of hydration (Figure 1). This probably corresponds to the fraction of water-soluble protein in the feeds. Free amino acids leak from formulated larval feeds at a higher rate than protein, as can be expected from their lower molecular weight. More than 50% of radio-labelled amino acids leaked from a micro-bound diet (Diet 1, Figure 1) after 1 min of hydration and, after 5 min, less than 10% was left in the feed (Figure 2). Similar leakage rates have been found for water-soluble vitamins (K. Hamre, unpublished) and will probably apply to minerals as well. Most commercial larval feeds are micro-bound, and leakage rates similar to those shown here may be expected. With the present feeds, care should be taken to reduce to a minimum the time the feed stays in the water before it is eaten by the larvae. Further, it is necessary to develop the technology of larval feed production with the aim of reducing leakage.
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Lopez-Alvarado et al. (1994) incorporated free amino acids into different particle types and measured leakage after 2 min of hydration (Table 2). Leakage from the micro-bound feeds was between 81% and 91%, corresponding with the results in Figure 2. Protein encapsulation reduced the leakage to 59% and additional coating with tri-olein gave a further reduction to 39%. Lipid walled capsules were the ones with the lowest leakage (47% and 1.4%). However, the fatty acid composition of the tri-acylglycerol in the lipid wall was important, with tri-palmitin giving the lowest leakage rate (Table 2). From a nutritional point of view, tri-palmitin is far from optimal, since fish larvae need long-chain polyunsaturated fatty acids, and the high melting point of tri-palmitin will cause low digestibility at the low temperatures used for culturing cold-water marine fish larvae. Further, lipid walled capsules have a thick wall which may comprise as much as 95% of the particle weight (Lopez-Alvarado et al., 1994). They may therefore be applicable to encapsulation of micronutrients for further incorporation into complex particles, but not to whole diet formulations. A considerable improvement in lipid encapsulation of micronutrients has recently been achieved by Önal and Langdon (2004). Using methyl-palmitate, a wax with relatively low melting point, they managed to include 15–18% riboflavin in lipid spray beads. The leakage rates of riboflavin from the beads were 2–8% in 2 min, depending on the production method, and the beads were digested by larvae of zebrafish.
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Lipid in formulated feeds |
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TopIntroductionFeeding regimes and growthDeformitiesNutrient composition of live...Development of the digestive...Leakage from formulated feedsLipid in formulated feedsOptimal macronutrient...Summary and conclusionsReferences |
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Marine fish larvae that are fed diets with tri-acylglycerol as the main lipid source develop extensive lipid vacuolization in the intestinal mucosa and, in some cases, also in the liver (Fontagné et al., 1998; E. Kjørsvik, pers. comm.). Larvae fed diets in which the tri-acylglycerol is partially exchanged with phospholipids do not show similar symptoms. The effect is most likely the result of impaired endogenous synthesis of phospholipids in the larvae, which will lead to impaired lipoprotein synthesis, hindering the transport of lipid away from the intestine into the blood. This will reduce lipid utilization and growth (Geurden et al., 1998; Cahu et al., 2003). The effect was discovered by Japanese scientists in 1981 (Kanazawa et al., 1981) and shown in cod in the early 1990s (Kjørsvik, unpublished; Olsen et al., 1991).
Increasing the level of soybean lecithin in the diet for sea bass (Dicentrarchus labrax) larvae, to provide between 2.7% and 11.6% phospholipids of dry weight, gave an increase in growth rate from 4% to 14% per day (Cahu et al., 2003). The feed contained 26% lipid and the trial lasted from 9 to 40 dph. Survival, deformities, and the activity of a range of digestive enzymes were also positively affected by increasing the level of phospholipids. Thus, 45% of the lipid was supplied as phospholipids, and yet the optimal level was not found. Previously, a supplementation of phospholipids to 2–4% of dry diet was considered sufficient (Geurden et al., 1998), although Sargent et al., (1999) recommended 10% phospholipids based on the composition of marine fish eggs.
Because marine phospholipids are not commercially available, most researchers and feed companies use either soybean lecithin or egg lecithin to supply phospholipids. If the optimal fraction of plant based, non-marine phospholipids is high, the fatty acid composition of the feed may become unfavourable. Thus, when adding such phospholipids to formulated larval feeds, one must find a compromise between optimal fatty acid and lipid class composition. Further, MacQueen Leifson et al. (2003a, b) found swollen mitochondria in the enterocytes of turbot larvae fed formulated diets supplemented with between 5% and 15% soybean lecithin just after first-feeding. This symptom was not seen in larvae fed rotifers or a formulated diet supplied with marine phospholipids. This matter should be studied further to confirm the results of MacQueen Leifson et al. (2003a, b) and to examine if older larvae and larvae of other marine fish species will show this reaction also.
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Optimal macronutrient composition for young cod juveniles |
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TopIntroductionFeeding regimes and growthDeformitiesNutrient composition of live...Development of the digestive...Leakage from formulated feedsLipid in formulated feedsOptimal macronutrient...Summary and conclusionsReferences |
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Determining the optimal composition of macronutrients in young cod juveniles can supply information on formulating weaning diets, both for early and late weaning, and can also be used to evaluate live feeds. The extrapolation from juveniles to larvae should be done with care, however. We have performed two experiments with juveniles of 0.26 and 0.23 g wet initial weight, respectively. The aim of the studies was to answer the following questions:
- – What is the protein requirement of the young cod?
- – Does cod have a specific requirement for lipid, and thereby energy supply?
- – How much carbohydrate can be tolerated?
- – Does cod have a specific requirement for lipid, and thereby energy supply?
In the first experiment (Hamre and Mangor-Jensen, in press), we used a three component mixture design, where protein, lipid, and carbohydrate were varied from 53% to 83%, 5% to 30%, and 0% to 15% dry diet, respectively, with 5% intervals. Each diet was fed to fish in one tank, allowing us to scan a large range of compositions. Finally, we chose an area of variation that we thought would cover the range of good practical diets for cod, based on results from the literature (Lie et al., 1988; Hemre, 1992; Hamre et al., 2003). The experiment lasted for 2 months. We found that fish growth increased with an increase in lipid and carbohydrate to the upper limit of supplementation, i.e. high levels of protein resulting in reduced growth. The differences in growth arose during the first month of the experiment and, during this period, the mortality was also high. During the last period there was almost no mortality and no effect of the different diets on growth. The results also showed that the protein requirement was less than 53% of dry diet and that the fish tolerated up to 15% carbohydrate and 25% lipid. Additionally, fish fed less than 15% lipid had a high rate of cannibalism during the first part of the experiment.
In the second experiment (Hamre and Mangor-Jensen, unpublished), we fed cod juveniles with iso-energetic diets varying from 36% to 67% protein exchanged with 10–30% carbohydrate. The diets were fed in triplicate, and the experiment lasted for 1 month. Again, growth was lowered at high protein levels and in the fish fed 36% protein. Thus, the protein requirement was set to 41%. The question of whether the fish tolerated up to the 25% carbohydrate used in this experiment will be studied using further analyses.
In both experiments, the lowered growth found in fish fed diets high in protein was unexpected, but this may have to do with the fact that the stomach was not fully developed at start of the experiment. The diets were not supplemented with hydrolysed protein, and we have seen in other experiments that the protein sources used (saithe fillet and squid mantle) seem to have low availability in cod larvae (Kvåle, unpublished; Hamre and Opstad, unpublished). Further studies on the macronutrient composition for cod juveniles using hydrolysed protein will be conducted.
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Summary and conclusions |
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TopIntroductionFeeding regimes and growthDeformitiesNutrient composition of live...Development of the digestive...Leakage from formulated feedsLipid in formulated feedsOptimal macronutrient...Summary and conclusionsReferences |
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Cod larvae cultured by intensive methods, using rotifers and formulated diets or Artemia as feed, have lower growth rates and higher frequencies of deformities than larvae fed on copepods in extensive and semi-extensive rearing systems. Sub-optimal nutrition is one possible reason for these differences. There are many differences in nutrient composition between copepods and rotifers. Rotifers seem to have sufficient protein levels, and lipid levels, partly dependent on culture procedures, are usually within anticipated safe limits. Fatty acid composition can be manipulated easily to match the requirements of cod larvae. Of the micronutrients, iodine and vitamin A may be critically low, and enrichment procedures for these nutrients are not well established. Further, thiamine and vitamins C and E must be added to rotifer diets but, in this case, there is ample information about enrichment methods. Leakage of water-soluble nutrients from formulated micro-particulate diets is a problem that has not been solved yet and which will lead to reduced delivery of soluble protein, free amino acids, water-soluble vitamins, and minerals to larvae fed formulated diets.
To optimize the bioavailability of protein and lipid from formulated diets to fish larvae, protein given as fishmeal should be partly hydrolysed to make the protein more water soluble, and lipid should be given partly as phospholipids to facilitate the transport of lipid from the intestine and into the larval body. The optimal levels of phospholipids and hydrolysed/water-soluble protein for cod larvae are not known. Higher leakage of water-soluble, compared with structural and insoluble protein, complicates the problem of producing micro-particulate diets with good availability of protein.
Although much progress has been made in culturing cod larvae by intensive methods, the actual knowledge within nutrition is very limited. Further developments will lead to optimization of diets that may result in growth rates and deformity frequencies approaching those found in (semi-)extensively cultured larvae.
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References |
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TopIntroductionFeeding regimes and growthDeformitiesNutrient composition of live...Development of the digestive...Leakage from formulated feedsLipid in formulated feedsOptimal macronutrient...Summary and conclusionsReferences |
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Cahu C. and Zambonino Infante J. (2001) Substitution of live food by formulated diets in marine fish larvae. Aquaculture 200:161–180.[CrossRef][Web of Science]
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Dedi J., Takeuchi T., Seikai T., Watanabe T., Hosoya K. (1997) Hypervitaminosis A during vertebral morphogenesis in larval Japanese flounder. Fisheries Science 63:466–473.[Web of Science]
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