© 2005 International Council for the Exploration of the Sea
Effect of enriched rotifers on growth, survival, and composition of larval Atlantic cod (Gadus morhua)
a Faculty of Marine Bioscience and Technology, Kangnung National University Kangnung 210-702, South Korea
b Ocean Sciences Centre, Memorial University St. John's, Newfoundland, Canada A1C 5S7
*Correspondence to V. Puvanendran: tel: +1 709 737 3026; fax: +1 709 737 3220. e-mail: puvy{at}mun.ca.
Recently, the nutritional requirements of marine finfish larvae have received considerable attention, and studies have shown that docosahexaenoic acid (DHA) affects the growth and survival of marine finfish larvae. We investigated the effects of different rotifer diets containing variable amounts of DHA on the growth and survival of larval Atlantic cod (Gadus morhua L.). Four different commercial rotifer enrichment formulations were used: spray-dried whole cells composed of Crypthecodinium sp. (ED1), spray-dried whole cells of Schizochytrium sp. (ED2), an oil emulsion (ED3) and ED1, and dried Chlorella at a 7:3 ratio by weight (ED4). The resultant rotifers contained a similar concentration of DHA (1.1–1.6% DW), but the level of DHA differed in proportion to EPA for each enrichment, and was designated ER1–4. Twelve 30-l aquaria were used with three replicates per treatment. Larvae were fed with rotifers from 3 to 43 days post-hatch (dph) at 4000 prey l–1. At the end of the experiment, no significant differences were found in body length and dry weight between the larvae reared on ER1 and ER2. However, larvae reared on ER3 were significantly smaller (both in length and weight) than larvae reared on ER1 and ER2. Larval survival on the ER2 treatment at 43 dph was significantly higher than on the other three treatments. Our results showed a positive effect of rotifer DHA proportions on growth and survival of cod larvae, and demonstrated that Atlantic cod larvae require a high ratio of dietary DHA to EPA.
Keywords: Atlantic cod larvae, DHA proportion, DHA/EPA ratio, enrichments, rotifer
Received 13 June 2004; accepted 6 October 2005.
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Introduction |
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 Top Introduction Material and methodsResultsDiscussionReferences |
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Lipids and amino acids are major sources of metabolic energy during the embryonic and pre-feeding larval stages in fish. At hatch, yolk-sac larvae have high levels of these energy sources, but they are dramatically reduced during the endogenous feeding stage (Evans et al., 2000). Thus, start-feeding larvae require a live feed that provides sufficient levels of these energy sources. Studies have shown that essential fatty acids (EFA), such as docosahexaenoic acid (DHA, 22:6n-3), eicosapentaenoic acid (EPA, 20:5n-3), and arachidonic acid (ARA, 20:4n-6) are also important in larval fish nutrition (Takeuchi, 1997; McEvoy et al., 1998; Estevez et al., 1999; Sargent et al., 1999). These fatty acids, as components of phospholipids (PL), are critical structural and physiological components of the cell membranes of most tissues. However, the live feeds commonly used for the first-feeding larval stages, such as rotifers and Artemia, are naturally poor in these fatty acids, so enrichment of live foods with lipids rich in EFA is necessary to achieve better growth and survival through metamorphosis (Rainuzzo et al., 1997).
Recently, absolute and relative levels of DHA, EPA, and ARA in the diets of marine fish larvae have received considerable attention (Sargent et al., 1999; Harel et al., 2002; Bell and Sargent, 2003). DHA, which has a competitive relationship with EPA, is particularly important for normal neural development and function, including that of retina and brain (Sargent et al., 1999). Studies have shown that the DHA requirement in the diet differs among fish species, especially in cold-water fish species such as yellowtail flounder (Limanda ferruginea) and Atlantic halibut (Hippoglossus hippoglossus), which require high levels of dietary DHA (McEvoy et al., 1998; Copeman et al., 2002). However, Planas and Cunha (1999) reported that turbot larvae (Scophthalmus maximus) require lower levels of DHA in their diet for better growth and survival. Other specific benefits of feeding DHA-enriched diets to fish larvae include successful metamorphosis, reduced pigmentation problems, enhanced vision capabilities, improved neural development and stress resistance (Watanabe, 1993).
Atlantic cod (Gadus morhua L.) is emerging as an alternative aquaculture species in the North Atlantic region (Brown et al., 2003). In order to develop this species as a new candidate for aquaculture at a commercial scale, a consistent production of juvenile fish must be achieved. Understanding the nutritional requirements of early larval cod, especially of EFA such as DHA and EPA, is important for successful mass production. This study investigated the effect of DHA level and DHA/EPA ratio in four rotifer-enriched diets on growth and survival of Atlantic cod larvae.
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Material and methods |
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TopIntroductionMaterial and methodsResultsDiscussionReferences |
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Experimental enrichments
Four different commercial enrichment formulations were used: spray-dried whole cells composed of Crypthecodinium sp. (ED1), spray-dried whole cells of Schizochytrium sp. (ED2), an oil emulsion (ED3) and ED1, and dried Chlorella at a 7:3 ratio by weight (ED4). Three of these enrichments, ED1, ED2, and ED3, were high (52.2–66.9%) in triacylglycerol (TAG) and relatively low (4.2–12.7%) in PL (Table 1). The Chlorella, however, was high in phospholipids (75.8%) and contained no TAG. The fatty acids in ED1, ED2, and ED3 were composed of 43.8%, 51.1%, and 24.2% DHA in total fatty acid, respectively, and they had less than 0.7% of EPA (Table 2). The Chlorella contained very low proportions of DHA and no EPA.
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Rotifer enrichment
Rotifers were cultured with baker's yeast and Culture Selco® (INVE, Dendermond, Belgium), and were enriched with ED1, ED2, ED3, and ED4 in 10-l vessels at a density of 4 x 105 rotifers 1–1. The resultant rotifers were designated as enriched rotifer 1 (ER1), enriched rotifer 2 (ER2), enriched rotifer 3 (ER3), and enriched rotifer 4 (ER4). Preliminary enrichment trials showed better results in 24-h enrichment than with 8- and 16-h enrichments. Each batch of rotifers was enriched at 0.08 g of enrichment material l–1 of rotifer culture for 24 h. Enrichment diets were divided into three portions and added at times: 0:00, 6:00, and 12:00. Water temperature and salinity for enrichment were 22°C and 31 psu, respectively. Once a week, from weeks 1 to 4, samples of 24-h enriched rotifers were taken from each enrichment vessel for lipid analysis.
Larviculture
Larviculture was conducted following the guidelines of Puvanendran and Brown (1999, 2002). Spontaneously spawned, fertilized eggs were collected from captive broodstock maintained at the Ocean Sciences Centre, Memorial University. Eggs were incubated in a 250-l flow-through silo system (2–3 l min–1) with aeration. Water temperature was maintained at 5–6°C. Lights remained on around the clock at an intensity of 300–400 lux. Dead eggs were siphoned out daily. Once completely hatched, 1500 larvae (50 larvae l–1) were transferred to each of the twelve 30-l rectangular glass aquaria (three replicates per treatment) that were randomly placed in a thermo-regulated water bath. This was considered day 1 of the experiment. The sides of the aquaria were covered with opaque black plastic.
The water temperature in the experimental tanks was maintained at 11°C and monitored twice daily. A flow-through saltwater system (31 psu) was provided at an initial flow of 150 ml min–1 (7.2 exchanges d–1), and gradually increased to 350 ml min–1 (16.8 exchanges d–1). Dissolved oxygen was monitored weekly, and remained at around 10.2 mg l–1. Light was provided around the clock at 2000 lux (Puvanendran and Brown, 2002).
Rotifer concentration (4000 prey l–1) was chosen from a previous study (Puvanendran and Brown, 1999), and larvae were fed four times a day. Each experimental tank was aerated, which ensured a homogenous distribution of prey within the tank. To maintain the desired prey concentration within each experimental tank, just before feeding, a 10-ml water aliquot was sampled from each tank. The number of prey in each sample was counted, and prey concentrations were adjusted as needed.
Data collection
Five larvae (15 per treatment) were randomly sampled for morphometric measurements from each experimental tank on 1, 8, 15, 22, and 29 dph. On days 36 and 43, ten larvae were sampled from each experimental tank (30 per treatment). As greater size variation among larvae was observed after the fourth week, the number of larvae sampled from each tank was increased at day 36 to reduce the effect of size variation between treatments. Each larva was photographed, and later, larval length was measured using an image analysis software (Matrox Inspector, Matrox Electronic Systems Ltd, Quebec, Canada). Once photographed, larvae were washed with distilled water to remove salt, and five larvae were placed on a 1.0-cm2 pre-weighed aluminium foil. This method resulted in three samples (six samples at 36 and 43 dph) of five larvae being analysed for dry weight tank–1 week–1. The foils were dried at 60°C for 48 h. Foils were then stored in a desiccator and weighed again using a microbalance (UMT2, Mettler Toledo, Switzerland). Survival at 43 dph was determined by counting all larvae from each tank.
Lipid samples and lipid analysis
Triplicate samples, consisting of approximately 10 mg dry weight of larvae, were taken from each tank at hatching and at 22 and 43 dph. Samples were placed directly in chloroform and stored under nitrogen at –20°C until extraction. Lipids were extracted in chloroform/methanol according to Parrish (1999), using a modified Folch procedure (Folch et al., 1957). Lipid classes were determined using thin layer chromatography with flame ionization detection (TLC/FID) with a MARK V Iatroscan (Iatron Laboratories, Tokyo, Japan), as described by Parrish (1987). Extracts were spotted on silica gel-coated Chromarods, and a three-stage development system was used to separate lipid classes.
The first separation consisted of a 25-min and a 20-min development in 99:1:0.05 hexane/diethyl ether/formic acid. The second separation consisted of a 40-min development in 80:20:1 hexane/diethyl ether/formic acid. The last separation consisted of two 15-min developments in 100% acetone followed by two 10-min developments in 5:4:1 chloroform/methanol/water. After each separation, the rods were scanned, and the three chromatograms were combined using T-Data Scan software (RSS, Bemis, Tennessee, USA). The signal detected in millivolts was quantified using lipid standards (Sigma).
Fatty acid methyl esters (FAME) were prepared by trans-esterification with 10% boron triflorate (BF3) in methanol at 85°C for 1.5 h (Morrison and Smith, 1964; Budge, 1999). A Varian model 3400 Gas Chromatograph (GC) equipped with a Varian 8100 Auto-Sampler was used for fatty acid analysis (Varian, California, USA). An Omegawax 320 column, 30 m long, 0.32 mm i.d., 0.25 µm film thickness (Supelco, Bellefonte, Pennsylvania, USA) was used for separations. Hydrogen was used as the carrier gas, and the flow rate was set at 2 ml min–1. The column temperature profile was as follows: 65°C for 0.5 min, hold at 195°C for 15 min after ramping at 40°C min–1, and hold at 220°C for 0.75 min after ramping at 2°C min–1. The injector temperature was increased from 150°C to 250°C at 200°C min–1. Peaks were detected by flame ionization with the detector held at 260°C. Fatty acid peaks were integrated using Varian Star Chromatography Software (version 4.02), and identification was made with reference to known standards (PUFA 1, 3 and 37 Component FAME Mix, Supelco Canada, Ontario, Canada).
Statistical analysis
All data were tested for normality to satisfy the assumptions of ANOVA. Two-way ANOVAs with the Bonferroni multiple comparison test were used to determine the statistical significance of age and treatment on dry weight and standard length of cod larvae. One-way ANOVAs with the Duncan multiple comparison test were used to compare differences in survival of larvae and lipid class, and fatty acid composition of rotifer and larvae between treatments. Differences were considered significant at the p < 0.05 level.
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Results |
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TopIntroductionMaterial and methodsResultsDiscussionReferences |
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Rotifer enrichment
The total protein content of enriched rotifers in all treatments was significantly lower (F = 3.38, d.f. = 4, p < 0.037) than in the initial rotifers, and in ER4, it was significantly higher than in the ER2 and ER3 treatments (Table 3). The total lipid content of rotifers increased in all treatments after 24-h enrichment, but it was significantly higher (F = 12.6, d.f. = 4, p < 0.0001) in the ER2 and ER3 treatments (98.7 mg g–1 DW) than in the ER1 and ER4 treatments. The protein/lipid ratio of the enriched rotifers in all groups was lower than in the initial rotifers. However, ER4 showed a significantly higher ratio than ER2 and ER3. TAG content of the ER2 and ER3 treatments was significantly higher than in the initial rotifers (25.8 ± 5.2%), while the PL level was significantly lower than in the initial sample (59.0 ± 8.1%). Meanwhile, no significant differences were found in TAG and PL contents among the ER1 and ER4 treatments and the initial rotifer samples.
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All treatments resulted in higher levels of DHA than EPA, and the highest levels of DHA (F = 174.4, d.f. = 4, p < 0.0001) and EPA (F = 10.3, d.f. = 4, p < 0.0001) were 23.4% DHA in the ER1 treatment and 3.5% EPA in the ER3 treatment (Table 4). However, no significant differences were found among enriched rotifers from all four treatments when considering the DHA content as a proportion of total lipid dry weight. Levels of n-3 polyunsaturated fatty acids (PUFA) were significantly higher (F = 89.2, d.f. = 4, p < 0.0001) in the ER1 treatment (28.2%) than in the other three treatments. Rotifers from ER1 and ER2 had significantly higher DHA/EPA ratios than rotifers from ER3 and ER4. Contents of DHA per dietary dry weight (DW) were significantly higher (F = 19.9, d.f. = 4, p < 0.0001) in all treatments, compared with the initial content.
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Growth and survival of larvae
The effects of enrichment (F = 38.3, d.f. = 3, p < 0.0001) and age (F = 2821, d.f. = 6, p < 0.0001) on standard length of cod larvae were significant. From 29 dph to the end of the experiment, larvae reared on ER1 and ER2 (except for 22 dph) were significantly larger than larvae reared on ER3 (Figure 1a). Throughout the experiment, except for 22 dph, no significant difference was found in larval standard length between the ER1 and ER2 treatments. Except for 22 and 43 dph, no significant difference was found in larval standard length between the ER3 and ER4 treatments.
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Enrichment (F = 13.1, d.f. = 3, p < 0.0001) and age (F = 1572, d.f. = 6, p < 0.0001) also had a significant effect on the dry weight of larval cod. Initially (15, 22, 29, and 36 dph), larvae reared on the ER2 treatment were significantly larger than those reared on the ER3 treatment, but no significant difference (p = 0.056) was found at the end of the experiment (Figure 1b). In general, no significant difference was found in the dry weight among the ER1, ER2, and ER4 treatments at the beginning and end of the experiment. However, from 29 to 43 dph, larvae reared on the ER1 treatment were significantly larger than larvae from the ER3 treatment.
The survival rate of larval cod in the ER2 treatment was higher than in the other treatments, although no significant difference was found (F = 2.21, d.f. = 3, p > 0.165) among treatments (Figure 2). Larval survival in one of the tanks in the ER2 treatment was lower (20%) than in the others (34%). Cochran's test for variance outliers (Kanji, 1994) was used to determine outliers in the data, and a significant critical value (p < 0.05) was found for the ER2 survival data. When the data were analysed after removing this outlier, larval survival in the ER2 treatment was significantly higher (F = 6.56, d.f. = 3, p < 0.019), while no significant difference was found among the other three treatments.
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Larval lipids
The total lipid content of cod larvae at 3 and 6 weeks was not significantly different (96.6–123.8 mg g–1 DW) among all treatments (Table 5, Figure 3a). The major lipid class of cod larvae was PL in all treatments, and the PL content in the ER3 treatment was significantly lower (F = 41.1, d.f. = 3, p < 0.0001) than in ER1 and ER4 at 43 dph (Figure 3b). Concomitantly, the TAG level in the ER3 treatment was significantly higher than in all other treatments at 43 dph (Figure 3c; F = 25.3, d.f. = 3, p < 0.019).
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The percentage of ARA in larvae from all treatments at 22 and 43 dph was higher than the initial value (1.3 ± 0.0%). The ARA level in larvae reared in ER2 was significantly higher (F = 793, d.f. = 3, p < 0.0001 and F = 89.7, d.f. = 3, p < 0.0001 at 22 and 43 dph, respectively) than in the other three treatments (Table 6, Figure 4). The level of EPA in all treatments, at 22 and 43 dph, was lower than initially (16.7 ± 0.1%), and it was significantly lower (F = 351, d.f. = 3, p < 0.0001 and F = 593, d.f. = 3, p < 0.0001 at 22 and 43 dph, respectively) in the ER2 treatment.
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DHA levels of 22- and 43-dph cod larvae fed with rotifers from ER1 and ER2 treatments were similar to those of 1-dph cod larvae (initial: 32.7 ± 0.1%), while DHA levels in the ER3 and ER4 treatments were lower than the initial values. DHA levels in the ER1 and ER2 treatments were significantly higher than in ER3 and ER4, but not significantly different between each other. The ratio of DHA/EPA was highest at 22 dph (10.4) and 43 dph (12.5) in the ER2 treatment, and lowest at 22 dph (5.4) and 43 dph (5.6) in the ER3 treatment.
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Discussion |
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TopIntroductionMaterial and methodsResultsDiscussionReferences |
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Several studies have suggested that PL rather than TAG are the preferred vehicle for delivering PUFA to marine fish larvae (Kanazawa, 1997; Sargent et al., 1997; Harel et al., 1999). In our study, rotifers from ER1 had significantly higher levels of PL than rotifers from ER2 and ER3 treatments, while rotifers from the ER3 treatment had a significantly higher level of TAG than the remaining treatments. The lack of a significant difference in growth between cod larvae reared in the ER1 and ER2 treatments suggests that PL levels in the diet may not have a dominant influence on the growth of cod larvae. Nonetheless, our results agree with other studies, which indicate that TAG may not be a preferred vehicle for delivering PUFA, which has an important role in the growth of marine finfish larvae (Kanazawa, 1997; Sargent et al., 1997; Harel et al., 1999).
A relationship between DHA levels and DHA/EPA ratios of rotifers, and the growth and survival of cod larvae was found in the present study. Takeuchi et al. (1994) investigated the effect of DHA levels of rotifers on the growth, survival rate, and abnormalities of larval cod (Gadus macrocephalus), and suggested that the appropriate level of DHA that should be contained in the rotifers was around 1% DW. In their study, any amount higher than 1% DHA resulted in a high percentage of abnormal fish, together with high mortality. However, in our study, larval cod fed ER2 containing 1.6% DW of DHA had higher growth and survival than larvae fed with ER3 and ER4 with 1.1–1.2% DW of DHA. Sargent et al. (1999) suggested that species-specific requirements for DHA exist among marine finfish larvae. Several other studies suggested that much higher levels of DHA (or n-3 highly unsaturated fatty acids – HUFA) could reduce larval survival (Planas and Cunha, 1999). Izquierdo et al. (1992) showed that, in larval Japanese flounder (Paralichthys olivaceus), lower (or higher) DHA content (1.5%) of Artemia did not affect survival, but larvae were significantly larger when fed Artemia containing a higher percentage of DHA (up to 3.5%). However, Salhi et al. (1994), in their study with gilthead sea bream (Sparus aurata), showed that larvae fed with a lower DHA microdiet (>0.5%) had a significantly lower survival than larvae fed with a higher DHA microdiet (1.2–1.3%). They suggested that the growth of larvae was affected by a combination of DHA content and total dietary lipid. In our study, however, the ER2 (1.6% DW of DHA) treatment gave a significantly higher survival than the ER3 (1.2% DW of DHA) and ER4 (1.1% DW of DHA) treatments. Thus, it seems that the requirement of dietary DHA levels of marine finfish larvae is species dependent.
Rodriguez et al. (1997) reported that a higher DHA/EPA ratio (1.4:1–0.3:1) during the rotifer stage improved the growth and survival of gilthead sea bream. Copeman et al. (2002) found that yellowtail flounder fed high DHA/EPA (8:1) had a higher growth and survival than those fed a DHA/EPA ratio of 1.9:1. However, there was no significant difference in the growth of Japanese flounder and turbot larvae when they were fed with different dietary ratios of DHA and EPA (Estevez et al., 1999; Furuita et al., 1999). Harel et al. (2002) investigated the effect of commercial enrichment materials on early development of three larval fish. They reported no significant difference in growth between striped bass (Morone saxatilis) and gilthead sea bream larvae fed with Artemia enriched with Algamac 2000® or PL-Cr (DHA-rich phospholipid extract of Crypthecodinium sp.). However, the growth of halibut larvae fed Artemia enriched with DHA Selco® was lower than the growth of larvae fed with PL-Cr. Our studies also showed that cod larvae fed high DHA diets (ER2) showed better growth and survival than those fed low DHA diets (ER3 and ER4). On the other hand, cod larvae fed ER1, which had an equivalent level of DHA compared with ER2, had a lower survival than those fed ER2 treatments. All these studies, including our own, suggest the existence of species-specific requirements for the DHA/EPA ratio for growth and survival of marine finfish larvae.
The lipid composition of eggs/yolk has been suggested as an indicator for determining the nutritional requirements of first-feeding larvae. Typically, a dietary DHA/EPA ratio of 2:1 is found in marine species, and has been suggested as adequate for larval feeding (Tocher and Sargent, 1984; Sargent et al., 1999). However, in our experiment, growth and survival of larval cod improved with increasing ratio. Similar to other experiments (Tocher and Sargent, 1984), newly hatched cod larvae in our experiment had a DHA/EPA ratio of 2:1. DHA/EPA ratio of larval cod increased as the larvae grew, irrespective of the rotifer enrichment. However, increase in the DHA/EPA ratio was significantly higher in the ER1 and ER2 treatments, which yielded better growth than the other two treatments. From our results, it seems that larval cod require a higher DHA:EPA ratio than some other marine finfish species. Copeman et al. (2002) suggested that larval yellowtail flounder require higher dietary DHA levels for better growth. Thus, our results indicate that the presence of high DHA and lower EPA levels in the diet may be important for better growth of cold-water marine finfish larvae.
It has been suggested that the ideal diet for fish larvae should provide a similar lipid class composition as the yolk of eggs or yolk-sac larvae (Sargent et al., 1999). In our study, lipid class constituents of newly hatched cod larvae were 12.9% TAG and 66.0% PL. The lipid class of larval cod reared on the ER3 treatment was similar at 43 dph to that of newly hatched larvae. The lipids of cod larvae reared on the remaining treatments had lower TAG and higher PL levels than those of larvae from the ER3 treatment and the newly hatched larvae. Therefore, our study suggests that increased PL levels and decreased TAG levels of older cod larvae, compared with newly hatched larvae, could be an indicator of better larval condition.
Watanabe (1993) suggested that the DHA content of Atlantic cod larvae could be reduced rapidly during larval development after hatching. In our study, the DHA levels of Atlantic cod larvae in ER1 and ER2 treatments did not change compared with the initial value. Meanwhile, the DHA levels of larval cod fed with ER3 and ER4 were lower than the initial value. This suggests that the DHA levels of cod larvae should be kept close to the initial levels for better larval growth, and that this can be accomplished by feeding diets with a relatively high DHA level and high DHA/EPA ratio. Copeman et al. (2002) found that supplementing diets with high EPA levels was not effective for the growth of yellowtail flounder. Similarly, in our experiment, EPA levels of larvae at 22 and 43 dph were very low in all treatments, compared with initial levels (newly hatched larvae), and had no effect on the growth of cod larvae.
Recently, studies have indicated that arachidonic acid (ARA) levels in marine fish larvae may be important for stress tolerance, pigmentation, growth, and survival (Bell and Sargent, 2003). In particular, the competitive interactions between EPA and ARA are important in the formation of eicosanoids (Harel and Place, 2003). In our study, ARA levels in larvae were higher in all treatments than in their respective diets. Thus, larval cod appear to have the ability to selectively incorporate dietary ARA into their body tissues. Similarly, Copeman et al. (2002) found that yellowtail flounder larvae have the ability to increase the dietary ARA levels in the body tissue in spite of lower dietary ARA levels (as low as 2.2% of total fatty acid). Zheng et al. (1996) reported that prey enriched with higher ARA provided no improvement in survival for Pacific cod larvae. Similarly, in our studies, ARA levels in rotifers did not affect growth and survival of Atlantic cod. However, studies have shown that dietary ARA levels are important for improved growth and survival in gilthead sea bream (Bessonart et al., 1999; Koven et al., 2001). Sargent et al. (1999) suggested that both the concentration and ratio, not only between DHA and EPA, but also between EPA and ARA, are important in larval marine fish nutrition. Thus, it appears that the ARA levels in diet have a species-dependent effect on cold-water marine fish.
Chemical composition of the enrichment diets used in our experiment differed not only in essential fatty acids, but also in phospholipids, proteins, and micronutrients. Although our results showed that DHA, EPA, and DHA:EPA ratio had significant effects on the growth, survival, and composition of larval cod, differences in other nutrients could have also affected our results. Unfortunately, it is difficult to control all the nutrients in studies involving commercial enrichments and live prey, and could only be achieved using formulated inert diets.
In conclusion, high dietary levels of DHA, relative to EPA, with ratios up to 10:1 in rotifers promoted growth in Atlantic cod. ER1 and ER2 had higher levels of DHA compared with ER3, and larvae fed with these two rotifers (ER1 and ER2) had a better growth than larvae fed with ER3. Enrichment of rotifers with a DHA-rich diet appears effective in improving the nutritional value of rotifers for the improvement of growth and survival of Atlantic cod larvae.
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Acknowledgements |
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We thank Danny Boyce, Cathy Williams, and other ARDF personnel for their help. This work was supported by the Postdoctoral Fellowship Programme of Korea Science & Engineering Foundation (KOSEF) to HGP and the Natural Sciences and Engineering Research Council of Canada (NSERC) strategic project (JAB and VP). We thank Newfoundland Aqua Ventures and Department of Fisheries and Oceans for providing cod eggs.
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References |
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