Kjell Inge Reitan

The significance of lipids at early stages of marine fish: a review

The present work reviews the significance of lipids at different early stages of marine fish larvae. Lipids in broodstock nutrition are considered to be important for the quality of the larvae. Lipids affect the spawning and the egg quality of many fish species and a deficiency in (n−3) highly unsaturated fatty acids (HUFA) in broodstock negatively affects fecundity, fertilization rate and hatching rate of the species studied. Lipids as a source of energy at the embryonic and larval stage (before first-feeding) are evaluated in relation to other sources of energy such as protein and carbohydrates. After hatching and prior to first-feeding, some marine species show a preference in catabolizing phosphatidylcholine, whereas phosphatidylethanolamine tends to be synthesized. The effect of long-term (LT) and short-term (ST) enrichment techniques on the lipid composition of rotifers has been documented using various marine oils/emulsions. The quantitative and qualitative lipid class and fatty acid composition of diets influenced the lipid and fatty acid composition of both LT- or ST-enriched rotifers. The nutritional improvement of Artemia is also important and may follow the general methods used for rotifers. The functions of docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA) during early stages of marine fish larvae are apparently different. High amounts of EPA in relation to DHA may create an imbalance in the structural composition of the phospholipids, which could affect the normal growth and the quality of the larvae. Turbot larvae tended to exhibit lower pigmentation success with lower DHA:EPA ratio in the total lipid fraction of the larvae, especially when the absolute amounts of EPA were high compared to those of DHA (in the total lipid and phospholipid fraction of the larvae). Considerable research is necessary to clarify many aspects regarding the function of these fatty acids, especially how their content at the egg stage can affect further requirements for normal growth and survival.

EFFECT OF NUTRIENT LIMITATION ON FATTY ACID AND LIPID CONTENT OF MARINE MICROALGAE1

Phaeodactylum tricornutum and Chaetoceros sp. (Badllariophyceae), Isochrysis galbana (clone T‐Iso) and Pavlova lutheri (Prymnesiophyceae), Nannochloris atomus (Chlorophyceae), Tetraselmis sp. (Prasinophyceae), and Gymnodinum sp. (Dinophyceae) were cultured at different extents of nutrient‐limited growth: 50 and 5% of μmax. The lipid content of the algae was in the range 8.3–29.5% of dry matter and was generally higher in the Prymnesiophyceae than in the Prasinophyceae and the Chlorophyceae. Increasing extent of phosphorus limitation resulted in increased lipid content in the Bacillariophyceae and Prymnesiophyceae and decreased lipid content in the green flagellates N. atomus and Tetraselmis sp.

A review of the nutritional effects of algae in marine fish larvae

In the first-feeding of larval, turbot (Scophthalmus maximus) and halibut (Hippoglossus hippoglossus), microalgae are used in the production of rotifers (Brachionus plicatilis) in order to transfer essential nutrients from the algae to the live food. In addition, the algae may be given directly to the larvae along with the live food. In this circumstance they act both as food for the fish larvae and for the live food. Microalgal addition to the first-feeding tanks along with the rotifers improved growth and survival of larvae, whereas short-term enrichment of rotifers with algae did not improve growth and survival of larvae in tanks without algae added. The algae in larval tanks tended to modify and stabilize the nutritional quality of the rotifers in the period before they were consumed by the larvae. The lipid content and fatty acid composition of the rotifers reflected the composition of the algal diets, and the algal species used may be an effective tool to control the fatty acid content (especially DHA, 22:6n−3, docosahexaenoic acid, and EPA, 20:5n−3, eicosapentaenoic acid). The content of n−3 polyunsaturated fatty acids varied between algal species, and to some extent, with the growth limitation of the algal cells. Young stages of marine fish larvae ingested microalgae, but the algal cells were assimilated to a different extent in halibut and turbot. The ingested microalgae may have triggered the digestion process or contributed to the establishment of an early gut flora. The algae in larval tanks most probably modified the bacterial flora of the water and the rotifers. In addition, the algae in larval tanks may have modified the light milieu for the larvae.

Nutritional effects of algal addition in first-feeding of turbot (Scophthalmus maximus L.) larvae

Abstract The effect which microalgal addition and rotifer enrichment with algae had on survival, growth rate and fry viability during first-feeding were examined for turbot (Scophthalmus maximus L.). Addition of the microalgae Isochrysis galbana or Tetraselmis sp. together with rotifers Brachionus plicatilis, grown on yeast and oil emulsion, and Artemia greatly improved rearing success, whereas short-term enrichment of the rotifers with Tetraselmis sp. gave only improved viability. The algae modified the relative fatty acid composition of rotifers. Rotifers with I. galbana exhibited and increased level of 22:6n-3 and a lower level of 20:5n-3, whereas the opposite change occurred when Tetraselmis sp. was used. Addition of either of the two algal species at 1 mg cl−1 to the larval tanks resulted in a constant lipid level and high egg ratio of the rotifers, and thereby also high individual biomass content. Without algal addition, the lipid content of the rotifers decreased by 20% day−1 in the early phase, and their egg ratio became close to zero. In addition to improved nutritional conditions of the larvae, some other factor, e.g. some trigger mechanism or changed microbial or light conditions, also can operate to explain the increased early appetite of larvae with microalgae added and must be taken into consideration.

Copepods as live food organisms in the larval rearing of halibut larvae (Hippoglossus hippoglossus L.) with special emphasis on the nutritional value

In the natural food web, zooplankton constitutes a major part of the diet for marine fish larvae and it is generally believed that copepods can meet the nutritional requirements of fish larvae. In this study, different copepod species were analysed for total lipid content, fatty acid and protein content and used in first feeding experiments with halibut larvae (Hippoglossus hippoglossus L.) together with enriched nauplii and juvenile stages of Artemia franciscana. Special attention was paid to the DHA content in both the live food organisms and the fish larvae. Copepodid and adult stages of the marine copepods Temora longicornis and Eurytemora sp. had a total lipid content varying between 7% and 14% of dry weight (DW). Copepodid stages I, II and III of Calanus finmarchicus had a lipid content varying between 10% and 19% of DW, increasing to 22–25% in copepodid stages IV, V and the adult stages. The predominant fatty acids of all copepods were docosahexaenoic acid (DHA; 22:6n−3), eicosapentaenoic acid (EPA; 20:5n−3) and the saturated fatty acid 16:0. In T. longicornis and Eurytemora sp. DHA constituted 26–42%, EPA 15–24% and 16:0 8–12% of total fatty acids. In C. finmarchicus, the DHA content ranged between 21% and 32.5%, whereas the content of EPA and 16:0 was 15–21% and 9–15% of total fatty acids, respectively. The sum of n−3 HUFA was highest in T. longicornis and Eurytemora sp. (55–62% of total fatty acids) and lowest in C. finmarchicus (38–47%). The loss rate (% day−1) of the total lipid and fatty acid content during starving conditions was 18 and 24% day−1 in T. longicornis, and 12 and 16% day−1 in Eurytemora sp., respectively. In both copepods, the relative content of DHA (% of total fatty acids) increased with time during starvation, reaching 43% in T. longicornis and 49% in Eurytemora sp. The quantitative content of DHA decreased steadily although at a lower rate than that of other fatty acids. The protein content of various copepods varied between 52.4% and 57.6% of dry weight (DW), and was significantly higher than in A. franciscana (41% in newly hatched nauplii and 34% after 24-h enrichment). There was a close relationship between the percent DHA content in 28–36-day-old halibut larvae and the live food organisms they were fed on. High percent DHA content in the live food organism resulted in a high percent DHA content in the fish larvae and vice versa. The relatively high DHA content in the larvae was significantly reduced after 8 days when given enriched A. franciscana. For the rest of the feeding period, the DHA content decreased steadily when fed on A. franciscana, whereas it remained at the initial high level in larvae-fed copepods.

Influence of lipid composition of live feed on growth, survival and pigmentation of turbot larvae

The effect of different lipid compositions of live feed on the survival, growth rate and pigmentation success of turbot larvae, Scophthalmus maximus (L.), was investigated. Rotifers, Brachionus plicatilis, together with the algae Tetraselmis sp., were administered until day 12, and Artemia was fed until day 27. The experimentally treated live feeds were enriched with four formulated emulsions, resulting in a gradient in the relative contents of Ω3 HUFA (highly unsaturated fatty acids) and in DHA (docosahexaenoic acid, 22:6 Ω3)/EPA (eicosapentaenoic acid, 20:5 Ω3) ratios in both the rotifers and Artemia.There were no differences in larval growth rate, and only small differences in survival rate throughout the feeding experiment, probably because of satisfactory levels of Ω3 HUFA in the live feed to sustain growth and survival. A correlation was obtained between the percentage of completely pigmented 27 d old turbot and the DHA/EPA ratio in the total lipids of 12 d old larvae, which again was correlated with the corresponding ratio in the live feed used. The results suggest that normal pigmentation in turbot requires dietary DHA in the early larval feeding period, and that this requirement cannot be replaced by EPA.

Protein and carbon utilization of rotifers (Brachionus plicatilis) in first feeding of turbot larvae (Scophthalmus maximus L.)

Abstract The effect of three different rotifer enrichments was examined on growth, survival, pigmentation and viability of first feeding turbot larvae. The diets differed in rotifer content of protein, lipid and ratio of protein/lipid. The diets were fed to turbot with or without algae ( Isochrysis galbana ) added to the larval tanks. The turbot larvae were fed rotifers for 10 days and thereafter the same Artemia diet was fed to all treatments for the rest of the experimental period. Growth and survival of fish larvae were higher in tanks containing algae than in tanks where no algae were added. Independent of algal addition, the highest growth rate and survival was obtained by feeding rotifers containing the highest protein content. Larvae reared in greenwater consumed higher numbers of rotifers during the stagnant period than larvae kept in clearwater conditions, while analysis of the larval gut contents showed lower rotifer numbers in the gut of larvae reared in greenwater conditions. This must imply longer residence time of the food in the larval gut, and presumably also higher digestion and assimilation efficiencies of larvae maintained without algae than in larvae maintained with algae. Calculation of protein and carbon conversion efficiency showed higher utilization in larvae maintained without algae (18–28% for protein, 12–19% for carbon) than in larvae maintained with algae (6–9% for protein, 4–7% for carbon). No significant differences in pigmentation rate and stress sensitivity were observed among the larvae of the various treatments.

Egg and larval quality criteria as predictive measures for juvenile production in turbot (Scophthalmus maximus L.)

Abstract Although much effort has been made to evaluate criteria for marine egg and larval quality, the significance of poor egg quality for the results in the final juvenile production has not been clarified. The aim of our experiments with turbot ( Scophthalmus maximus ) was to determine whether the initially observed egg quality affected offspring viability and normal development up to the juvenile stage. Eggs from each of six individual females were divided into three replicate groups, and the eggs in each replicate was quality evaluated according to fertilization rates and the ratio of normal cell cleavages (blastomere morphology) at the 8–32 cell stage. The condition of yolk-sac larvae was tested by an acute high salinity stress test, and the larvae were fed according to our standard feeding regime with microalgae, enriched rotifers and Artemia nauplii. Significant positive correlations were found between observed egg quality characteristics and hatching rates, larval tolerance to acute stress, and to survival, metamorphosis and juvenile pigmentation. Larvae hatching from poor quality egg groups thus had a lower viability and were less able to develop normally to the juvenile stage than larvae from good quality egg groups. Our conclusion is that egg quality observed as fertilization rate and rates of normal blastomeres at early egg stages may be a useful predictive tool for evaluation of the potential juvenile production in turbot.

Comparative study on the fatty acid and lipid composition of four marine fish larvae

Abstract 1. 1. Fatty acid and lipid class composition were determined in larvae of four marine species: Atlantic halibut (Hippoglossus hippoglossus L.), plaice (Pleuronectes platessa), cod (Gadus morhua) and turbot (Scophthalmus maximus) at hatching and prior to first feeding. 2. 2. Total fatty acid content decreased in the four species with up to 50% reduction in one of the halibut groups. Docosahexanaoic acid (22:6 n-3) was especially utilized. 3. 3. Low lipid utilization was found in turbot in relation to the other three species. 4. 4. Water environmental temperature may explain some of the differences in the fatty acid utilization and the source of metabolic energy between cold water species (halibut, cod, and plaice) and temperate species (turbot), in the period from hatching to prior to first feeding. 5. 5. Relative amounts of neutral lipids and phospholipids were similar in plaice, cod and halibut, approximately 25% and 75% of total lipids, respectively, and were approximately constant during the yolk-sac stage. Neutral lipids were dominant for turbot at hatching, accounting for 53–55% of the total lipids, while phospholipids predominated prior to first feeding, being 56–59%. 6. 6. Phosphatidylcholine was catabolized in halibut, plaice and cod but not in turbot, while phosphatidylethanolamine tended to be synthesized in all four species.

Lipid composition in turbot larvae fed live feed cultured by emulsions of different lipid classes

Fatty acid and lipid class compositions were determined in eggs and larvae of turbot (Scophthalmus maximus L.). The larvae were fed on rotifers and Anemia fed on various lipid emulsions. Starving larvae were also studied. The lipid class compositions of the emulsions affect the fatty acid composition of the rotifers. Ethyl ester-based emulsion exhibited the highest assimilation by the rotifers and Artemia and were possibly incorporated in their triacylglycerol fraction. During larval starvation docosahexaenoic acid and arachidonic acid levels tended to be conserved whereas eicosapentaenoic acid was highly reduced. Moreover, the relative proportion of phosphatidylethanolamine increased whereas that of phosphatidylcholine decreased in starved larvae. Increasing the n-3 highly unsaturated fatty acid levels in rotifers and Artemia did not influence the survival and growth of the turbot larvae. A positive correlation was found between pigmentation success and the ratios of docosahexaenoic/ eicosapentaenoic acids in the total and polar lipid fractions of the turbot larvae. Furthermore, the amounts of these polyunsaturated fatty acids in the phosphatidylethanolamine fraction of the larvae are suggested to be of particular importance in turbot pigmentation.