C. Fernández-Díaz

Influence of co-feeding larvae with live and inert diets on weaning the sole Solea senegalensis onto commercial dry feeds

The senegal sole (Solea senegalensis) is a potential culture species in salt water ponds in south Spain and Portugal. However, growth of an industry around this species is constrained by poor survival during weaning from live Artemia onto artificial diets. This paper describes the influence of larval conditioning with inert diets on weaning performance. Mixed diets of commercial formulated feed and live prey (rotifers and Artemia) allowed larvae to develop from first feeding to metamorphosis without significant differences (P>0.05) in specific growth rate (SGR) (18.8%±1.2 and 18.1%±2.2) and survival (61.5%±10.6 and 61.1%±9.4) compared to larvae fed live feed alone (SGR of 18.4%±1.1 and survival of 71.6%±25.9). In metamorphosed fish, however, significant differences (P 0.05) after weaning (day 70) to that obtained for fish that remained all the time on Artemia (5.3%±0.1). The SGR after weaning significantly decreased (3.4%±0.3, P<0.05) when the larval co-feeding regime had a higher proportion (2:1) of live feed. Survival for the weaning period was significantly lower (P<0.05) in weaned fish (39.0%±4.2 and 34.0%±11.3 for 2:1 and 1:1 live:inert larval co-feeding) compared to fish that remained on live feed (78.5%±5.0). This study demonstrates the feasibility of early weaning of S. senegalensis and that larval co-feeding enhances growth and survival after weaning.

A highly efficient microencapsulated food for rearing early larvae of marine fish

Abstract This paper describes the process of elaboration and the characteristics of an inert microencapsulated diet which provides much improved growth and development in the rearing of gilthead seabream larvae. The microencapsulated diet was composed of casein, fish protein hydrolysate, octopus meal, dextrin, emulsified lipids and a vitamin mixture. Following 4 days feeding on rotifers, three rearing treatments were used from day 8 to day 15 after hatching: Regime A (control): larvae fed on rotifers in semi-open system; Regime B: larvae fed on microcapsules and reared in a semi-open system; Regime C: larvae fed on microcapsules and reared in a closed recirculating water system. Growth rate and survival were similar in the three treatments (growth rates: 0.13, 0.09 and 0.12 day −1 , respectively), and notably higher than those obtained with the previous prototype. The results clearly show that the present microencapsulated diet was able to substitute for live food during the early stages of larval rearing and hence can be used as an excellent tool for investigation of the nutritional requirements of marine larval fish.

Free amino acid leaching from a protein-walled microencapsulated diet for fish larvae

The leaching loss of free amino acids (FAA) from an experimental protein-walled microencapsulated diet (MC) for larval fish after immersion in water was measured and compared with leaching loss from a gelatin microbound diet (MB) containing the same dietary ingredients. The loss of FAA (as a percentage of total dietary FAA) was significantly higher in the MB diet compared to the MC diet. After 5 min of re-hydration, 22% of FAA were leached from the MB diet compared with 8% from the MC diet. After 60 min, 85% of FAA from the MB diet were leached into the water compared to 17% from the MC diet. The addition of free lysine to protein-walled microcapsules (MC-L) was also investigated to determine the suitability of MC for delivering specific FAA. Lysine was incorporated into the particles with an efficiency of 7.5% and accounted for approximately 60% of the total FAA in the diet. The loss of this amino acid from MC-L after 60 min of immersion was 1.4%. The loss of specific FAA from MB and MC diets was found to be negatively and positively correlated to the hydrophobic character (hydropathy index) of each FAA, respectively. These results support the use of protein-walled microcapsules as a vector for specific dietary amino acids using a macronutrient-balanced diet.

Detecting growth in gilthead seabream, Sparus aurata L., larvae fed microcapsules

Abstract The ability to grow Sparus aurata fed a prototype microencapsulated food in relation to larval age was examined. The contribution of live prey in co-feeding experiments (live + inert food) was also determined. To attain these objectives several feeding regimes differing in the time that inert food was introduced were tested during the first 2 weeks of larval life. Larvae fed microcapsules alone from first feeding did not increase their dry weight during the experimental period, but after the point of irreversible starvation (day 8 after hatching), the larvae were able to grow (specific growth rate 0.034) and recover their initial dry weight. The larvae fed microcapsules from day 8 or day 12, after previously being fed rotifers (from day 4 to day 8 or 12), showed similar growth rates (specific growth rate 0.053 and 0.034, respectively), although in these cases the final dry weight was higher. Survival of larvae fed only microcapsules ranged from 11%, when the capsules were added from first feeding, to over 50% when pre-fed rotifers. The addition of live prey (5% of the total food supplied on dry weight basis) improved the survival (42%) when the microcapsules were supplied from the start of feeding. Feeding incidence on microcapsules was similar to that obtained with rotifers and the physiological status of the surviving larvae at end of the experiments was acceptable. Water quality was similar between the controls fed rotifers and the treatments fed microcapsules, indicating that no negative effect on larval growth could be attributed to water quality. This study indicates that S. aurata larvae are able to grow when fed only microcapsules, although with a low growth rate probably due to a lower assimilation of the diet. Further experiments testing other sources of protein and additives are needed in order to understand the factors that are limiting larval growth.

Feeding rates of gilthead seabream (Sparus aurata), larvae on microcapsules

Abstract Food consumption rates of gilthead seabream larvae from first feeding to 20 days of age were studied by feeding larvae on two types of microcapsule (hard- and soft-walled) having diameters ranging from 25 to 400 μm. Experiments were carried out in 15-litre beakers using larvae cultured with standard techniques and fed on rotifers and Artemia sp. nauplii. Ingestion rate ranged from 3 to 35 particles · larva−1 · h−1. but, except for some increase during the first days of feeding, no consistent trend was observed in relation to larval weight. However, when values were converted to volume or dry weight units, food consumption rate increased progressively with increasing larval weight. On a volume basis, the rate of this increase was different for the two microcapsule types, being generally higher with soft microcapsules, probably due to its better accessibility and to their easier progress through the digestive tract. However, on a dry weight basis the rates of food consumption were similar with both types of microcapsule because of the higher density of hard-walled microcapsules. Ingestion rates increased from 0.5–3 μg · larva−1 · h−1 at first feeding up to 18–25 μg · larva−1 μg · h−1 in older larvae. The daily specific ration increased from 0.5 to 2.5 μg food · μg larva−1 · d−1 at first feeding to a maximum of 3–5 μg food · μg larva−1 · d−1 at a larval dry weight of 70–110 μg. Above this larval weight, the specific ration tended to remain constant. The results show that gilthead seabream larvae can be transferred directly from live to microencapsulated food without appreciable variation in the incidence of feeding or in the rate of food consumption from first feeding onwards.

Delivering bioactive compounds to fish larvae using microencapsulated diets

The efficient delivery of nutrients and hormones has special relevance to the development of rearing technologies for fish larvae and juveniles. The main aim is to find an effective and measurable way to administer them into the body of small aquatic animals. In this study, three different compounds (hormones, amino acids, and vitamins) were incorporated into protein-walled microencapsulated diets. Specifically these microencapsulated diets were examined for (a) the kinetics of incorporation of estradiol in Sparus aurata larvae, (b) absorption and leaching patterns of the free amino acids (FAA), and (c) growth results and tissue incorporation of vitamins in relation to the supplementation of vitamin C in larvae of S. aurata and Solea senegalensis. The efficiency of inclusion was relatively low, but the capsules were able to retain enough of these compounds when immersed in water and to deliver them into the digestive tract of the larvae. There are noticeable differences among the nominal amount of a given substance in the ingredient mixture, the actual amount in the microparticle and the amount delivered in the larval gut. It is therefore necessary to examine carefully whether the ingredient is reaching the digestive tract for achieving suitable conclusions in nutritional studies. These results indicate the applicability of these microencapsulated particles in nutritional studies of small aquatic animals.

Capacity of gilthead seabream, Sparus aurata L., larvae to break down dietary microcapsules

Abstract The capacity of hatchery-reared gilthead seabream larvae to disintegrate mixed-walled protein/ carbohydrate microcapsules in their midgut was studied. The response of feeding larvae to eight types of microcapsules was examined. The effects of the following factors, which could potentially influence capsule breakdown, were evaluated: (a) larval age; (b) addition of enzymes to the diet; (c) concentration of cross-linking agent; and (d) procedure for isolating microcapsule after formation. The results revealed that larvae belonging to the same population exhibited substantial individual variability in their capacity to disrupt the microcapsule wall. Despite this, there were clear differences in the ability of the larvae to break down different types of microcapsules, depending principally on the technique followed during the isolation phase of microcapsule elaboration. Thus, capsules isolated in gelatin (typeG) were more easily broken down than those isolated in alcohol (type A), irrespective of larval age ( P P > 0.05) in the degree of capsule breakdown. Likewise, the concentration of cross-linking agents used to form the capsule walls had no effect on capsule disruption by the larvae. The present results suggest that seabream larvae are able to digest inert food from the onset of exogenous feeding. Their capacity to do so is, however, influenced by the thickness of the capsule coating and by their age. The results of this study also contribute to our knowledge of the behaviour of a cultured larval population in the presence of microencapsulated food.

Pilot evaluation of freeze-dried microalgae in the mass rearing of gilthead seabream (Sparus aurata) larvae

Abstract The replacement of live microalgae by freeze-dried Nannochloropsis gaditana (B3 strain) and Isochrysis galbana (T-ISO strain) in the mass rearing of larval seabream ( Sparus aurata ) was studied. Total substitution of live N. gaditana by freeze-dried cells in 1 m 3 larval tanks produced similar growth and survival in larvae reared from first feeding until day 43 with three different types of rotifer enrichment. Differences in growth of larvae were only due to the type of enrichment used to feed rotifers prior to their addition to larval tanks. This was regardless of the presence of live or freeze-dried N. gaditana in the larval tanks. Specific growth rate ( G ) of larvae was significantly enhanced ( G =0.106±0.001; P N. gaditana ( G =0.099±0.003) as rotifer enricher. Feeding the rotifers with freeze-dried I. galbana produced the same larval growth rate ( G =0.109±0.002; P >0.05) as the commercial enricher, indicating the high nutritional value of these algae in a freeze-dried state. Larval survival was similar in all treatments. Water quality, in terms of dissolved oxygen and pH, was also similar in the different treatments. Ammonia concentration was higher ( P N. gaditana was added to the larval tanks, but only during the first 15 days of culture, when no water exchange was used. Nitrite did not vary ( P >0.05) during the first 15 days, but increased more for live N. gaditana from day 16 onwards. Results of this study indicate the potential for a complete replacement of live microalgae by freeze-dried microalgae throughout the whole process of mass rearing seabream larvae.