Ph. Dhert

Variation of ascorbic acid content in different live food organisms.

Abstract Ascorbic acid (AA) is an essential nutrient both in particle and live aquafeeds. In order to better assess the needs for this nutrient during larviculture the AA content of algae, rotifers and Artemia was studied with respect to their suitability at startfeeding. In general, the microalgae evaluated were rich in AA (1000–4000 μg AA/g DW), but showed a considerable variability among the different species: e.g. the concentration in Isochrysis and Chlorella reached values 3-to 4-fold the percentage of Tetraselmis (0.11 % of DW). Brachionus routinely cultured on Chlorella contained 2300 μg AA/g DW. Cysts of various batches and strains of Artemia differed considerably in ascorbic acid-2-sulphate (AAS) concentration (296–517 μg AA/g DW). The amount of AA available in the freshly-hatched nauplii reflected exactly the AAS reserve present in the cysts, what evidences the complete conversion of AAS to free AA during completion of embryonic development into nauplii. Boosting techniques both for Brachionus and Artemia using ascorbyl palmitate (AP) as the vitamin C source were established. The addition of 20% AP in the diet of Brachionus enhanced their AA content 10-fold over 3 days of culture. Supplementation of the enrichment emulsion for Artemia with 20% AP increased the AA content up to 2000 μg/g DW after 24 h enrichment. This lipophilic derivative of AA appeared to be a stable form of vitamin C for enhancing AA levels in the live diets during culture and/or enrichment. This bioencapsulation method provides a tool for hatcheries to build up high AA concentrations in the live prey administered to first feeding larvae of aquaculture organisms in case of specific requirements (e.g. with respect to handling stress, deformities). A survey of commercial hatcheries indicated that a wide range of products is used for the cultivation and boosting of rotifers, which consequently affect their AA levels. In general, the AA content in the algae and, consequently, the algal enrichment of Brachionus tended to score lower in the hatcheries than under lab conditions.

Selection of bacteria enhancing the growth rate of axenically hatched rotifers (Brachionus plicatilis).

Abstract The effect of bacterial strains on the growth rate of rotifers, Brachionus plicatilis , was determined under monoxenic conditions. The first objective was to obtain sterile rotifer cultures starting from rotifer resting eggs using merthiolate or glutaraldehyde as disinfectant. Sterile rotifer cultures were obtained, without affecting the hatching ability of the resting eggs, when 0.05 μl/l glutaraldehyde was used. This disinfection procedure was used to examine the effect of 20 bacterial strains, isolated from well-performing live-feed production systems, on the population growth rate of rotifers cultured under monoxenic conditions. Five out of the 20 bacterial strains tested were able to improve significantly the asexual reproduction of rotifers. The population growth rate ( μ pop ) of rotifer cultures treated with GR 12 and GR11 (respectively 0.664±0.043 and 0.622±0.062) was significantly higher than the μ pop of the control treatment (0.512±0.101). Overall, the egg ratio after 48 h was significantly higher in the cultures inoculated with the bacterial strains than in the axenic control treatment. The results show that it is possible to control the microbial community in rotifer cultures started from disinfected resting eggs by adding bacterial strains which have a positive effect on the population growth rate.

Technical and economical feasibility of a rotifer recirculation system

Abstract A feasibility study was performed on the use of a recirculation system for the mass culturing of rotifers at industrial level. Rotifer culture systems with a culture volume of 750 l were operated at three different stocking densities (3000, 5000 and 7000 individuals ml−1) in a completely closed recirculation system. At all operating rotifer densities, a reliable production of 2.2 billion rotifers could be obtained on a daily basis during 3 weeks. Excellent water quality was maintained by the use of protein skimmers, the use of ozone and a submerged biofilter. The microbial counts remained stable during the whole culture period (106 CFU ml−1 on marine agar and 104 CFU ml−1 on TCBS after 15 and 23 days, respectively). No difference in HUFA and protein content were obtained between rotifers harvested from the recirculation system or from a conventional batch culture system. Compared to a commercial batch culture system, the use of a recirculation system can contribute to a 43% saving on the capital investment and the annual operation cost. By using this system, capital investment cost is considerably reduced by 46%. Savings are also made on labour cost (65%) and feed cost (21%) during a 1-year production. In general terms, it can be stated that by using a simple recirculation system, a cost-effective technology and a reliable rotifer culture can be obtained.

Possible applications of modern fish larviculture technology to ornamental fish production

There has been rapid development in the marine foodfishlarviculture technology in Europe since the early eighties,especially in the flat fish, turbot and halibut, and the bass andbream species. The most significant improvements in the eightieswere the introduction of light control, artificial reproductiontechniques, appropriate water treatment and the use of rotifersand Artemia nauplii of specific sizes and in the late eightiesand early nineties the quality enhancement of live food organismsusing specific enrichment techniques. Present research is focusedon the implementation of several microbial techniques to improvethe hygiene of live prey and fish.Many of the modern larviculture techniques being used in marine foodfish could be adapted for application in the ornamental fish industry. For examples, research in the Onamental Fish Section, Primary Production Department, Singapore has demonstrated that the use of freshwater rotifers and Artemia nauplii would enable artificial rearing of Discus in the absence of the parent fish and improve the larval performance of Gouramis and Tetra larvae. The use of such small live food organisms is likely to facilitate breeding of new fish species with small larvae. Research has also indicated that the use of diets containing vitamin C and certain immuno-stimulants improved the stress resistance of guppy. Such techniques would have important application in the fish transport, an important aspect in the ornamental fish industry

Use of ongrownArtemia in nursery culturing of the tiger shrimp

Juvenile and adultArtemia produced in a semi flow-through culture system were used as food for postlarval shrimp. The growth performance of shrimp reared on such ongrownArtemia live prey is identical to the growth obtained when feeding newly hatchedArtemia. However, a significantly better stress resistance is obtained when the postlarvae are exposed to a low salinity in a stress test. Besides nutritional and energetic advantages, the use ofArtemia biomass for feeding postlarval shrimp also results in improved economics as expenses for cysts and weaning diets can be reduced.

A simple semi flow-through culture technique for the controlled super-intensive production of Artemia juveniles and adults

Abstract A simple semi flow-through Artemia culture unit is described for possible integration in marine fish and shellfish hatcheries as source of a cheap nursery diet. The system does not require the use of feeding pumps and involves minimal care. Food preparation and addition to the feeding tank is reduced to one or two manipulations per day during the meta-nauplius stages (day 1–3) and the juvenile stages (day 4–14), respectively. Biomass productions are superior to those reported for stagnant culture systems and are comparable to those demonstrated for flow-through culture units. This simple rearing technique offers the possibility of producing brine shrimp populations with a uniform size. Furthermore, by varying the feeding regime with the Artemia density at the start of the culture, specific Artemia prey sizes corresponding to the daily physical requirements in shrimp and fish hatcheries can be obtained.

Production, disinfection and evaluation for aquaculture applications of rotifer resting eggs from Bohai Bay, P.R. of China

The dynamics of resting egg production of the rotifer Brachionus plicatilis originating from a wild population in the Tanggu Saltworks (P.R. China) was investigated. In the natural environment as well as in semi-controlled rearing conditions an increased resting egg production was noticed with declining food availability. Processed resting eggs had a hatching efficiency of 3 x 10 6 rotifers per gramme irrespective of their origin. Hatching started 22 h after the initiation of incubation and was completed after 36 h. Rotifers obtained from resting eggs could be further cultured on Culture Selco® and enriched with Super Selco®. The fatty acid profile of these rotifers were not divergent from reference rotifers (originating from the Laboratory of Aquaculture and Artemia Reference Centre) demonstrating that this strain was not catabolizing essential fatty acids and could be used for enrichment purposes. Storage of resting eggs at 4°C resulted in a 50% lower hatching after 1 year but remained stable during the next 2 years. The resting eggs used for storage could easily be disinfected without affecting their hatching characteristics. These results indicate that this material could be used as inocula for mass cultures of live food for commercial hatcheries.