Application of immunostimulants in marine larviculture: possibilities and challenges.
Microbial interferences are one of the main problems in the rearing of marine larvae, and it is therefore a main goal to develop methods for establishing microbial control. In addition to counter measures directed towards the environment, measures to improve the resistance of the larvae against bacterial infections must also be included. Immunostimulation represents methods where the capacity of either the specific or the non-specific immune system is increased. Limitations in the application of immunostimulation in a specific case are set by the developmental stage of the immune system, target organisms, type of immunostimulant used, and administration procedures.
Although information is limited, we may conclude that larvae of both fish and shrimp have a poorly-developed immune system, and primarily have to rely on the non-specific immune system. Immunostimulation of larvae must therefore be directed against this part of the immune system. Several substances have been shown to stimulate the general immune system of animals, and examples of their use and potential with fish are presented. Work on immunostimulation of larvae is very limited, even though the potential of their use may be enormous. The use of immunostimulation in larviculture requires the development of methods for administration and adaption of methods for the detection of response on the immune level. This last point is a particular challenge due to the small size of larvae.
It is hypothesized that immunostimulation, together with other methods aimed at obtaining microbial control, will severely reduce the probability of microbial problems in larviculture. As a result, increased and more stable survival and growth is anticipated, and hence production of high quality larvae for the on-growth period.
(SINTEF Applied Chemistry, Center of Aquaculture, N-7034 Trondheim, Norway)
Characterization and possible role of the Vibrio flora involved in marine larviculture.
Vibrios constitute the majority of culturable bacteria associated with marine organisms and are part of their normal gut microflora and of the environment. A range of Vibrio species are reported to be pathogenic to fish and shrimp, but only few reports clearly constitute pathogenic strains for the larval stages. Because of the specific problems related to the omnipresence of Vibrio and of the small size of the hosts, isolation and identification of pathogens and especially the confirmation of their pathogenic nature is difficult. Surveys to relate environmental conditions to health status of fish and shrimp larvae result in large numbers of isolates, for which reference systems based on traditional biochemical testing and/or serology are too cumbersome and thus unfeasible.
In order to overcome these problems we have made use of a combination of BIOLOG metabolic fingerprinting, analysis of fatty acid methyl esters (FAME) and the PCR based AFLP® technique for the characterization of Vibrio strains. The following topics will be addressed: (1) the diversity of Vibrio species; (2) the identification of pathogenic Vibrio strains for shrimp larvae; (3) the relationship between the microflora in live feed, rearing water and fish larvae; (4) the selection and characterization of probiotic strains useful in fish and shrimp larval rearing; (5) the possible role of probionts in larval health status.
Based on the experience gained so far recommendations are formulated for improved zootechniques in fish and crustacean larviculture.
(Laboratory of Microbiology, Ghent University, Ledeganckstraat 35, B-9000 Gent, Belgium)
The impact of development on larval nutrition.
The design of adequate larval rearing and feeding strategies and the formulation of dry larval diets need a comprehensive analysis of the ontogenetic changes occurring during the early life stage of the species under concern. Differences between larval and juvenile nutrition can be explained both by size differences (scaling factors) and by ontogenetic factors. It is hypothesized that differences in nutritional aspects between different larval species may be less rigid when regarded from an ontogenetic perspective. In this regard, the onset of stomach functions and its physiological consequences in the larvae seems to be a key parameter to understand some basic aspects of larval nutrition. Most fish larvae lack a functional stomach at the onset of exogenous feeding. This may have an effect on the protein dynamics in the larvae. Nevertheless, from a quantitative point of view, differences in protein requirements between larvae and juveniles can be related to a scaling effect. The impact of the stomach functions seems to be more of a qualitative nature. In fast growing animals such as fish larvae, any imbalance in the influx of dietary amino acids to the body free pool may cause increased protein degradation and decrease the growth efficiency. The presence or absence of stomach functions may affect the affinity of dietary protein substrates for intestinal lysis and thus affect the growth efficiency of the animal. In larval nutrition, a correct definition of developmental stages is crucial for the design of experiments and for the interpretation of results. The problem however is how to scale and translate developmental aspects into measurable units which can be used. In the present paper, several of these parameters will be discussed. Most studies on larval nutrition use age as a marker for development. Size (length and/or weight) may be a better parameter for staging development. Temperature is the single most important factor determining the speed of metabolic processes in fish, and thus also the rate of development. In the present paper the relation between temperature, growth and the design of sampling schemes in larval studies is discussed.
(Dept. of Fish Culture and Fisheries, Wageningen Agricultural University, P.O. Box 338, NL-6700 AH Wageningen, The Netherlands)
