For five years, the authors have been microbially maturing culture water with high bacterial density having low numbers of fast-growing and opportunistic bacteria, evidenced by fairly good postlarval survival even during the rainy seasons.
The sun-dried larval rearing tank with drop-line aeration system (strictly without airstones) is filled with pre-settled, alum-treated, slow sand-gravity filtered seawater passing through stacks of coconut shell charcoal kept in trays fitted to the tank inlet pipe, with a flow rate of 2 tons per hour.
Maintained at 29 ppt (a salinity which keeps the incidence of luminous vibrios in check) larval rearing water is treated with 1 ppm dried Skeletonema microfine powder, 1 ppm lactobasivite powder, 0.2 ppm microfine ricebran and 0.1 ppm sugarcane molasses-based yeast.
After 24 hours of strong aeration in the dark at a constant water temperature of 31.6 C, the fresh water-dipped prawn nauplii are stocked in the tank. The bacterial biomass bred in the culture water provides a highly proteinaceous feed for the larvae. In 1996, Sunil Kumar Mohamed fed autochthonously obtained, non-pathogenic heterotrophic marine bacteria to larval monodon shrimp.
Once the nauplii started moulting to zoea, 40 micro bag-harvested (fast-flowing straining of long chains) early to mid-log phased green-celled young Skeletonema chains are collected and chain-distorted by passing the microalgal liquid through a bubble-fractionator device. These are fed to the early larvae.
A fluorescent illumination of 2,000 lux is directed onto the larviculture tank surface to keep the algal cells in the growing phase. The ascorbic acid content of Skeletonema costatum ranges from 11-700 fg per cell, depending on the photosynthetic rate, optimal carbon: nitrogen levels, the growth phase (early stationary) and cell size (Brown & Miller, 1992). Rupturing of Skeletonema chains by centrifugal pumping is not advisable.
Excess
During mysis II and III stages, before feeding the rotifers, an indigenously-prepared Skeletonema extract called "Skelon" is bioencapsulated into the live feed (rotifers). Preparation of the Skeleton liquid involves lysis of the bag-harvested Skeletonema chain cells in a blender and mixing with 0.01 ppm vitamin B12, 0.001 ppm seamud extract and sprouted-cum-fermented barley extract, frozen at 6 C.
Skelon liquid is administrated to the metamorphosing larvae from zoea I until PL 7 harvest, but only on alternate days at 0.2 ppm level for zoea, 0.4 ppm for mysis and 1 ppm level for postlarvae. No exchange of culture tank water is done until postlarvae I, but seawater is added to the larviculture tank starting from zoea I up to PL 1. Postlarval stages should have water exchanged every other day. Temperatures are kept at 31.8 C for zoea and 31.5 C for mysis. Postlarvae are held at normal temperatures of 30 C.
Postlarvae with luminescence in the gut need a continuous drip of Skelon liquid to help them recover from oral inflammation. Feeding of live cells of young greenish Skeletonema cells is done to maintain
a cell density of 300 cells/ml during zoea stages; 150-200 cells/ml during mysis stage and 75-100 cells/ml during postlarvae stages. Maintenance of Skeletonema cell density for postlarvae serves as a water conditioner only. The principal feed for postlarvae is frozen krill.
It is likely that the establishment of microflora in the intestinal tract of the fish larva is influenced by microalgae in the rearing water, either indirectly, or directly by ingestion of microalgae affecting the colonisation of the mucosae.
We hypothesise that the extracellular metabolites and cellular protoplast of Skeletonema cells are able to perform bacteriostatic activity and competitive exclusion of the luminous vibrio.
We suggest it is time for marine biotechnology firms to produce a freeze-dried product derived from the very young greenish cells of Skeletonema costatum which combat luminosis, reduce post-ablation shock, inhibit luminous biofilms on spawners and postlarvae and also condition the pre-treated seawater in the hatchery reservoir for use in shrimp hatcheries.
Unlike chemotherapy, which develops potentially-resistant luminous strains, a natural cure is the best ecofriendly measure in larviculture for challenging luminosis now and in the future.
(excerpts from article in Fish Farmer (international file), November/December 1997)