BACTERIAL MANAGEMENT STRATEGIES FOR STOCK ENHANCEMENT OF WARM WATER MARINE FISH - A CASE-STUDY WITH COMMON SNOOK (CENTROPOMUS-UNDECIMALIS)

Accurate decisions about microbial health of released fish require det ermination of normal and pathogenic flora for each species released. A s a first step toward assessing juvenile health and formulating a pred ictable standard for successful stocking, we monitored but did not man ipulate bacteria of larval common snook (Centropomus undecimalis) in t wo culture systems. Snook survived better in an open (flow-through) sy stem than in a closed (recirculating) system. Fish from the open syste m had a higher diversity (at least nine genera) of bacteria than those from the closed system (four genera). After salinity was reduced from 32 to 2-4 parts per thousand, the open-system flora further diversifi ed and shifted from gram-negative bacteria dominated by vibrios to gra m-negative environmental and grampositive staphylococcal bacteria. The closed-system fish did not undergo a salinity or floral change. Fewer potentially pathogenic bacteria were found in fish from the open syst em, both before and after the salinity change. The presence of certain bacteria (Bacillus sp. and gram-negative environmentals) seems respon sible for the exclusion of pathogenic strains and, therefore, better o verall fish health. During the second part of the study, inoculation o f culture water with a probiotic isolate (Bacillus no. 48), combined w ith salinity reduction, apparently eliminated vibrios from juvenile sn ook. All snook from four rearing cycles in the open system that were t ransported by truck for 6 h, and then held in tanks for another 12 h, survived. Survival of 2368 juveniles stocked in saltwater ponds was 89 -96% after 2 mo. On the basis of these preliminary results, our experi ence, and a review of the literature, some recommendations are made fo r microbial management of juvenile snook and similar fish. Most bacter ial infections of marine fish larvae probably begin in the gut. Good b acterial selection by both the culturist and the fish, immunological p reparation of fish (e.g., prior exposures, immunostimulants, vaccines) before they are released, and reduction of stress (e.g., temperature, handling) during release will contribute to better survival.