Accurate decisions about microbial health of released fish require determination of normal and pathogenic flora for each species released. As a first step toward assessing juvenile health and formulating a predictable standard for successful stocking, we monitored but did not manipulate
bacteria of larval common snook (Centropomus undecimalis) in two culture systems. Snook survived better in an open (flow-through) system than in a closed (recirculating) system. Fish from the open system 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‰, the open-system flora further diversified and shifted from gram-negative bacteria dominated by vibrios to gram-negative environmental and gram-positive 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 system, both before and after the salinity change. The presence of certain bacteria (Bacillus sp. and gram-negative environmentals) seems responsible for the exclusion of pathogenic strains
and, therefore, better overall fish health. During the second part of the study, inoculation of culture water with a probiotic isolate (Bacillus no. 48), combined with salinity reduction, apparently eliminated vibrios from juvenile snook. All snook from four rearing cycles in the open
system that were transported 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 experience, and a review of the literature, some recommendations
are made for microbial management of juvenile snook and similar fish. Most bacterial infections of marine fish larvae probably begin in the gut. Good bacterial selection by both the culturist and the fish, immunological preparation 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.
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