Bacterial necrosis was the only bacterial disease syndrome seen during monitoring of 8 larval production runs at 2 commercial hatcheries during the study. Results indicated a possible causal relationship between concentrations of one or more components of the "presumptive Vibrio" populations in rearer tank water, as detected by dip slide TCBS agar, and outbreaks of bacterial necrosis. The source of these putative pathogens and the means by which they induce lesions was not determined.

There was no evidence that outbreaks of bacterial necrosis were causally associated with changes in other variables measured, including concentrations in rearer water of total bacteria, total heterotrophic bacteria, unionised ammonia, nitrite or nitrate. There was no convincing evidence that antibiotics, in the concentrations used at he commercial hatcheries during the study, were effective in reducing concentrations of the putatively pathogenic presumptive Vibrios.

Dip slides proved to be a convenient method of monitoring bacterial concentrations in larval rearing tank water, Artemia cultures and algal cultures. While providing useful information on hatchery hygiene and serving as an indicator for possible outbreaks of bacterial necrosis, the 1-day incubation requirement means such monitoring has no predictive value; during the 2-day risk period relevant to the dip slide result, hatchery operators can more accurately determine the disease status of the larvae by direct examination.

The epifluorescence technique did not appear to be a useful predictor of changes in larval disease status.

Thirteen species of bacteria were identified from larval rearing water containing diseased larvae, and from algae and brine shrimp culture samples. The most common isolate was Vibrio alginolyticus. This organism was subsequently found not to cause disease when larvae in the bioassay system were challenged. Nine bacterial species were isolated and identified from bacterial necrosis lesions on diseased larvae. No single species was common, but included Vibrio tubiashi and Vibrio alginolyticus.

In order to study the relationship between rearing environment and bacterial pathogens, a small-scale larval rearing system was developed. It was based on one litre Imhoff cones in a temperature-controlled water bath. Penaeus monodon larvae were used for all trials. Ultrafiltered, u/v sterilised seawater was exchanged daily in the larval cultures. Chaetoceros gracilus and Frippak microencapsulated diets were the source of food for the larvae. It was found that between batch and between replicate survival was highly variable. This reduced the sensitivity with which differences between treatments could be detected on analysis. The bioassay system studies showed that low nutrition, trauma (heavy aeration) and high stocking densities increased the prevalence of bacterial necrosis lesions and tended to reduce larval survival. In the one trial where treatment larval cultures were exposed to a daily cold-water shock, survival appeared to increase.