It was found that a s ingle dose of live algae or indeed the algal water (AW) therefrom promotes higher survival of F. indicus larvae. Sterilised algal water still promotes a positive effect indicating that this effect was not due to a si mple transfer of beneficial bacteria. It was shown that live algae or the AW reduced the total bacterial counts over the initial 36h. However no apparent pattern was obvious for the presumptive Vibrio counts. It was furthermore found that the bacterial flora of F. indicus culture vessels vary significantly after only 12 h. The bacterial flora of a Tetraselmis chuii culture was found after 6 days not to consist of any presumptive Vibrios. It was shown that Pofysiphonia Lanosa and Delisea pulchra extracts have a s ignificant effect on the AHL dependent bacterial responses of swarming, protease production and bioluminescence and T. chuii and 4 other marine algae modify at least I AHL controlled process (swarming). The extract was a lso shown to be temperature tolerant and could be extracted using dichloromethane, water, or heated water furthermore finely ground P. Lanosa alone is shown to be effective. Polysiphonia Lanosa extract was s hown to inhibit growth of some bacterial pathogens in vitro however no bacterial growth was observed in vivo. Inhibition of bacterial growth by extracts of kale, Ascophylum nodosum, Polyides cotundos, Heterosiphonia plumosa, Plocamium cartilagium and Desmartia acculata plants were found for at least one bacterial pathogen. It was also demonstrated that the inclusion of P. Lanosa extract significantly increases the surviva l of F. indicus larvae when challenged with 2 different Vibrio pathogens. It was also observed that when the water quality was poor (low Secci disc depth reading), treatments supplemented with P. Lanosa extract achieve consistently higher survival rates. Finally it was shown in rearing experiments that the inclusion of a 0.5 ml daily dose of P. lanosa extract s ig nificantly increased the survival compared to the microencapulated feed alone. Using state of the art recirculation technology in a commercial clownfish hatchery is was shown the design was not optimal s ince bacterial disease still occurred. The sterilisation of the water was in fact very short lived, the culture water in the rearing tanks always favored high bacterial activity. T he reservo ir was s hown to act as a s ink with high bacterial concentrations but apparently low species diversity possibly indicating that it might be a source of infection. It was demonstrated that clownfish larvae inoculated with a bacterial pathogen (L. anguillarum) resulted in a significant rise in mortality which was dependent on pathogen dose but also dependent on the age of the larvae. Treatments supplemented with P. Lanosa extract achieved s ig nificantly higher survivals when challenged with the pathogen. The present work also shows that it is possible to experimentally infect bivalve larvae and juveniles with bacteria and cause mortality. However it was also shown that it is possible to s ignificantly reduce mortality caused by the bacterial pathogen Vibrio tubiashii upon inclusion of P. Lanosa extract of both Mytilus edulis larvae and Pecten maximus juvenile. DNA fingerprinting (AFLP) reveals that the swarming bacteria used in present work are indeed different strains. Using the same technique on bacteria and pathogens isolated from the clownfish hatchery it was shown that the bacteria present in the water at times of no disease are similar to those at times of disease. Finally the bioactive compound was isolated from the algal extract using GC-MS . No AHL like molecule was found instead a new pteridine like molecule. These findings suggests that the mode of action in this present work maybe bacteriostatic preventing Vibrios reaching a critical concentration in culture where quorum sensing can occur.