Sources of Vibrio bacteria in mollusc hatcheries and control methods:: a case study

Results of a 1-year intensive monitoring of Vibrio bacteria, as thiosulphate citrate billar salt sucrose (TCBS) counts, in a hatchery for molluscs, are presented. Surface seawater, well seawater, tap water from a freshwater well, microalgae system, including pasteurized well seawater, f/2 medium, starter cultures of eight micro algal species, mass-produced microalgae and compressed air were studied as possible sources of Vibrio. Additionally, recently collected broodstock of the catarina scallop Argopecten ventricosus (Sowerby II 1842) was also subjected for investigation for their possible role in discharging Vibrio during spawning. Efficiency of a cartridge-ultraviolet (UV)-filtration system and of a steam gun employed for sterilizing tanks, hoses and utensils was also determined. Results indicated that Vibrio were present in surface seawater throughout the year. In well seawater these bacteria were only found during the first 5 months soon after construction of the well and before commencement of continuous operations, but not after this period. Vibrio was never detected in pasteurized well seawater, f/2 medium, starter cultures of microalgae and compressed air. However, mass-produced microalgae contained Vibrio, possibly as a result of the use of contaminated tap water for washing and rinsing the fibreglass culture tanks. Scallop broodstock was found to be an important source of Vibrio in the hatchery. Biopsy on female gonads of live organisms demonstrated higher numbers of colony-forming units than male gonads. At spawning, Vibrio was released into water, producing bacterial blooms in tanks after 24 h. The cartridge-UV-filteration system was not sufficient to eliminate the bacteria from seawater, but the steam gun proved to be a good tool to eliminate Vibrio biofilms on tank walls or tools used in the hatchery. Some recommendations are given, which could be useful for other mollusc hatcheries.