Stimulation of heterotrophic bacteria associated with wild-caught blue mussel (<i>Mytilus edulis</i>) adults results in mass mortality

Although hatchery technology is available and although hatcheries have several advantages over collection of natural spat, hatchery production of blue mussel (Mytilus edulis) larvae currently is not economically feasible. In contrast to other bivalves such as oysters and clams, no mortality episodes due to pathogenic bacteria have been described for adult blue mussel thus far. In this study, we aimed at investigating whether opportunistic pathogens are associated with wild-caught adult blue mussel, as we reasoned that environmental conditions that are beneficial to opportunistic pathogens might be responsible for mass mortalities of mussel larvae under hatchery conditions. The growth of heterotrophic bacteria associated with wild-caught blue mussel adults was stimulated by the addition of organic matter (tryptone and yeast extract) to the rearing water. The addition of organic matter resulted in a 3 log increase in total heterotrophic bacterial counts in the rearing water, and resulted in complete mortality of the animals after 6 days, and this could be prevented by the addition of antibiotics. Water quality parameters (TAN, nitrite, oxygen and pH) were monitored regularly, and were all within the acceptable range throughout the experiment, excluding water quality deterioration as the cause of mortality. These data might give a first insight into the reason why blue mussel larviculture still is problematic