In bivalve hatchery, broodstock founds one of the most important bacterial sources for larval cultures due to the vertical transmission of bacteria. In Amsa shellfish station (INRH_Tétouan, Morocco), the discharge of bacterial load from reared broodstock originated from natural beds is one of the main challenged goals to resolve pathogenic bacteria causing massive mortality during early stage larvae. Hence, this study has been carried out to reduce the bacteriological load (particularly Vibrio sp) of mussel broodstock (Mytilus galloprovincialis), without any further use of antibiotics in order to ban or minimize their application in aquaculture.
The trials were applied on adult specimens of the Mediterranean mussels collected between May and July 2021 from a natural bed located in M’diq, Morocco (35°40’55.9"N 5°18’36.0"W). Three different water flows (3, 10 and 30 L. min-1) were investigated herein to perform the effect of a purification system after 24h, 48h, 72h and 96 h. This study was held in 400 liters rectangular tanks with an open circuit of microfiltred (0.2 µm) and sterilized (Ultraviolet) seawater at optimal physicochemical conditions (temperature, pH, salinity and dissolved oxygen) with a stocking density of 1 kg. L-1.
The results showed that the highest reduction was obtained after 96 hours of purification by reaching 99.83%, 99.64% and 94% of bacterial discharge at 30, 10 and 3 L. min-1 respectively. However, the lowest bacterial reduction was obtained after 24 hours and reaches 90.9%, 42.2% and 30.10% respectively at water flows of 30, 10 and 3 L. min-1. The statistical analysis (ANOVA) revealed that flow rates have significant effect on the bacterial discharge in mussels at 24h (F=134.6, p < 0.001), 48h (F=55.02, p < 0.05), and 72h (F=47.13, p < 0.05), except 96h where no significant influence was observed between flow rates (F=1.18, p = 0.3936). As regards the effect of purification period, the bacterial load was reduced significantly using 3 and 10 L/min (p < 0.001) among all tested periods. As results, the bacterial depuration of mussels during conditioning could be held using high flow-through (30 L. min-1) during a short period (< 24h) or low to moderated flow (3 to 10 L. min-1) during long period (> 24h).
The originality of this study is based on proposing a reliable and sustainable practices during controlled shellfish spat production that can be easily transferred to commercial and industrial purposes.