The use of mussels for mitigating the noxious effect of phytoplankton spring blooms on farmed fish

International audienceThe possibility of using the natural biofiltration power of blue mussels (Mytilus edulis, 0.37 ± 0.08 g ind−1 dry weight) to dampen the potential detrimental effect of phytoplankton blooms on juvenile farmed sea bass (Dicentrarchus labrax) was tested in a fish farm during a 35-day mesocosm experiment. Mussel effective clearance rates averaged 41.15 ± 14.19 m3 h−1 and led to a 6.3–13.1-fold reduction of the phytoplankton abundance as well as comparable decreases in chlorophyll a and turbidity. This improvement in seawater quality significantly enhanced fish physiological performances: weight-based growth rates were significantly higher (2.87 ± 0.43% d−1) compared to control exposed to non-filtered (bulk) seawater (2.55 ± 0.44% d−1). The same observation holds for the Fulton condition index and the metabolic activity (RNA:DNA ratio). For fish reared in bulk seawater, diatoms embedded in gills (Rhizosolenia imbricata, Thalassiosira sp.) and mucus overproduction indicated a stress (i.e. mechanical damages) induced by phytoplankton exposure which, in turn, may have affected fish energy balance. The use of mussels as a satisfying mitigation tool reducing phytoplankton bloom impacts is discussed with regard to phytoplankton bloom magnitude and ashore marine fish farming in coastal ecosystems