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

• Sea bass were exposed to natural and mussel filtrated seawater during a 35-d mesocosm experiment.
• Mussel filtration efficiently dampened phytoplankton biomass and abundance.
• Higher growth rate, condition K index and RNA:DNA ratio were recorded for fish reared in biofiltered seawater.
• Gills irritation (mucus overproduction) linked to needle-shaped phytoplankton species.

The 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.