Pesticides commonly used in aquaculture: Interactive and repetitive effect of azamethiphos and deltamethrin on the physiology and growth of the mollusc Mytilus chilensis.

The intensive use of antiparasitic pesticides in salmon aquaculture has raised concerns regarding their impacts on non-target marine organisms. In Chile, the organophosphate azamethiphos and the pyrethroid deltamethrin are widely applied to control infestations of the sea louse Caligus rogercresseyi, yet their combined and repetitive effects on ecologically and economically important species remain poorly understood. This study evaluated the physiological and growth responses of juveniles Mytilus chilensis exposed to concentrations of azamethiphos (15 and 100 μg L-1) and deltamethrin (1 and 2 μg L-1), applied individually and in combination. Mussels were subjected to a treatment regime mimicking salmon farm practices (1 h daily exposure for 7 consecutive days followed by 7 days of recovery), repeated over a seven-week period. Clearance rate and oxygen consumption rate were measured at weeks 1, 3, 5, and 7, while shell growth and soft tissue biomass were assessed at the end of the experiment. Repeated exposure to both pesticides, alone or combined, significantly reduced clearance rates over time, with the strongest effects observed during mid-experiment. Oxygen consumption rates exhibited transient increases during early exposure, followed by declines consistent with metabolic depression. Shell growth was significantly reduced in several pesticide treatments, particularly under combined exposures, whereas soft tissue biomass was not significantly affected. These results demonstrate that chronic and repeated exposure to azamethiphos and deltamethrin induces sublethal physiological stress and growth impairment in M. chilensis. Such effects may have ecological consequences for natural mussel beds and economic implications for mussel aquaculture in regions overlapping with salmon farming.