Sensitivity of Atlantic salmon to the pesticide pirimiphos-methyl, present in plant-based feeds

The use of plant-based feeds has introduced undesirables, such as pesticides, that have previously not been associated with seawater farming of fish species such as Atlantic salmon. Earlier wide-scope chemical screening showed that the organophosphate (OP) pirimiphos-methyl (PM-m) is one of the most prevalent pesticides present in commercially produced Atlantic salmon feed. Information on the safe upper limit of background levels of PM-m in Atlantic salmon feeds with regards to possible adverse effect on fish health is lacking. Therefore, Atlantic salmon (132 ± 25 g) were fed graded levels of PM-m spiked feeds (0, 0.35, 1.5, 5, and 22 mg kg⁻¹) in triplicate for 3 months. Adverse effects were assessed on OP target toxic exposure such as plasma choline esterase (ChE), and secondary toxic responses such as lipid metabolism and oxidative stress, as well as general adverse effect parameters (plasma biochemistry, haematology, and growth). Safe limits were set by model-fitting the effect data in a dose-response (lower bound) bench mark dose (BMDL) regression analysis. Fish fed 1.5 mg kg⁻¹ and higher had a significant (p < 0.05) dose-dependent growth reduction, oxidative stress as seen from reduced glutathione ratio, liver damage as seen from plasma alkaline phosphatase, and reduced sum neutral and TAG liver lipids. Inhibition of ChE was observed in fish fed 5 mg kg⁻¹ and above. Disturbance in phospholipid (PL) fatty acid composition and reduced level of liver phosphatidyl choline (PC) occurred at a lower exposure level (0.35 and 1.5 mg kg⁻¹), however this was not dose-dependent as at higher exposure levels (5 and 22 mg kg⁻¹), no significant differences were observed. A safe feed limit (as BMDL) for dietary PM-m was set at 0.14–0.46 mg kg⁻¹ ww feed (daily dose 0.49–1.62 ng kg BW⁻¹ day⁻¹), based on inhibited growth, ChE, glutathione ratio, and sum neutral and TAG liver lipids. When including an estimated uncertainty factor (UF) of five for inter-species variability and extrapolation to chronic exposure, the safe limits are 0.028–0.092 mg kg⁻¹, which is lower than the highest PM-m levels observed in commercially produced salmon feeds (0.038 mg kg⁻¹). Thus indicating a potential risk for disturbance in lipid metabolism when fish are fed plant-based feeds. For the most sensitive lipid response, disturbance in PL metabolism, no certain BMDL could be assessed due to lack of a clear dose-response relationship.