Toxicity of two pyrethroid-based anti-sea lice pesticides, AlphaMax® and Excis®, to a marine amphipod in aqueous and sediment exposures

• Amphipods were exposed to anti-sea lice pyrethroid pesticides in water or sediment.
• Immobility and delayed mortality occurred after acute aqueous exposure.
• Toxicity was observed at aqueous pesticide concentrations expected in the field.
• Mortality occurred at sediment concentrations above environmental concentrations.
• There is low potential risk from sediment exposure for this amphipod species.

Pyrethroid pesticides used to control ectoparasitic sea lice in salmon aquaculture are released in effluent plumes from cage sites and have the potential to adversely affect non-target organisms. Pyrethroids have been shown to be highly toxic to crustaceans, but the toxicity of the pyrethroid-based anti-sea lice pesticides AlphaMax® (active ingredient (a.i.) deltamethrin) and Excis® (a.i. cypermethrin) has not been thoroughly studied for non-target, marine benthic crustaceans such as amphipods. The amphipod Echinogammarus finmarchicus, which is ubiquitous in near-shore environments in the northern Atlantic Ocean, was collected from the field for use in laboratory toxicity tests. Amphipods were exposed to the two pesticides in 1- and 24-h water-only, single-pulsed exposures and 10-day spiked sediment tests. In water-only tests, immobility occurred within 1- or 24-h of exposure to the highest test concentrations and delayed mortality and immobility were observed following exposure to lower concentrations as well. Effect thresholds ranged from 6.7–70 ng/L for deltamethrin and 20–220 ng/L for cypermethrin. Organisms exposed to sediment were affected within 2–4 days; resulting 10-d LC50s were 16 and 80 ng/g (dry weight) for deltamethrin and cypermethrin, respectively. Results suggest that amphipods in an effluent plume may be exposed to aqueous concentrations of pyrethroids sufficient to cause adverse effects, including delayed toxicity. In contrast, the 10-day sediment LC50s were much higher than (limited) reported environmental concentrations in sediment. Overall, these results suggest a low potential for risk from sediment exposures for a northern Atlantic species of amphipod that inhabits the near-shore environment where cage aquaculture sites are located.