Salmon aquaculture is among the fastest growing food-producing industries worldwide, growing from 12,000 tonnes in 1980 to over 3.2 million tonnes by (Asche et al., 2013FAO, 2020). Canada is now the fourth largest producer of farmed salmon behind Norway, Scotland and Chile producing over 100,000 tonnes of farmed salmon in 2019 (DFO, 2018a). This industry relies on permeable open net pen structures placed directly into nearshore coastal waters to hold the salmon (Asche et al., 2013). These open net pens are efficient for rearing salmon as the open transfer of seawater provides exposure to natural fluctuations in water conditions such as salinity, temperature, nutrients and dissolved oxygen.Outbreaks of parasitic sea lice (Lepeophtheirus salmonis and Caligus spp.) are a major issue in open net pen salmon farming worldwide (Costello, 2006;Krkošek et al., 2005). These ectoparasites feed on the host's skin, mucus and underlying tissue, leaving the host fish susceptible to secondary infection and disease (Costello, 2006).Outbreaks of sea lice represent the greatest source of mortality and economic losses to fish farms, causing estimated global losses of $500 million annually, accounting for 6% of product value (Costello, 2009;Mustafa et al., 2001).

Early life stages of Pink salmon (Oncorhynchus gorbuscha) are at risk of exposure to the active ingredients of chemotherapeutant formulations (hydrogen peroxide [HP], azamethiphos [AZ], emamectin benzoate [EB], cypermethrin [CP] and deltamethrin [DM]) used to control sea lice in salmon aquaculture. LC50 values (95% con dence intervals) for acute 48-h water exposures in order of least to most toxic to seawater-adapted pink salmon fry were: mg/L), EB (1090EB ( [676-2006 µg/L), µg/L), ] µg/L), and DM (980 [640-1800] ng/L). In subchronic 10-d lethality sediment exposure tests, LC50 values (95% con dence intervals) in order of least to most toxic were: EB (2065 [1384-3720] µg/kg), µg/kg), and DM (1035DM ( [640-2000 ng/kg). Alterations in behaviour varied between chemicals; no chemical attracted pink salmon fry; sh avoided HP to a limited extent at 50 mg/L), as well as EB (300 µg/L), and AZ (50 µg/L). Signi cant concentration-dependent decreases in olfactory responsiveness to food extract were seen following AZ, CP and DM exposures that occurred at lower concentrations with longer exposure periods (10 µg/L, 0.5 µg/L and 100 ng/L thresholds at 168 h). Following 10-d sediment exposures, olfaction was only affected by CP exposure at 50 µg/kg. Signi cant decreases in swimming performance (Ucrit) occured for HP, AZ, CP and DM at concentrations as low as 100 mg/L, 10 µg/L, 2 µg/L and 200 ng/L, respectively. This study provides comprehensive data on the lethal and sublethal effects of aquaculture chemotherapeutant exposure in early life stage pink salmon.