Physiological effects of pesticides on different life stages of Atlantic salmon (<i>Salmo salar</i>)
A recent decline in Atlantic salmon (Salmo salar) populations have led to their listing as an endangered species and is related to the contamination of rivers due to the high application rates of pesticides in agriculture. Four experiments were designed to study the effect of pesticides on several life history stages of Atlantic salmon: (i) yolk-sac larvae (YSL) were exposed for 21 d to sub-lethal levels (<100 μg l-1</super>) of hexazinone (HEX) and atrazine (ATZ) in fresh water (FW). After a year smolts were exposed to 24 h seawater challenge (SW); (ii) smolts were exposed for 21 d to sub-lethal levels (<100 μg l-1</super>) of HEX and ATZ in FW. After 21 d smolts were exposed to 24 h SW; (iii) YSL were exposed for 12 d to 10 μg l-1 of ATZ, HEX, chlorothalonil (CTL), and phosmet (PHO) at pH 6.5 and 5.0; and (iv) YSL were exposed to either 1 or 10 μg l-1 of ATZ, CTL, PHO and HEX or binary combinations of each. We measured the hormones involved in smolt development (cortisol, GH, IGF-I, T4 and T3) and cholinesterase activity in larvae, plasma ions (Cl-, Mg2+, Na+, Ca2+) in FW and after SW challenge, and Na+, K+-ATPase activity at both life stages. In conclusion ATZ causes ionoregulatory, growth and endocrine disturbance and reduces salinity tolerance of Atlantic salmon smolts. Plasma cortisol was affected in smolt exposed to ATZ and HEX as YSL and after a second ATZ exposure. Exposure of YSL to ATZ, HEX and low pH caused faster opercular movement, which suggests a higher energetic demand and/or a respiratory impact. Low pH causes ionoregualtory disturbance in YSL. Low pH and combinations of pesticides causes few synergistic effect and in most cases the effect was additive. Body size was smaller in YSL exposed to low pH and to a combination of pesticides. Disruption of nerve transmission was found in YSL exposed to the insecticide PHO alone or in combination with low pH, ATZ, CTL or HEX. In the wild these impacts may compromise their growth, competitive ability and predator avoidance affecting their survival and population recruitment.