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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) 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) 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.

Episodic acidification and its associated aluminum (Al) toxicity has been identified as a possible cause of Atlantic salmon decline in the northeastern United States including Maine where several salmon rivers are listed as endangered. During precipitation events such as snowmelts and storms, rivers and streams in this region experience episodic pulses of low pH and elevated inorganic Al which can damage the gill epithelium of fish leading to ion regulatory disturbances. To date, the impacts of episodic acid/Al on the physiology of Atlantic salmon undergoing critical life-stage transitions such as the parr-smolt transformation remain largely unknown. In this dissertation, I have used both laboratory and field studies to demonstrate that Atlantic salmon smolts are particulary vulnerable to ion regulatory disturbances during episodic acid/Al exposure. In particular, short-duration (days) exposures to acid and low levels of inorganic Al can impair the seawater tolerance of smolts in the absence of detectable impacts on freshwater ion regulation demonstrating the extreme sensitivity of the smolt hypoosmoregulatory system. I have also presented evidence that loss of seawater tolerance occurs through alterations in gill ion transporter expression, chloride cell dynamics, and several endocrine systems including the growth hormone-insulin-like growth factor I, interrenal and thyroid systems. Many of these alterations are likely involved in the upregulation of ion uptake mechanisms as part of acclimation to acid/Al in freshwater which may come as a direct cost to the ability to maintain ion homeostasis in seawater. The results presented here have important implications for salmon populations in regions affected by episodic acidification. Smolts with compromised seawater tolerance may experience delayed migration, decreased seawater preference and increased susceptibility to predation. This is likely to increase mortality during downstream migration, seawater entry, and marine residence ultimately leading to population level effects. Furthermore, these results support the idea that the freshwater experience of smolts may have a direct impact on survival in the marine environment.

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