Impacts of episodic acid and aluminum exposure on the physiology of Atlantic salmon, <i>Salmo salar</i>, smolt development
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.