The Effects of Acid Rain Mitigation Efforts on the Behaviour-Dependent Movement of Migrating Atlantic Salmon (Salmo salar, L.) Smolts

Acidification has led to changes in the chemistry of fresh waters and has impacted the survival of Atlantic Salmon (Salmo salar), an endangered anadromous fish species. Liming, which has been implemented to restore freshwater ecosystems, has been demonstrated to restore a suitable pH for Atlantic Salmon, however, it remained unknown to what extent liming can restore the underlying behavioural traits and migration behaviour of smolts. We captured 116 smolts from two freshwater river treatments in Nova Scotia, Canada: acidified Little River (n=17) and lime-treated West River (n=99). We performed stream-side behavioural assays and discerned five behavioural metrics: (1) Activity and Food Motivation, (2) Willingness to Explore, (3) Shelter Use, (4) Anti-predator Response, and (5) Foraging Flexibility. Activity and Food Motivation differed significantly between treatments, whereby smolts from West River were less active and food-motivated, which we suggest is an adaptive trait given the risk this behaviour would pose in the daytime. We implanted acoustic transmitters in a subset of smolts (Little River, n=17; West River, n=54), and found that residency times, reversal behaviours, and survival did not differ between treatments. Smolts with high Activity and Food Motivation had marginally extended residency in the inner estuary, potentially increasing conspicuousness and energetic expenditure. Survival declined to 0.79/km within this habitat, underscoring the importance of this habitat for conservation measures. Liming may be a useful conservation method for restoring behavioural types of migrating Atlantic Salmon and should continue to be implemented and its long-term effects investigated further.