Smoltification and extensive migration characterise the anadromous Atlantic salmon (Salmo salar) and sea trout (Salmon trutta). Migration from a freshwater to saline environment is essential for individuals to rapidly grow in the richer feeding grounds offshore, optimising their growth rate and future reproductive output. When salmon parr reach a threshold size (100-200 mm), they undergo physiological pre-adaptations to life in a saline environment through a process called smoltification.
The smolt run, occurring over two-three months in spring, marks the mass migration of smolts to the sea. Smoltifcation and migration present numerous risks, which include an increased risk of predation, increased nutrient competition amongst smolts and osmotic pressures once they reach their new environment. Numerous studies have reported these risks as having resulted in high mortality rates during the run. A review by Thorstad et al. (2012) reported the mortality rates started at 0.3%, and rose as high as 7.0%/km (median = 2.3%/km). High variation in mortality rates occurs due to the differentiating river conditions and predatory hotspots. |