Part 1: Three groups of fish were tested: 1) Wild Post-Smolt: Ten post-smolt collected in traps in the River Tay as seaward migrating smolt and held for one year in a 1.5 m diameter circular tank indoors (LT1 Tank 3). Standard Length (SL) 273.5 ± 11.3 mm (mean ± SE) and Fork Length (FL) 288.7 ± 12.1 mm. 2) Captive Post-Smolt: Ten post-smolt reared since hatching in captivity from wild stock eggs and held outdoors in a 1.5 m diameter circular tank outdoors (Outdoor Tank 16). SL 294.0 ± 11.4 mm, FL 312.0 ± 11.4 mm. 3) Captive Adults: Ten adult salmon reared since hatching in captivity from wild stock eggs and held in a 15 m long tank (Dumbell). SL 379.0 ± 15.0 mm, FL 401.5 ± 15.7 mm. To assess hearing sensitivity of fish from the three test cohorts, the Auditory-Evoked Potentials (AEP) technique was used, which is a non-invasive electrophysiological measure of the synchronized brain response to auditory stimuli. Part 2: 1. Behavioural Experiment - 40 adult Atlantic salmon raised in captivity at the Marine Laboratory following collection as smolts in the wild (trials 1–16) or from a farm (Landcatch Ltd., Ormsary, trials 17-20). The salmon were housed in two outdoor tanks (tanks 2 and 20, respectively). Wet weight was 3595.10 g ± 151.67 (mean ± SE); TL (Total length) 677.63 mm ± 9.04. The outdoor tanks received ambient lighting conditions during the experimental period (May – June). 2. Physiological Experiment - 26 marine-phase salmon that had been fertilised and hatched at the Marine Laboratory from eggs and milt from broodstock at Aultbea, were housed in one indoor tank (tank room two) for the duration of the experiment (June-September). The indoor tank was lit by low intensity green lamps. Wet Weight was 1319.48 g ± 62.82; TL 480.17 mm ± 5.37. All holding tanks received running seawater with a constant temperature of 10?C, with aerators maintaining dissolved oxygen levels. Acoustic conditions in the holding tanks were measured using a calibrated omnidirectional HTI-96-MIN hydrophone (frequency response = 2–30,000 Hz, sensitivity = -165 dB re 1V/µPa), positioned 10cm above the bottom of the tank (water depth: 1 m), and a Sony PCM-M10 24-bit recorder (96 kHz sampling rate). Root Mean Square (RMS) noise levels, analysed using SASLab Pro v4.5.2 (Avisoft Bioacoustics, Berlin) were 134.54 (Outdoor tank two) and 130.77 (Indoor tank 2) dB re 1µPa (full spectra, 1 sec averaging, 15 sec recording).