SUMMARY1. The effect of a range of ecologically relevant broader spectrum street light intensities on the dispersal timing of Atlantic salmon (Salmo salar) fry was investigated to assess the efficacy of a proposed management tool, the dimming of lamp brightness, for reducing the ecological consequences of artificial night light on aquatic ecosystems. Dispersal timing under treatment street light intensities of 8, 4, 2 and 1 lux was compared to that under a control night light intensity of 0.1 lux, representative of approximately half that experienced from a full moon. 2. Dispersal timing was significantly delayed (by 1.4 to 2.2 days), and its diel pattern significantly disrupted under the treatment street light intensities. However, the dose-response for both delay and disruption effects was not linear, with a strong effect apparent at 1 lux, and little or no additional impact seen when the light intensity was increased further. Under control conditions, the mean time of dispersal was 3:58 h after dusk, with very few fry (<4%) dispersing during daylight hours. For the treatment street light intensities, the mean time of dispersal of fry was significantly later (5:31 h after dusk at 1 lux) at night, and a much wider distribution of fry dispersal times was apparent with many more fry (19% at 1 lux) dispersing during daylight hours. 3. Survival to dispersal in aquarium conditions was high (≥97.8%) and comparable in the control and treatment street light intensities. However, in the wild, the period between fry dispersal and the establishment of feeding territories is considered to be of critical importance in the dynamics of salmonid populations and any disruption may significantly increase predation and reduce fitness. 4. The findings of this aquarium-based investigation suggest that the dimming of lamp brightness has little potential as a successful management strategy to reduce the disruptive impact of street lighting surrounding freshwater ecosystems. We therefore recommend that the best course of action is to maintain and increase natural unlit areas.

Artificial light at night (ALAN) is known to disrupt the nocturnal dispersal behaviour of Atlantic salmon fry, yet the underlying physiological mechanism is unknown. Here novel methodologies are utilised to examine whether behavioural disruption seen in dispersing fry is mediated via a cortisol stress response. Fry showed the capacity to mount a stress response, however, the two methods for sampling cortisol provide conflicting results as to whether ALAN acts as a stressor to dispersing fry.

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