Street lighting delays and disrupts the dispersal of Atlantic salmon (Salmo salar) fry

There has been a decline in the abundance of Atlantic salmon (Salmo salar) despite significant conservation measures designed to reduce fishing mortality. Populations at the southern edge of their historical distribution, where anthropogenic impacts on the freshwater environment may be greater, have suffered the largest decline. In this investigation, we compared the timing of Atlantic salmon fry dispersal from incubators in an aquarium under control and ecologically relevant broad spectrum street-lit conditions (median night light intensity = 12 lx). Fry dispersal occurred 2.8 days later (F = 82.9, df = 1,8, p < 0.001), and on average the fry were smaller at dispersal (0.017 g, se = 0.0012, p < 0.001, n = 730), in the incubators exposed to street lighting. Significant disruption to the diel pattern of fry dispersal was also observed. Dispersal under control conditions was significantly directed around a mean time of 4:17 h after dusk (p < 0.001, r = 0.76, n = 1990) with very few fry (<2%) dispersing during daylight hours. Under street lighting, the dispersal of fry was significantly delayed (mean time 6:38 h after dusk; p < 0.001, r = 0.39, n = 2413) with a significant proportion (32%) dispersing during daylight hours. Survival to dispersal in the controlled aquarium conditions was not lower under street-lit conditions (p = 0.21, n = 5000 eggs across 10 incubators). However, in the wild, the period between fry emergence and the establishment of feeding territories is considered to be of critical importance in the dynamics of salmonid populations and any disruption may reduce fitness.