Road culverts are widespread barriers to fish movement, yet the influence of light on fish passage remains poorly understood under natural field conditions. We investigated how changes in entrance lighting affected the movement and behaviour of estuarine fishes through a 50 m multi-cell culvert beneath the M1 Motorway at Saltwater Creek, southeast Queensland. Five lighting treatments were evaluated during the spring migration period: ambient daylight (control), 50% shadecloth, dappled light (simulated using leafy tree branches), complete darkness, and darkness with low-level internal LED illumination. Fish passage was quantified using upstream fyke nets while underwater video documented behavioural responses at culvert entrances. A total of 368 fish representing 12 estuarine species were recorded.
Fish passage differed markedly among lighting treatments. Culverts with low-level internal LED illumination supported the highest passage (154 fish), followed by dappled light (95 fish) and partial shade (77 fish), whereas complete darkness resulted in the lowest passage (19 fish). Responses were strongest among small-bodied and juvenile fishes, particularly estuary glassfish (Ambassis marianus) and gobies (largemouth goby, Redigobius macrostomus and Gobiidae spp.), while larger-bodied species, including yellowfin bream (Acanthopagrus australis) and common toadfish (Tetractenos hamiltoni), exhibited little response to lighting conditions. Video observations revealed hesitation, probe-and-retreat behaviour, and delayed entry at abrupt light transitions, indicating that sharp changes in illumination can act as behavioural barriers, particularly for visually oriented and early life-stage fishes.
These findings provide rare field-based evidence that the light environment within culverts can influence fish movement and contribute to reduced habitat connectivity. Managing light transitions through riparian shading, modified culvert entrances, or low-level internal illumination offers a practical means of improving passage for light-sensitive species. Incorporating behavioural responses to light into culvert assessment and retrofit guidelines could enhance the ecological performance of fish passage infrastructure.