Picture a poorly designed city with enormous city blocks and few roads. It would probably be difficult to get around, so you might try to avoid going across town if you could. Now, if alleyways were created throughout this city as shortcuts, you might be more likely to walk across town. But what if those alleys were dark, sketchy and had a guy in a trench coat sharpening a knife? Even though you technically can use these alleys as shortcuts, you would almost certainly be too afraid to walk through them. Craig Franklin and colleagues from the University of Queensland in Brisbane, Australia, wondered whether riverine fish are also afraid of using dark alleys. Except the alleyways used by fish are called culverts and they allow rivers and streams to pass underground beneath streets. Culverts can vary in structure, from corrugated plastic tubes to rectangular channels, and function to redirect waterways, reduce erosion near infrastructure and physically connect upstream and downstream environments, but do they also alter the way fish move up and down rivers?
The team wanted to know if the abrupt transition between a sunny stream and a dim culvert would act as a barrier to fish movement, so they made an experimental culvert: a 12 m long fish tank with water flowing through it and a light over one side while the other half was blacked out to simulate a natural culvert. In addition, the team included a viewing window in the middle to watch fish travel between the two sides. They then placed several native Australian fish species – one species at a time – in the experimental culvert at different locations to see if the light levels affected their movements.
Overall, the team found that the experimental culvert affected fish behaviour. Australian smelt and fly-specked hardyhead strongly preferred the light half, rarely venturing to the dark half. These small species are most active during the day and rely heavily on sunlight to navigate their environment. Meanwhile, the Australian bass joined the dark side of the culvert, rarely visiting the light side. These fish are crepuscular, which means that they are mainly active at dawn and dusk, using their large eyes to see in low light conditions when ambushing unsuspecting prey.
While there probably are no cane toads in trench coats sharpening knives in Australian culverts, fish that are active during the day might be afraid of predators – such as Australian bass – lurking in the shadows, reducing their ability to migrate through streams. This means that while culverts may be created with the best intention of connecting fish habitats, they may effectively perform as barriers if they are too dark. So, how much light do day-active fishes need to encourage them to pass through culverts?
After testing various light levels, the team found that the light threshold for the daylight-loving species was very low. They do not need much light to find their way around in culverts. However, the researchers measured the light levels in various Australian culverts and found them to be dimmer than the minimum that the fish require, suggesting they may be hindering fish migrations. Fortunately, there is a simple solution to the problem: lights. The scientists suggest that dim lighting could be installed in culverts to help shy fish complete their journeys. Just like everyone in the world right now, daytime fishes also want to see the light at the end of the tunnel.