Fish the lights at night

Light pollution is changing the day-night cycle of some fish, dramatically affecting their feeding behaviour, marine scientists from the University of New South Wales (UNSW) have discovered.

Not that this will come as news to everyone, mind you. Astute anglers have long regarded the lights as the best place to fish at night.

Lights around bridge pylons, wharves, underwater lights on boats and spotlights over your cockpit are all known to attract baitfish, prawns, squid, worms and other small marine critters that in turn attract big predators. Fish the lights!

But in one of the first studies of its kind, UNSW scientists recently found that increased light levels in marine habitats, associated with large coastal cities, can significantly change predator-prey dynamics on a much larger scale.

They used a combination of underwater video and sonar to spy on these communities and record how their behaviour changed. Like us, the animals in our study slowed down at night. Predatory fish became sluggish and had little appetite.

But when the lights went on some of these same predators disappeared, while others feasted on the well-lit underwater buffet. Overall, there was much greater predation on seafloor-dwelling communities when the night waters were lit.

Using LED spotlights, the scientists manipulated the light patterns underneath a wharf in Sydney Harbour, illuminating sessile (attached to the seafloor and wharf) invertebrate prey communities to fish predators. They recorded fish numbers and behaviour under different lighting scenarios (day, night and artificially lit night), and the prey communities were either protected or exposed to predators.

Despite different changes in different species, overall we found that more animals were getting eaten. The main predators were yellowfin bream (Acanthopagrus australis) and leatherjackets (Monocanthidae). The prey being consumed included barnacles, bryozoans (encrusting and arborescent), ascidians (solitary and colonial), sponges and bivalves.

When they turned on the lights, the scientists found prey communities changed to more closely resemble communities exposed to predation during the day. This increase in predation pressure highlights the effect prey communities face under a brightening future, possibly leading to shifts in prey structure with flow-on effects to ecosystem functioning.

Scientists are only now beginning to understand the effects of artificial light on the natural world around us, but there is still a long way to go — especially in the underwater realm. World populations continue to grow and increasing pressure is placed on our coastal fringes to support this growth, so we need to find solutions to reduce our impact wherever we can.

LEDs are increasingly being used because they are effective and cheap to run, but they emit a broad spectrum with peaks in blue and green wavelengths, which penetrate to great depths underwater. Moving towards other spectra, such as red which doesn’t penetrate as far, could reduce the problem, scientists say.

This study was published by Damon Bolton is an Associate Lecturer in coastal resource management and environmental impact, UNSW.

Meantime, for anglers who like to fish at night, the obvious message is to work the lights! Here are some good tips on safe night boating and navigation.