Strange Attraction: Underwater Lights and the Environment

23 February 2009 By Kate Hubert

Ports and anchorages around the world are aglow with a green tint; superyachts seem to float on liquid light, as the sea is bathed with other-worldly illumination.

Generators purr to feed these lights, releasing a steady stream of carbon dioxide. To be fair, advances in technology, including the use of low-energy LED lights, are helping to reduce this carbon footprint – but that’s not the only environmental impact made by your yacht’s underwater lights.

Fishermen have known for years that lights in the water can attract certain fish, squid and shrimps. (While yacht crews typically use this power of attraction to entertain the guests rather than for angling, in Thailand there was at least one crew who were horrified to find the previously pristine white stern of their yacht splattered with black ink after their Thai workers had made the most of the lights for an all-night squid-jigging session.)

Blue or green light tends to be the most attractive – to marine life as well as to us. Red lights would probably be ignored by sea life and yellow aren’t so alluring, but then these colors aren’t popular with many owners, either!

So why are creatures like squid attracted to underwater lights like, well, moths to a flame? Probably because squid gather to breed near the surface around the time of the full moon; it’s their way of finding each other in the vastness of the ocean. So the bright lights glowing from yachts’ hulls must look like an invitation to a tentacular love-in!

But it’s not just fishermen that squid need to worry about. Bright lights also attract predators. The weird orange eye-shine of barracudas is often seen at night under these conditions. But just what makes these predators hone in on bright lights?

It all has to do with the greatest wildlife migration on earth. Wildebeest galloping across the Serengeti? Nope, sorry. In terms of biomass – literally the combined mass of the creatures involved – by far the greatest migration happens in the sea every day as plankton rises up to the surface in the daytime, then sinks back down at night. These tiny organisms, which get their food from photosynthesis, need the sunshine during the day.

Underwater lights attract plankton, which are followed by hungry zooplankton (animal plankton), which in turn are followed by tiny shrimps and fish, then bait fish, then larger predators. So far, this doesn’t sound like too much of a problem, but artificial light can have an effect on all this wildlife.

All sea creatures evolved in a world with a regular rhythm of light and dark. For many, the length of the day is the most reliable way of telling the seasons – and so a lot of breeding, migratory and other vital behaviors are triggered by changes in day length. Similarly, the moon creates behavioral signals at night, and many creatures like squid rely on those cues to get together....

Underwater lights can upset these natural rhythms and some creatures may get so confused they don’t feed or they even exhaust themselves.

Of course, a few superyachts shining their underwater lights in a harbor are not going to have a massive long-term influence – unless they are there all the time, with lights blazing all night long. Offshore oil rigs are a much bigger problem, as are squid-fishing boats – they are not just the bane of officers on watch trying to preserve their night vision, but also of sea life and migrating birds as well.

So you shouldn’t feel too guilty for making the boat look all pretty at night...but next time you flick the switch, just think – is it really necessary? Not only will you save on fuel costs and lower your carbon footprint – you’ll also leave the squid to make love by the light of the moon, and stop some dazzled little fish becoming an easy meal for a barracuda.