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March 16, 2016 | By:  Jessica Carilli
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Why lights attract ocean life at night

My son is 4 years old now, and one of his favorite activities is listening to true stories about things that have happened to people he knows. "Tell me a story - a real story," he says as I settle him into the carseat for our commute to preschool and work. Sometimes it's hard to come up with stories that are both interesting and age-appropriate on the spot - so I often tell him about misadventures or natural things I observed during fieldwork.

This morning, he also requested a long long long real story, so I started in on describing the time I was lucky enough to join an expedition through the northern Line Islands in the central Pacific. We were based on a beautiful ice-breaker expedition yacht, the Hanse Explorer. At night, lights that shone off the ship into the water below incited a mêlée of activity, with large predators including sharks, tuna and dolphins feeding on prey with gusto.

Dolphins and sharks have extra senses - dolphins can echolocate and sharks can sense electrical cues - which allow them to hunt for prey in the dark. However, these predators use vision to hunt as well. It may be energetically easier to hunt visually (or possibly allow higher precision in prey capture), so when artificial lights provide extra illumination at night, these predators can take advantage. In addition, predators that only hunt visually like tuna can also feed in these illuminated regions.

But why didn't we see these feeding bonanzas during the day - when there is much more light from the sun?

Well, I'm not totally sure, to be honest. But here is my informed guess: the diel vertical migration. At night, zooplankton like copepods rise vertically through the water column to feed on phytoplankton near the ocean's surface (since phytoplankton need sunlight to grow, they are concentrated here). They feed under the cover of darkness, thinking that they may not be eaten because their predators - small fish, for instance - can't see them. However, once a human introduces an artificial light - Bam! Little fish can see the copepods, and will start eating them. Larger fish can also see these smaller fish, and eat them - and ultimately the top predators can come in and have a nice feast.

People who fish have learned to take advantage of this phenomenon, and there are many products designed to be used for night fishing by anglers. The squid fisheries also take full advantage of this and rig their boats with strong lights to attract their target organisms during night fishing. Groups of squid boats can even be seen from space at night!

Another suggestion to explain why fish are attracted to lights at night could be that they have evolved to prey on naturally bioluminescent organisms, which give off light at night as well. Can you think of other reasons fish and other marine life might be attracted to lights at night?

Gannon, D. P., Barros, N. B., Nowacek, D. P., Read, A. J., Waples, D. M., & Wells, R. S. (2005). Prey detection by bottlenose dolphins, Tursiops truncatus: an experimental test of the passive listening hypothesis. Animal Behaviour, 69(3), 709-720.

Kalmijn, A. J. (1982). Electric and magnetic field detection in elasmobranch fishes. Science, 218(4575), 916-918.

Norris, K. S., Prescott, J. H., Asa-Dorian, P. V., & Perkins, P. (1961). An experimental demonstration of echo-location behavior in the porpoise, Tursiops truncatus (Montagu). Biological Bulletin, 120(2), 163-176.

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