Question: Do sharks behave differently towards divers at night? - Candice L, Scuba Diving Magazine

Answer: It’s “common knowledge” that sharks are more active at night than during the day…but there isn’t actually much scientific evidence demonstrating this. That said, I’ve seen sharks do things on night dives that I’ve never seen them do during the day, and so have many other divers.

Many animals are more active at night than during the day, and as everyone who was a nature nerd in elementary school knows, these animals are called “nocturnal.” Being nocturnal has many advantages, especially if you’re a predator. For example, prey animals may be sleeping, or they might otherwise not be able to defend themselves for many reasons, including not being able to see very well. A predator with great night vision can eat very well.

Related Reading: Go Big or Go Home! Best Dive Spots for Big Animals

It is “common knowledge” that many sharks are nocturnal, but are they really?

During graduate school, my lab hosted a symposium called “Fish at Night.” As part of this, our team reviewed 166 peer-reviewed scientific journal articles about the behavior of sharks and rays at dawn, dusk and nighttime. We found almost no evidence that sharks and rays are more active at night in terms of hunting, reproduction or general moving around their habitat.

However, we did find lots of evidence that many species of sharks and rays are “crepuscular,” which refers to animals that are more active at dawn and dusk than during the day or the middle of the night. The advantages of this are also clear—it doubles your available prey to include the animals that are out mostly at night and those that are out mostly during the day.

However, an important scientific truism is that “absence of evidence isn’t evidence of absence,” which means that just because something hasn’t been scientifically proven doesn’t mean that it’s impossible that it can happen. After all, scientific studies of shark behavior are performed by humans, who sleep at night, and therefore, depending on the tools they use, potentially aren’t documenting things that happen when we’re not looking.

Related Reading: Fiji After Dark: One Diver’s Journey Into Night Diving

The scariest encounter I’ve ever had with a shark underwater was on a night dive. A bull shark was apparently following right behind and above me, and as soon as my dive light illuminated a grouper, it rushed by me so close that my mask came off. I’ve never seen sharks do that during the day, and I know many divers who have similar stories of behavior they’ve only ever seen at night.

So, do sharks behave differently towards divers at night? Anecdotally it sure seems like it, but this is a case where divers’ experiences appear to be ahead of the state of scientific study.

Ask a Marine Biologist is a monthly column where Dr. David Shiffman answers your questions about the underwater world. Topics are chosen from reader-submitted queries as well as data from common internet searches. If you have a question you’d like answered in a future Ask a Marine Biologist column, or if you have a question about the answer given in this column, email Shiffman at [email protected] with subject line “Ask a marine biologist.”

Dr. David Shiffman is a marine conservation biologist specializing in the ecology and conservation of sharks. An award-winning public science educator, David has spoken to thousands of people around the world about marine biology and conservation and has bylines with the Washington Post, Scientific American, New Scientist, Gizmodo and more. Follow him on @WhySharksMatter on Twitter, Facebook and Instagram, where he’s always happy to answer any questions about sharks.

The views expressed in this article are those of David Shiffman, and not necessarily the views Scuba Diving magazine.