Why Some Islands Have More Sharks Than Others

Reef sharks and rays are in trouble.

Around the world, their numbers continue to drop, with overfishing being the biggest reason for their decline. But in order to bring them back, scientists need to know where these animals naturally belong and what shapes their different communities. A new study led by shark ecologist and University of Washington post-doctoral researcher Dr. Naomi F. Farabaugh is helping do just that by focusing on the coral reefs of French Polynesia, home to some of the most protected waters in the world. The study was published in Marine Ecology Progress Series.

As the world’s largest shark sanctuary, French Polynesia offers scientists a place to study sharks and rays without the intense fishing pressure found in most other parts of the world. By surveying 34 coral reefs across 17 different islands using baited remote underwater video stations (also known as BRUVSs), the research team was able to get a clearer picture of how species vary between islands and what might be influencing those patterns. These underwater cameras record which animals swim by without the noise usually associated with a person at the end of that capture, providing snapshots of reef life that are difficult to capture otherwise. Between 2016 and 2017, the team documented 16 different species (10 sharks and six rays). At least one shark appeared on every reef. Rays, on the other hand, were absent from nearly half of the reefs surveyed, likely due to the specific types of habitat studied.

But even in this protected sanctuary, shark and ray diversity wasn’t spread out evenly. Some island groups had unique combinations of species, and a few species were found only in certain locations. This raised an important question: If not fishing, then what’s causing these differences?

The answer lies in the islands themselves. Physical features like the shape of the island, the kind of reefs present and even latitude all played a role in determining which species were found where. The Marquesas Islands — a remote and rocky group in the northeast of French Polynesia — had the highest average number of species per reef. In contrast, the Austral Islands in the south had the fewest. The team of scientists believe that the Marquesas’ productive waters, high latitude and rocky underwater structure give sharks and rays more shelter, better hunting grounds and a wider range of resources to thrive. Latitude also appeared to matter; the farther south the reef, the fewer species were found, likely because of colder water temperatures or less productive environments. Islands farther north, closer to the equator, had higher diversity, hinting at the influence of environmental conditions like warmth and nutrient availability. Even the geology of the islands seemed to make a difference! High volcanic islands with steep, rocky shorelines tended to support more diverse shark and ray populations. These environments offer crevices to hide in, slopes to hunt along and a range of habitats packed into a small area. Meanwhile, low-lying islands with flat fringing reefs (where coral hugs the shore) hosted fewer species.

Guess the saying, “Location, location, location” also applies to animals in the marine world!

Understanding these patterns is more than just academic, or having a stake in the marine housing market. It has real-world conservation implications. “If we want to rebuild reef shark and ray populations, we need to understand which species naturally live in which places, and why,” said Farabaugh. Similarly, she says the relationships between species also matters. How sharks and rays interact with each other — whether they compete for food, avoid one another or even benefit from sharing space — is still poorly understood. Yet, these interactions may influence where different species can live, and how many of them a reef can support. Florida International University marine ecologist Dr. Michael Heithaus points out that conservation often overlooks these dynamics. “Despite the dramatic global declines in shark and marine predator populations, management rarely considers their potentially important ecological roles,” the dean of the College of Arts, Sciences & Education (CASE) and Professor in the Department of Biological Sciences at FIU said. “That’s partly because we don’t yet know enough about how these animals interact with each other and their environment.”

By factoring in island shape, reef type and location, scientists and policymakers can make smarter, more effective decisions. Farabaugh argues that recovery efforts that don’t consider these environmental details risk setting unrealistic goals or protecting areas that were never meant to support certain species in the first place. In a time when global populations of reef sharks and rays are shrinking fast, having this sort of place-based knowledge could mean the difference between failure and a real path to recovery. She just hopes those in charge listen before it’s too late.