Nature's blueprint defines species makeup for sharks, rays

Populations of reef sharks and rays continue to dramatically decline all across the world, largely because of overfishing. To help bring them back, scientists need to understand which sharks and rays belong where.

The coral reefs of French Polynesia are home to an abundance of sharks. A new study led by shark ecologist and FIU alumna Naomi F. Farabaugh found the type of species and number of sharks and rays differed between islands. The reason? Natural factors. The physical shape of each island, latitude and the type of islands and reefs together play key roles influencing these communities.

“As the world’s largest shark sanctuary, French Polynesia offers a unique chance to study sharks and rays, outside of the overwhelming human pressure present in much of the rest of the world,” Farabaugh said.

The study was published in Marine Ecology Progress Series.  

Farabaugh and FIU Institute of Environment researchers including Mark Bond, Simon Dedman, Alastair Harborne, Jeremy Kiszka and Mike Heithaus, surveyed 34 reefs across 17 islands using baited remote underwater video stations (BRUVSs). These underwater cameras gave scientists a clear view of how species vary depending on the island and environment.

Between 2016 and 2017, the team recorded a total of 16 species – 10 sharks and six rays. At least one shark was spotted on every reef. Rays were missing from 16 of the 35 – likely due to the specific habitat studied.

The Marquesas Islands – one of the most isolated island groups in the world – had the highest average number of species per reef, while Austral Islands had the fewest.  Scientists say the Marquesas’ rocky reefs, productive waters and high latitude may account for the increased species richness. Some island groups had distinct species groupings, while some species were found unique to specific island groups.

Latitude also had a hand in species richness. The southern islands had the lowest diversity, while the number increased toward the northern islands, likely due to environmental factors such as temperature.

Tall volcanic islands with rocky reefs supported the most diverse species as it gave sharks and rays more places to hide, hunt and thrive. The coral reef growing directly along the shoreline of the high islands supported fewer species.

The findings highlight the importance of incorporating habitat features when creating conservation baselines and management targets, said Farabaugh, who currently is a researcher at the University of Washington.

The relationship between different species – whether they work together or compete for resources – could influence where they live and needs further investigation.

“Despite the dramatic global declines in shark and marine predator populations, management rarely considers their potentially important ecological roles and the possibility that their populations influence one another,” Heithaus said. “One reason for this gap is how little we know about interactions among sharks, rays and other large marine predators. Understanding these dynamics will improve our successes in restoring healthy marine ecosystems.”