Reef shark numbers in the Pacific Ocean have declined by more than 90 per cent. Researchers from the University of Miami's Rosenstiel School of Marine & Atmospheric Science have found that reef shark populations have plummeted in the past three decades. They found that more than 90 per cent of reef sharks in the Pacific Ocean were killed.
Researchers believe that the main reason behind the decline in the reef shark population is excessive harvesting - for their fins, as an incidental catch of fisheries targeting other species, and in recreational fisheries. Until now, they did not know how many reef sharks have been killed because of lack of data.
The researchers used a new method called towed-diver surveys to find out about the shark population. Towed-diver surveys were designed specifically for the census of large, highly mobile reef fishes like sharks. The surveys involve paired scuba divers recording shark sightings while towed behind a small boat.
"Towed-diver surveys are key to our effort to quantify reef shark abundance," said Ivor Williams, researcher at the University of Miami's Rosenstiel School of Marine & Atmospheric Science, in a statement. "Unlike other underwater census methods, which are typically at an insufficient spatial scale to properly count large, mobile species, these surveys allowed our scientists to quickly record shark numbers over large areas of reef."
Researchers had also conducted underwater surveys over the past decade across 46 US Pacific islands and atolls as part of the NOAA's (National Oceanic and Atmospheric Administration) extensive Pacific Reef Assessment and Monitoring Programme. The team compared reef shark numbers at reefs spanning from heavily impacted ones to those among the most untouched reefs.
The towed-diver surveys data revealed that reef shark numbers drastically dropped in populated islands by more than 90 per cent compared to at those reef sharks at the most untouched reefs.
"We estimate that reef shark numbers have dropped substantially around populated islands, generally by more than 90 percent compared to those at the most untouched reefs," said Marc Nadon, scientist at the Joint Institute for Marine and Atmospheric Research (JIMAR) located at the University of Hawaii, in a statement.
"Our findings underscore the importance of long-term monitoring across gradients of human impacts, biogeographic, and oceanic conditions, for understanding how humans are altering our oceans," concluded Rusty Brainard, head of the Coral Reef Ecosystem Division at NOAA's Pacific Islands Fisheries Science Center.