Shark populations have recently fallen by up to 90 percent in some areas. Specifically, in areas near human populations. The loss has mostly been as a result of harvesting for the fins, as a by-catch of other types of fishing, and from sport fishing.
In a new study by The University of Miami’s School of Marine and Atmospheric Science, researchers have used underwater surveys conducted over the past decades to compare shark numbers at heavily human-impacted reefs to numbers near more isolated ones.
“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, lead author of the study and a scientist at the Joint Institute for Marine and Atmospheric Research (JIMAR) located at the University of Hawaii, as well as a PhD candidate with Dr. Jerry Ault at the University of Miami’s Rosenstiel School of Marine & Atmospheric Science. “In short, people and sharks don’t mix.”
The research was done by using a survey method called ‘tower diver surveys’. These surveys were designed for the counting of highly mobile reef animals, like sharks. They are performed by towing paired scuba divers with a boat while they record everything.
“Towed-diver surveys are key to our effort to quantify reef shark abundance,” said Ivor Williams, head of the team responsible for these surveys. “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.”
The survey numbers were combined with information on human population, habitat complexity, reef area, and satellite data on ocean temperature and oceanographic productivity.
The models show that humans have a very damaging effect on reef sharks.
“Around each of the heavily populated areas we surveyed — in the main Hawaiian Islands, the Mariana Archipelago, and American Samoa — reef shark numbers were greatly depressed compared to reefs in the same regions that were simply further away from humans,” Nadon said. “We estimate that less than 10% of the baseline numbers remain in these areas.”
“The pattern — of very low reef shark numbers near inhabited islands — was remarkably consistent, irrespective of ocean conditions or region,” added Williams.
“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, which conducted the surveys.