Reef Sharks Vanishing Near Populated Islands
The research is the first to provide a large-scale estimate of reef sharks in the Pacific, a group of species that includes the gray reef shark, the whitetip reef shark and the tawny nurse shark.
"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 study leader Marc Nadon, a doctoral candidate at the University of Miami's Rosenstiel School of Marine and Atmospheric Science. "In short, people and sharks don't mix."
Nadon and his colleagues pulled shark-sighting data from more than 1,607 dives at 46 reefs in the central-western Pacific, which included reefs near the Hawaiian islands and American Samoa as well as extremely isolated reefs nearly devoid of human influence. Though eight species of shark were seen on the dives, the researchers excluded sharks, such as hammerheads, that aren't dependent on reefs. That left them with five shark species to tally: gray reef sharks, blacktip reef sharks, whitetip reef sharks, Galapagos sharks and tawny nurse sharks. [ On the Brink: A Gallery of Wild Sharks ]Combining that data with information on human population, habitat complexity, availability of food and sea-surface temperatures, the researchers created models comparing the numbers of sharks at pristine versus human-impacted reefs.
"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 (farther) away from humans." Nadon said in a statement. "We estimate that less than 10 percent of the baseline numbers remain in these areas."
The devastation of sharks in areas near human civilization could be the result of illegal fishing, incidental killing or fishing for sport, the researchers report Friday in the journal Conservation Biology. Human impact on the reef fish that sharks call dinner could also play a role. Human influences were shown to outweigh natural influences, such as warmer water temperatures, the researchers found.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," said Rusty Brainard, head of the coral reef ecosystem division at NOAA's Pacific Islands Fisheries Science Center, which conducted the shark surveys.
These methods allowed them to fill in the blanks on the numbers of missing sharks. Their models showed that:
Densities of gray reef sharks , whitetip reef sharks (Triaenodon obesus), and other reef sharks increased substantially as human population decreased.
Densities of reef sharks increased substantially as primary productivity and minimum sea surface temperature increased.
The newly named species, Bythaelurus giddingsi, is a kind of catshark. Such animals had never been seen near the famed Eastern Pacific archipelago until researchers descended some 1,600 feet (500 meters) to the ocean floor.
"We looked out the window and saw this spotted catshark and said, 'What the heck is that?'" recalled John McCosker, chairman of aquatic biology at the California Academy of Sciences, and lead author on a paper describing the shark. "It was very exciting because we didn't expect that that genus had a species of shark living in the Galapagos."The veteran ichthyologist and shark researcher said he turned to the sub's pilot and said, "We gotta get that shark."
A farcical chase ensued, McCosker said, with the shark scurrying out of reach every time the sub's vacuum-like collecting tool got within reach.
The sub finally sucked in the evasive shark, and researchers eventually captured six more specimens: dainty, chocolate-brown sharks, speckled with pale spots about the size of their large eyes.
Unlike in many shark species, the spots appear to be distributed at random, with patterns unique to each animal, "which is quite remarkable," McCosker said. The species is found only near the Galapagos Islands, famed for their unique species both on land and in the sea, attributed to their extreme geographical isolation.McCosker estimated that the largest catsharks he observed were about 2 feet long, about average for catsharks. Limited by the submersible's collecting instrument, the largest shark the team retrieved was only about 17 inches lthough researchers are only now publishing the new species description, the newly described specimens were collected on diving expeditions in 1995 and 1998.They were stored in formaldehyde, which breaks down proteins and makes DNA sampling impossible; McCosker said that, with advances in DNA analysis, the team was hoping to get further shark specimens for genetic sampling before proceeding.
Unfortunately, he said, they weren't able to return to the region, and decided to publis
It was "almost like a sight from another planet," said expedition leader Alex Rogers, a professor of zoology at Oxford University. Even in the eye-popping world of deep-sea vents, the Antarctic discoveries stand out, with the unfamiliar species of crabs found crowded in piles around the warm waters emanating from the seafloor. Many of the animals found at the vents have never been found at hydrothermal vents in other oceans, Rogers said. "To see these animals in such huge densities was just amazing," Rogers told LiveScience.
In the dayless world of deep-sea vents, energy comes not from the sun but from the hydrothermal energy generated in the oceanic crust.
The yeti crabs seem to cultivate "gardens" of bacteria on their chests, which are covered with hairy tendrils. These bacterial mats almost certainly provides the crabs with sustenance, Rogers said. In turn, predatory seven-armed sea stars stalk the periphery of the vents, snacking on unfortunate crabs. [See video and photos from the vents]
"We were absolutely stunned to see the animal communities, because they were so different from the hydrothermal vents seen elsewhere," Rogers told LiveScience. He and his colleagues reported their results Jan. 3 in the journal PLoS Biology.Among the new species were the yeti crabs, crowded around the vents up to 600 per square meter.
"They're literally, in places, heaped up upon each other," Rogers said. Crabs normally don't tolerate cold temperatures well, so the vents may be a warm haven for these crabs, Copley said.Unlike vents in other oceans, the Antarctic vents lack tube worms, mussels and shrimp. Instead they harbor new species of barnacles and anemones, as well as a large brown spiral-shelled snail. The researchers even saw ghost-pale octopuses, which seemed drawn to the lights of the ROV.
"We were completely blown away by what we found," Copley said. "I've worked at vents in the Atlantic, the Pacific and the Indian Ocean, but these are the lushest, richest vents, in terms of life, that I've come across."
The discovery helps fill a gap in researchers' understanding of how deep-sea life disperses around the oceans, Rogers said. They had expected that the Southern Ocean would be a historical gateway for vent species to travel between the Atlantic, Pacific and Indian oceans, and there do seem to be some species, such as the yeti crab, that are related to species found at other vents. Those relationships seem to reach back into geological history, Copley said, when there was a connection between the Antarctic and the eastern Pacific.
But the vast differences between Antarctic vents and vents found elsewhere suggest that the area is not a gateway but a biological region in its own right, Rogers said. The cold Antarctic waters may act as a barrier to species that start their lives as swimming, feeding larvae, he said. On the other hand, larvae that carry their own food supply with them in eggs — known as lecithotrophic larvae — may be able to survive and disperse in the chilly Southern Ocean.
As humans increasingly exploit the deep seas for fish, oil and mining, understanding how species are dispersed is crucial, Copley said.
"Until we understand what governs the patterns of life at deep-sea vents, how interconnected their populations, how well life disperses from vent to vent, we can't make responsible decisions about how to manage these deep-ocean resources."