By Tim Radford
Climate News Network
Global warming could be bad for sharks, too. These ocean-going creatures that have survived 420 million years of natural climate change could be at risk from increasingly acidic seas, according to two entirely different scientific studies.
The sharks are already in trouble everywhere. They are pursued as food or feared as a threat, and the habitat they favor is gradually being degraded or destroyed.
But Danielle Dixson, a marine conservation biologist at the Georgia Institute of Technology in Atlanta, and colleagues report in the journal Global Change Biology that changes in the pH value of water – in other words, as the seas became more acidic – have interfered with a shark’s ability to smell food.
Behavior change
Dixson has already shown that increasing acidification, due to greater levels of carbon dioxide in the atmosphere, could change the behavior of reef fish, seemingly making them less afraid of predators because the acidic waters disrupt a specific receptor in the fish’s nervous system.
This time, she experimented with a shark known as the smooth dogfish (Mustelus canis), which is found in the Atlantic waters off the U.S. coast. She tested 24 sharks in a 10-meter tank with two currents, or plumes, of water. One was normal sea water, and the other was rich in the odor of squid. As expected, the sharks showed a distinct preference for the smell of food.
Then she and her colleagues enriched the water with carbon dioxide to levels predicted for mid-century as greenhouse emissions continue to rise and the seas become more rich in carbonic acid.
When released into the most acidic water, the sharks actually avoided the plume of squid odor. Once again, the change in the water’s pH seemed to have disrupted an all-important receptor, and thus the sharks’ interest in hunting.
“Sharks are like swimming noses, so chemical cues are really important for them in finding food,” Dixson said.
Less aggressive
Overall activity did not change significantly, but shark attack behavior did. The squid odor was pumped through bricks to give the sharks something to push against, but the sharks in the most acidic waters responded less aggressively.
“They significantly reduced their bumps and bites on the bricks, compared to the control group,” Dixson said. “It’s like they’re uninterested in their food.”
There is always the chance that, as acidity levels slowly rise, sharks will adjust or adapt. But increasing acidification may not even give them the chance to adapt.
In a second paper, this time in the Proceedings of the Royal Society, Rui Rosa, senior researcher at the Center for Oceanography in Cascais, Portugal, and colleagues considered the impact of warmer and more acidic seas on the survival of the newly-hatched tropical bamboo shark (Chiloscyllium punctatum), normally found in the intertidal zones of the western Pacific.
The researchers tested hatchlings in tanks at temperatures and pH values predicted for 2100, and found “significant impairment” in survival rates.
In their experiments at normal temperature conditions, mortality among the hatchlings was zero. In experimental conditions, behavior changes were apparent from the outset and, within 30 days, more than 40 percent had died.
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