Shipwrecks off the coast of North Carolina have long drawn divers and even treasure hunters. Now, species of tropical and subtropical fish are showing up, driven there by the impacts of climate change.
And that’s also bringing scientists to these sites, hoping to learn more about what fish are appearing, and how many.
On a recent day, Captain David Day used a GPS unit and sonar to steer the 48-foot dive boat “Midnight Express” to a precise spot in the Atlantic Ocean off Morehead City.
A diver leaped in to secure the boat to the bottom, more than 100 feet below, and reported back to Day, listening through headphones: “We’ve got 60 feet of visibility and plenty of sharks.”
That was all good news — even the part about sharks — for scientists in the back of the boat, who were hooking a winch to an ice-chest-sized underwater drone called a Remotely Operated Vehicle, or ROV.
Inside, some of the scientists monitored a video feed from the drone as it dived to the sea floor. Soon, it approached a pile of rusting steel that had been an oil tanker before it was torpedoed by a prowling German submarine in 1942.
Today, this site is home to a thriving community of fish. It is part of an increasing collection of “marine urbanization” sites that include man-made structures, like dock pilings, artificial reefs and the submerged parts of oil drilling platforms and offshore wind turbines.
At those sites, scientists have been finding fish typically found on the coral reefs of Southern Florida and the Caribbean. The fish seem to prefer shipwrecks to nearby rocky reefs that might, at least superficially, seem more like coral reefs.
“We still don't understand fully why they may be enjoying these artificial structures,” said Avery Paxton, a marine ecologist and conservation biologist with the National Oceanic and Atmospheric Administration.
Paxton and another NOAA scientist aboard the boat, Chris Taylor, have been studying the life on North Carolina’s famously numerous shipwrecks and the rocky reefs for years. But numerous unanswered questions remain about how life on artificial structures works.
One thing they haven’t tried to figure out yet, for example, is whether the populations of warmer-water fish are actually shifting north.
Even if they are trying, North Carolina’s waters might be a kind of barrier. This is a kind of offshore crossroads: Warm water moving north Gulf Stream collides with a cold current flowing south.
That means North Carolina has traditionally been the southern end of the range of some species, and the northern end for others.
“So we don't know the answer to whether the tropical fish are moving north or have come up against a hard stop,” Paxton said. “What we do know is that in this area, which is oftentimes at the northern or forward edge of their range, they are hanging out on these artificial structures in higher numbers.”
Paxton said that one of the thoughts moving forward is to use artificial structures to facilitate corridors that the species could use as they move poleward, or toward the poles.
There have been well-documented habitat shifts by some commercially-fished species already. Black sea bass and summer flounder, for example, used to be much more common off North Carolina’s coast, but have recently moved north.
This has caused problems for fishermen, and fisheries regulators, all along the Atlantic Coast.
Paxton and the other scientists on the “Midnight Express” are studying the shipwreck to learn more about how all kinds of species interact, not just those with commercial value.
They use the underwater drone to methodically move up and down wrecks, like the oil tanker. High-definition cameras bolted to the drone record video they will use later to count fish and identify species.
On this excursion, they saw numerous sand tiger sharks. The researchers use their presence as an important indicator.
“We're finding that certain types of fish really like to hang out with the sand tiger sharks, whereas others, you aren't usually going to see when the sand tiger sharks are present,” said Paxton. “So we're trying to put two and two together and figure out why.”
After the survey of the oil-tanker wreck was done, a diver went back in, ignored the sharks, and unhooked the anchor line. Then, Captain Day steered the “Midnight Express” a few miles northeast, for another survey, this one of the wreck of a German submarine, the U-352.
The water was still unusually clear, and more clouds of fish swirled in front of the drone’s cameras, and the distinctive, fin-like conning tower of the U-Boat was obvious.
Predators were again thick. There were so many, they kept bumping into the drone. At one point, Paxton laughed as the drone startled a barracuda and it bolted into a shark.
“Did anybody just see that?” she said.
Then another species grabbed their attention.
“That looks like another Spanish hogfish,” said Paxton, who was piloting the drone.
Spanish hogfish are one of the tropical species. Often during such surveys, researchers don’t see the small tropical and subtropical species until they download the high-definition recordings from the drone.
Later, on shore, they’ll start the painstaking scrutiny of all that video, to identify and count the fish.
What they’re looking for is a bigger and more nuanced picture of how life works on artificial reefs, and to understand things like how differences in the wide array of different man-made structures affect these communities.
The information could help guide decisions about what kinds of structures should go where to create artificial reefs.
“What we've learned in the study is that the type of structure that you put out there is likely to attract a different community of species,” Paxton said.
Eventually, researchers hope to determine if it’s possible to place the right kinds of structures in key places to help different species - whether it’s those that fishermen want to target or tropical fish that need stepping stones as they head north over otherwise inhospitable underwater terrain.