The science of North Carolina's artificial reefs

Sep 26, 2016

Diver records photographs of the benthic invertebrates and macroalgae growing on a flat rocky reef.
Credit J. McCord / UNC-CSI

Fishermen and divers can access a new, online interactive guide to learn more about the 64 artificial reefs in North Carolina.  These underwater sites enhance fisheries that the coastal economy and culture rely on.  Now, local scientists are involved in ongoing research to determine the best way to maximize fish production at artificial reefs. 

The coast of North Carolina is known as the Graveyard of the Atlantic, where hundreds of ships have run aground or sunk to the depths of the sea.  Can you imagine also that old train cars, aircraft, demolished bridges and construction waste lay on the ocean floor?  It’s not a dump site.  These manmade structures make up a collection of artificial reefs that serve as important habitat for fish and invertebrates.  Divers are drawn to these sites where colorful sponges and coral grow on the repurposed material.  Anglers frequent these fish oasis because of the variety and abundance of sea life they attract. 

Now, anyone with a curiosity about North Carolina’s artificial reefs can virtually explore them thanks to an online interactive reef guide recently unveiled by the North Carolina Division of Marine Fisheries.  Artificial reef biologist Amy Comer says people can learn more about the types of fish and materials at each of the state’s 42 ocean artificial reefs and 20 estuarine reef sites.

“You can zoom in on any of the reef sites in the entire state, look at the side scan sonar information which is really cool.  You can actually see how the material that we’ve put down at those reef sites fits on the site. You can click on different material layers and get information about when we put down the material, how much material.  Stuff like the ships, you can click directly on the ships and you can find out a little history about them.”

A school of jacks corrals baitfish on an artificial reef.
Credit J. McCord / UNC-CSI

Comer says the new website had over 1,800 hits in the first week.  The Division of Marine Fisheries is also providing a 131 page, full color reef guide printed on waterproof paper.  They’re available for free at their headquarters in Morehead City or at any of their division offices in eastern North Carolina. 

  Maintaining and creating artificial reef sites is economically, ecologically and scientifically invaluable. 

“Artificial reefs are intentionally deployed on the sea floor so they can mimic natural reefs.

Marine Ecologist Avery Paxton says artificial reefs host a biologically diverse ecosystem essential to many organisms for spawning, breeding and feeding.   

“And there still remain unanswered questions about if the artificial are successfully mimicking the natural reefs.  And that’s one of the questions that our research group is trying to answer.”

Paxton is involved in a two year project focusing on understanding how to use artificial reefs to enhance fisheries.  She’s also a Ph.D candidate at University of North Carolina’s Institute of Marine Sciences in Morehead City.  Natural reefs are formed from exposed rock, and are found in nearshore and at mid and outer-shelf areas.  They come in a variety of shapes and sizes.

A grouper glides above a rocky ledge that is encrusted with brilliantly colored corals.
Credit J. McCord - UNC-CSI

“Some of them are extraordinarily flat, like a parking lot.  Other ones have nooks and crannies or a little bit rubble-like.  And we also have some that are very, very complex habitats that rise up into expansive ledges.  They have overhangs and underhangs.”

Natural and artificial reefs may differ in appearance, but their purpose is the same. 

“These reefs are federally designated as what’s called essential fish habitat because they are so important. And the reason these reefs are important is because fish use them to eat there, they use the reefs to hide from predators, they use them as nursery grounds, and they even use them as stepping stones.  And what I mean by this is that when fish move from nearshore shallow waters when they’re younger to offshore deeper waters when they’re older, they use them as stepping stones to move along this corridor.”

As part of her research, Paxton is trying to determine the optimal distance between natural and artificial reefs to maximize fish production.  There’s currently a 200 foot buffer zone because the current line of thinking is that an artificial reef may upset the balance of a natural reef if it’s too close.  Paxton says this hasn’t been scientifically substantiated and she hopes to have an answer soon.

Diver counts fish that live on an artificial reef.
Credit J. McCord / UNC-CSI

“What we have found though is that the reef associated fish really rely on the structure itself of these objects on the sea floor. And we see a sharp drop-off in the number of fish as you move from the structure into the sediment that surrounds it.   And usually this drop off starts to occur around 30 meters, so around 90 feet from the structure.”

Paxton observed that the types of fish found on reefs are very different than those found in the sand in between.  This, she believes, is due to their preferences in food.

“Maybe the vegetarian fish are in one area and the carnivorous fish are in another area is one way we like to think of it.  So their location reflects their food resources.  Different food resources occur in the sediment or sand than on the structure.”

Another aspect of her research measures the effectiveness of different types of artificial reef structures.  She studied how well concrete pipes, reef balls, pods, train cars and other materials were at growing and producing fish. 

“The ships that are intentionally sunk seem to support much higher numbers of fish than both the naturally occurring reefs and other types of artificial structures made out of concrete.  This really surprised us but one of our hypothesis as to why this might be happening is that ships provide a really large structure that often times protrudes well up into the water column above the sea floor and may be able to offer a different type of habitat resource for these fish.”

Paxton adds that artificial concrete structures appear to support equivalent fish to the naturally occurring rocky reefs that they’re intended to mimic. 

Although there is no official name for this research project, it’s funded by the North Carolina Coastal Recreational Fishing License Grant Program and will continue through 2017.  Paxton says the findings could very well change the way artificial reefs are constructed and maintained off our coast, and potentially may influence artificial reef construction worldwide.

Video credit: A. Paxton/ UNC-IMS