How Shipwrecks Help Marine Predators

Artificial reefs become havens as natural reefs continue to decline.

reef shark in shipwreck
A reef shark swims in a shipwreck that was purposefully sunk in the Caribbean.

 Stephen Frink / Getty Images

As natural reef ecosystems across many of the world’s oceans continue to decline due to climate change, pollution, and development, large marine predators swim outside their typical habitats in search of new sources of food. Sharks, barracudas, mackerels and other large migratory fish typically hunt in the water around reefs.

But shipwrecks and other artificial reefs created as substitutes for eroding natural reefs can support concentrated populations of these predators, a new study finds. In fact, predator densities were as much as five times greater at 14 artificial reefs analyzed in the study compared to 16 nearby natural reefs.

Shipwrecks were their favorite. They particularly liked those that rose between 4 and 10 meters (13 to 32 feet) up into the water column, which is the column of water from the bottom of the sea up to the surface. Researchers found that in some areas, shipwrecks supported predators at densities 11 times greater than natural reefs or low-profile artificial reefs that were made of concrete.

“Artificial reefs are intentionally sunk on the seafloor to supplement existing reefs, but it is unknown whether artificial reefs benefit large predators, which are important for reef health. To help fill this knowledge gap, our team tested whether artificial reefs support large predators by conducting extensive scuba-diving surveys along the North Carolina coast,” lead researcher Avery Paxton, research associate with the National Oceanic and Atmospheric Administration’s National Centers for Coastal Ocean Science (NCCOS) in Beaufort, North Carolina, tells Treehugger.

“Artificial reefs composed of ships, as well as accidental shipwrecks, provide tall reef structure. Our study demonstrated that these tall artificial habitats can host high densities of fast-moving, water-column predators.”

The findings were published in the journal PLOS One.

Height Matters for Some Predators

For the study, scuba-diving scientists surveyed fish populations at 14 artificial reefs and 16 natural reefs from 10 to 33 meters (32 to 108 feet) deep along 200 kilometers (124 miles) of North Carolina’s continental shelf. They worked between 2013 and 2015.

The researchers made 108 surveying passes along the artificial reefs and 127 along the natural reefs. They visited most of the sites four times each year to track information such as seasonal changes in the fish populations and the types of species observed.

They found that tall reefs, like shipwrecks, attract more large migratory predators because their height makes them easier to see from far away. Once the predators have arrived at the artificial reefs, the added height complements their hunting style, by giving the quick-moving fish additional space to dart in and around the structure and up and down the water column as they go after their prey.

Although these water-column predators preferred the artificial reefs, researchers found that bottom-dwelling predators weren’t as choosy. Large bottom-dwelling fish like grouper and snapper were observed in similar densities in both artificial and natural reefs. This suggests that artificial reefs can support these fish, but not to the extent that they benefit sharks, mackerel, and barracudas.

Although the study concentrated on reefs in North Carolina, researchers also analyzed the results from studies at natural and artificial reef systems in other parts of the world and found similar patterns appear to be happening globally.

The findings suggest that artificial reefs made from shipwrecks (or made to look like them) could be placed near degraded natural reefs and along migratory routes between those reefs to “act as stepping stones for fish on the move due to climate change or other changes in the ocean,” says study co-author Brian Silliman, a marine conservation biology professor at Duke, in a statement.

And because viewing large marine predators is attractive for tourists, creating these artificial reefs could benefit coastal economies as new destinations for recreational diving as many reefs off North Carolina have already done, he points out.