When the Scottish ship Glanmire ran aground on a foggy July morning 101 years ago, it was probably a disaster for the Glasgow-based owners of the cargo-hauling steamship. But a century later, the sinking of this 242-foot-long vessel is an unlikely boon for some, namely corals and shellfish.
It turns out the hulking remains of the Glanmire have acted as an accidental refuge for sea creatures, sheltering them from a destructive fishing method known as bottom trawling that can scrape sensitive organisms from the seafloor.
“Outside of legal protection, only areas inaccessible to trawlers are offered any protection,” said Jenny Hickman, who studied the accidental ecological benefits of these wrecks as a master’s student at the University of Plymouth in the United Kingdom. That’s “why shipwreck sites are rarely subject to trawling pressure.”
When bottom trawling, fishers tether a weighted net to a boat and tow it across the ocean floor, scooping up low-lying fish and crustaceans. But the approach can also bulldoze habitat and bottom-dwelling species. Intensive bottom trawling has been found to alter ecosystems by reducing the presence of static filter feeders such as mussels and shrinking the mass of organisms living in the area. By one estimate, roughly half of the seabed surrounding the United Kingdom has been affected by fishing methods such as trawling.
While marine reserves that bar such fishing can counter these effects, these restrictions are uncommon. For instance, in a 650-square-kilometer marine conservation area created in the early 2000’s along the eastern coasts of Scotland and England, bottom trawling is permitted in much of the Scottish portion, except for a 26-square-kilometer patch of water.
But there is another factor at play in that area. Both in places closed and open to trawling, several shipwrecks dot the ocean floor, including the Glanmire. Such wrecks are known as biological hotspots, attracting marine creatures and human divers alike. They are also a headache for bottom trawling, threatening to entangle nets.
That split between places where bottom trawling is and isn’t permitted, as well as places with and without shipwrecks, created an opportunity to gain a better understanding of the ecological effects of the fishing and the interplay with these sunken vessels.
To understand these dynamics, Hickman and colleagues from the University of Plymouth and the London-based Blue Marine Foundation went swimming during part of the summers of 2021 and 2022. Armed with video cameras, they scanned five different shipwrecks and the surrounding underwater terrain. Three of the ships were outside the no-trawling zone, while two were inside. Then they scrutinized the footage, counting each species as well as the overall number of organisms.
The results revealed that shipwrecks in places where bottom trawling is permitted can act as biological oases. In those fishing zones, the sheer number of organisms was more than 300% greater around shipwrecks than a short distance away, the scientists reported Nov. 22 in Marine Ecology.
“It has long been thought that shipwrecks could be playing an important role in providing sanctuary for marine species to utilize. It is brilliant to see this proven in this study,” said Joe Richards, a co-author and manager with the Blue Marine Foundation.
In addition to population sizes, the species living around the shipwrecks inside fishing zones had more in common with those in protected areas, while trawled areas within shipwrecks were dominated by scavenging species such as lobsters and starfish.
At least some of the shipwreck benefit appears to be greatest in places where bottom trawling is allowed. In the no-go zones, the picture flipped, with fewer organisms found on shipwrecks than nearby.
Still, shipwrecks couldn’t do everything. The most trawling-sensitive species, such as a soft coral known as “dead man’s fingers” for its fleshy, finger-like lobes, were entirely absent from the trawled areas. That coral only turned up on wrecks in protected areas, such as the Glanmire.
No one is calling for a campaign to reduce fishing damage by creating more shipwrecks. While there are benefits, modern-day vessels can also taint the ocean with spilled oil and other toxic chemicals. But the benefits of existing sunken vessels could be significant, given the more than 30,000 such wrecks in the waters surrounding the UK alone.
Hickman, et. al. “Shipwrecks act as de facto Marine Protected Areas in areas of heavy fishing pressure.” Marine Ecology. Nov. 22, 2023.