By Malaka Rodrigo

COLOMBO – The MV Thermopylae Sierra, a 155-meter (508-foot) bulk carrier flying the Cypress flag, was anchored off Sri Lanka’s west coast by court order following a legal dispute. Neglected for three years without proper maintenance, the vessel eventually succumbed to the forces of nature and sank in 2012.

Though abandoned, the sunken ship has since transformed into an underwater oasis, providing shelter to a diverse array of marine life. A recent study off Sri Lanka’s west coast, which included surveys of coral reefs and shipwrecks like the Thermopylae Sierra, found that shipwrecks support more abundant fish assemblages compared with natural coral reefs near Colombo, the country’s commercial capital.

“We know that shipwrecks serve as important underwater habitats for reef fish, but there had been no proper study quantifying how fish assemblages on shipwrecks compare to natural reefs in terms of species composition, diversity, richness and density. Our research aimed to fill this gap,” said Nishan Perera, a marine scientist at Blue Resources Trust.

Underwater habitats

In addition to the Thermopylae Sierra, which lies at a depth of 23 m (75ft), the research team selected the Medhufaru wreck, at 30m (98ft), and the Nilgiri tugboat, lying 32m (105ft) deep off the west coast near Colombo. The Medhufaru was sunk in 2009 and the Nilgiri tugboat in 1997, both of which have become popular diving sites.

The team also surveyed three sandstone-based coral reefs located nearby — Barracuda Reef, and Anchor Point — to compare fish assemblages against the coral reefs.

The study recorded a total of 2,918 fish from 20 families and 30 genera, with 749 observed on reef sites and 2,169 on the wrecks, indicating a higher fish density on the wrecks. Perera attributed this result to the more complex structures of the shipwrecks compared with the natural reefs.

Snappers (Lutjanidae), known for aggregating in large schools, were abundantly found on the wreck sites. Perera pointed out that these commercially important fish underscore the significance of wrecks for local artisanal fishers.

Fish density

In contrast, higher abundances of cardinalfishes (Apogonidae) and fusiliers (Caesionidae) were recorded on wreck sites, while damselfishes (Pomacentridae) and barracudas (Sphyraenidae) were more common on coral reefs. These findings suggest that fish communities on wrecks and natural reefs can vary significantly.

The research team used standardized diver-operated video transect surveys to measure fish assemblages. “We would lay out a transect on a section of the wreck or reef, wait for the fish to resettle after the initial disturbance and then swim 25m [82ft] at a set speed with a camera,” explained Anjleen Hannak of Bremen University, the study’s lead author. This method allowed the researchers to review footage post-survey for more precise fish identification and count.

Hannak, who developed a passion for studying shipwrecks during her secondary education in Sri Lanka, shared that her favourite wreck site was Medhufaru due to its diverse structure, including the bridge, radio tower and cargo hold filled with concrete pipes. These sections are occupied by biodiverse marine communities using the substrate as a habitat.

The researchers also had remarkable experiences during their surveys. During one survey on the Nilgiri wreck, Hannak and Perera encountered a juvenile whale shark, likely feeding on plankton around the shipwreck.

The researchers emphasized the importance of studying these shipwreck habitats thoroughly as potential solutions to ongoing marine habitat degradation. They suggest that lessons learned from such studies could inform the design and development of artificial reefs that mimic shipwrecks to support marine communities and the livelihoods of coastal regions.

Sinking discarded buses 

In a bid to boost fish populations, the Department of Fisheries and Aquatic Resources has also sunk discarded buses and train compartments into the sea.

However, former National Aquatic Resources Research and Development Agency scientific officer Arjan Rajasuriya is sceptical about the long-term viability of these structures, noting that they may not withstand underwater conditions and could break down after a few seasons.

Not everything that drops into the sea becomes coral reefs, however, such structures could be strategically used to deter bottom trawling, a significant environmental issue, particularly in the northern oceans.

Shipwrecks are also key attractions for dive tourism, according to shipwreck explorer Dharshana Jayawardena, who authored a book on Sri Lanka’s shipwrecks. Sri Lanka currently earns about $3 million to $5 million from dive tourism, but Jayawardena said he believes this figure could increase tenfold.

He noted that Sri Lanka’s shipwrecks, many of which are nearly a century old, offer significant archaeological value. In terms of shipwrecks, Sri Lanka trumps Malaysia, the Maldives, Thailand and Indonesia, according to Jayawardena.

Among the notable shipwrecks are HMS Hermes, the world’s first purpose-built aircraft carrier sunk during World War II, and the Roman shipwreck at Godawaya, dating back to the second century BCE. The latter is believed to be the oldest known shipwreck in the Indian Ocean, serving as an underwater oasis for marine life even after 2,200 years, said Rasika Muthucumarana of the Maritime Archaeology Unit, Central Cultural Fund.

Blast fishing 

However, these shipwrecks face threats from blast fishing and illegal metal salvage. Jayawardena highlighted that blast fishing is particularly rampant on the East Coast, and illegal salvaging often targets the shipwrecks, even Thermopylae Sierra in Colombo.

The researchers hope to survey more of Sri Lanka’s shipwrecks to gain a complete understanding of their role in marine ecosystems. Sri Lanka launched the National Shipwreck Database in 2021 to share valuable information on its 115 known shipwrecks.

 -mongabay.com

A school of snapperfish swimming on the shelter of a shipwreck. – Nishan Perera/mongabay.com