1. Impacts of the wreck of a fishing vessel on the
coral reef of St. Brandon’s Rock, Mauritius
Du Preez M1, Choong Kwet Yive NS2, Minnaar K1, van der Schyff V1, Kylin H1,3,4, Raffin J5, Bouwman H1
1North-West University, Potchefstroom, South Africa; 2University of Mauritius, Mauritius; 3Linköping University, Linköping, Sweden;
4Norwegian Institute for Air Research, Fram Centre, Tromsø, Norway, 5Shoals Rodrigues, Port Mathurin, Rodrigues, Mauritius.
INTRODUCTION
Wreck section
We counted 979 individuals of 17 fish species associated with the wreck sections, most of them quite large (compared to the lagoon), as would be expected at wrecks.
The dominant coral species in the area were Poccilopora, Stylophora and corals of the Faviidae family. No coral settlement was observed on any part of the wreck. This is rather unusual, as Poccilopora takes approximately three months to settle on a wreck, and Poccilopora was common in the area. The coral directly underneath the wreck sections were clearly dead, but also blackened (as opposed to bleached; Figure 2C). Especially the stern section (with the engine room) was associated with extensive coral blackening. Coral colonies surrounding the wreck returned to apparently normal, 5-10 m away from this section.
Coral reefs are among the most diverse and important marine ecosystems in the world. Shipwrecks can be among the biggest sources of ocean pollution. Very little has been documented on the impacts and recovery of coral reefs following shipwrecks.
St. Brandon’s Rock (SBR) is an atoll 400 km North of Mauritius. There is about one ship wreck every two years on SBR. In 2012 a fishing vessel stranded, and in 2014 a yacht (participating in the Volvo Ocean Race) stranded not very far away.
The fishing vessel broke into three pieces due to storms; a stern section, a bow section, and the wheelhouse (Figures 1 a, b and c). We were made aware that this wreck caused a major visual impact on the biology of the coral reef at the wrecking site; as well as down-current from there. Green filamentous algae started to cover a large area of the lagoon, down-current of the wreck.
We investigated the impact of the wreck on the coral reef of SBR during our 2014 Mascarene Coral Island Expedition (MCIE).
MATERIALS AND METHODS
Fish and coral surveys and general observations were conducted by means of snorkeling around the three sections of the vessel.
Transects were done up- and down-current of the vessel.
The transects consisted of four, 50 m transects, 50 m apart. All fish and sea-cucumbers seen within a 10 m wide transect was recorded. All fish associated with the wreck were also recorded.
We used multivariate statistics (NMS) to investigate differences in relative proportions of the fish and sea cucumbers.
A B C
Figure 2. A – A coral head covered by green algae. B – a bunch of green algae attached to one head. C – Blackened coral and fish associated with one of the sections.
Comparative biodiversity of the three areas
Figure 3. NMS graph showing the differences in fish and sea-cucumber community compositions between the reference site (red triangle), the algae field (green triangle), and the wreck site (blue triangle).
A B C
Figure 1. The three sections of the wreck A: Bow; B: Stern; C: Wheelhouse.
RESULTS AND DISCUSSION
Algae-covered area in the lagoon
Approximately 20 ha of the lagoon was covered by green algae.
Hundreds of coral heads were covered by long strands (up to 10 m) of dense filamentous algae. Some of the coral heads were torn of their base due to the hydrological drag (Figures 2A and B).
We noted 376 individuals of 21 fish species in the un-affected reference area.
In the algae-affected area, we counted 123 individuals of only five fish species.
Three sea cucumber species were seen: 19 individuals in the unaffected area, and 218 in the algae-affected area.
From the boat, we counted at least 13 Hawksbill Turtles in the algae-affected area.
CONCLUSIONS / RECOMMENDATIONS
The wreck had a negative impact on the coral and fish community in the algae affected area, but stimulated sea cucumber numbers.
Oil and fuel could be the cause of the extensive blackening associated with the stern (engine) section.
It is possible that an antifouling agent in the hull paint inhibited the growth of new coral colonies on the hull.
The algae field seemed to attract endangered Hawksbill Turtles.
The impacts of the stressors that caused the blackening of the coral and growth of the algae may be harmful to the turtles and other marine life.
The impact of ship wrecks on coral reefs therefore seem to be quite extensive and long-lasting.
The cause(s) of the formation and long-term presence of the algae field should be investigated.
Impacts can be measured and changes over time should be monitored.
Recovery of the reefs can be tracked using fish and sea cucumbers, and probably coral communities.
Acknowledgements: We thank the National Research Foundation (NRF) of South Africa for funding.
Opinions expressed and conclusions arrived at are those of the authors, and are not necessarily to be attributed to the NRF.