Thorin S. - Créocéan
Setting the scene
A port extension project at Pointe-à-Pitre, Guadeloupe, French West Indies (building a new terminal for Panamax-specification vessels ships with a draught greater than 16m) required the dredging of an access channel (removing almost 7 million cubic metres of sediment over an 11-month period). The work included preserving a coral reef area by moving a substantial part of it to a new location.
The main impacts on marine biodiversity were:
- Destruction of coral reefs through rock breaking (0.8 ha of a 3 ha area where corals were located).
- Destruction of marine seagrass meadows (10,000 m3 backfilled or dredged).
- High turbidity during dredging, dumping, rock breaking and backfilling.
- Wildlife disturbance (e.g. on marine mammals and turtles).
Various surveys and mitigation measures were implemented including coral and seagrass transplantations. The French company CREOCEAN was in charge of all coral transplantation operations including preliminary studies assessing the feasibility of the project, evaluating the health status of the donor coral reefs, identifying and characterizing the best recipient sites and submitting a detailed methodology to authorities for the different phases of the work.
In view of these studies the State environmental services required a coral transplantation of a minimum of 4,150 coral colonies, taken from two sites located with an area of over 3 ha that was to be impacted.
The transplantation was performed from January to March 2015. A total of 4,188 coral colonies belonging to 22 species and measuring 20 cm to 1.5 m in diameter were relocated; this included massive and encrusting corals belonging to 7 families (mainly Favidae and Siderastreidae). The project required major logistics and organization (total cost: €700,000 (0.5% of the project’s cost), 20 divers and 3 vessels during a two-month period).
Monitoring of relocated and control-site corals is being undertaken and will continue over a 3-year period. The monitoring aims to gauge the success rate of the transplantation, coral growth rates, signs of bleaching or necrosis, coral recruitment, coral cover, and the status of associated fauna.
Monitoring is carried out using photoquadrats on 62 transplanted colonies selected at random and compared to reference sites. The sessile benthic community (including juvenile corals and sea urchins) and associated fish communities are being monitored as well (semi quantitative visual census).
The frequency of monitoring efforts is 1, 3, 6 and 12 months after transplantation, and then every 6 months for the 2nd and the 3rd year (from 2015 to 2018).
Figure 1. Illustrations of brain coral Pseudodiploria strigosa (A), boulder brain coral Colpophyllia natans (B), and massive starlet coral Siderastrea siderea (C). Photo credits to Créocéan.
Initial results and future perspectives
For the first year of monitoring, the survival rate of the 62 transplanted colonies chosen for monitoring was very good (98.4%). Only 1 mortality was observed one year after transplanting. Colonies showed a decline in their condition during the first 6 months post-transplant. This was mainly due to the development of small, localized tissue necrosis linked to damage from the transplanting methods. Moreover, a bleaching event occurred in October 2015 (6 months after transplanting) owing to warm waters.
During the second half of post-transplant period the majority of the colonies had recovered from the bleaching phenomenon and their state of health is improving.
These results show that the relocated corals were resilient to the stresses they experienced during relocation and the effects of subsequent seasonal warming. Boulder brain corals (Colpophyllia sp.), brain corals (Diploria sp.) and massive starlet corals (Siderastrea sp.) appear to be more resilient than others to the stresses of transplantation and the bleaching event. These observations will be refined during the continued monitoring campaigns. Future monitoring of the relocated corals during the contracted work and beyond may ascertain the resilience of these corals. Having the relocation occur during “optimal” environmental conditions would provide a buffer of time for corals to recover from any stresses during the relocation process and to acclimate to the biotic and abiotic settings of the relocation site.
Figure 2. Harvested corals are placed in crates to be hoisted to a vessel for transportation to the recipient site (A); large boulderbrain coral being harnessed to a line from the support vessel (B); harvested corals temporarily placed seawater holding tanks during transportation (C); periodic monitoring of transplanted coral colonies (D). Photo credits to Créocéan