Genetic connectivity and stock composition of coral reef associated fish in the Western Indian Ocean

Coral Reefs and their associated fisheries in the Western Indian Ocean (WIO) are threatened from over fishing and climate change, which have resulted in a pronounced decline in the abundance and diversity of corals. Fisheries management structures in the region are weakly enforced and fisheries policies are not adequately grounded on an appropriate level of scientific information. In an attempt to contribute to the base of scientific knowledge to guide fisheries management policy and practice in the region, the WIO-magnet (Western Indian Ocean Marine Genetics Network) has carried out a three year MASMA funded study on ‘Genetic Connectivity and Stock Composition of Coral Reef Associated Fish in the Western Indian Ocean.’ CORDIO East Africa, based in Kenya is the lead organization for this project, with regional partners including the Institute of Marine Sciences in Zanzibar and the Mauritius Oceanographic Institute in Mauritius. In addition, technical support for the project has been provided by Academia Sinica, Taiwan and Södertörn University in Sweden.  

Genetic studies help estimate amount of genetic diversity, which can indicate the potential for a species to adapt and survive in the face of environmental change. Furthermore, as it is impossible to monitor movements of fish and coral larvae over large distances in the oceans, genetic markers have been used to track dispersal of larvae between distant reefs, providing information on how distant reefs may be ‘connected.’ This is particularly important, since if the location from where a degraded or depleted population recruits is known, this can allow for protection of the source population, with benefits downstream. This concept has stimulated numerous studies on genetic connectivity and its implications for the design, i.e. number, size and placement, of marine protected areas. Genetic data is also used to identify distinct breeding stocks of fish that are isolated from other such groups of the same species. This is known as the degree of population structure. Strong genetic structuring implies many isolated groups instead of a single widespread interbreeding group. Intense harvesting from a local population leads to the collapse of genetically distinct and locally adapted stocks. Management of fisheries without prior knowledge of the genetic fish stock composition reduces the success of the fisheries.

We have focused on two species of coral reef associated fish, the Blue Barred Parrotfish, Scarus ghobban, and the White Spotted Rabbitfish, Siganus sutor for our research. Whereas each is relatively common in the Western Indian Ocean, Scarus ghobban is globally distributed and the Rabbitfish is regionally endemic. Both species are fairly important from an economic standpoint, particularly for traditional fisheries in which fish are caught from small non- motorized vessels with hook and line, or basket traps, and sold to middlemen at local fish landing sites. The study was carried out in Kenya, Tanzania, Mauritius, Seychelles and Rodrigues. We used two types of genetic markers, mitochondrial DNA and AFLP, to study dispersal and stock composition. Mitochondrial DNA was used to understand historical patterns of dispersal on an evolutionary timescale, and AFLP markers are better suited to understanding contemporary patterns of genetic structure on an ecological timescale of a few generations. AFLP is thus a better suited marker to aid management policy.

Results from mitochondrial DNA suggest that the Blue-Barred Parrotfish historically had high migration rates between the study locations. This is in line with the pattern of high gene flow often reported for fish with pelagic larvae, or larvae that drift large distances with currents. Long distance migration for fish is generally achieved during the larval phase when currents can transport the larvae across huge distances. Ocean currents, particularly the South Equatorial Current and the East African Coastal Current, exerted a strong influence on dispersal of the Parrotfish, and the Mascarene Plateau served as a stepping stone for fish larvae migrating between Mauritius and Seychelles in either direction. There was a high level of genetic diversity for the Parrotfish, and this bodes well for the long term survival and adaptability of the species. However, historical movement of the White Spotted Rabbitfish was more restricted, and the oceanic island sites of Mauritius and Rodrigues comprised a few dominant genetic types, called haplotypes. This suggests that the populations of the White Spotted Rabbitfish on the oceanic islands were isolated and are could therefore be more vulnerable to over fishing or environmental changes.

AFLP markers showed that the Blue-Barred Parrotfish, which is less intensely harvested than Siganus sutor, comprised a mixed stock fishery, especially along the East African coast. Most sites had multiple stocks, likely enhancing resilience against local depletion. However, we caution that in order for this situation to persist, suitable habitats must be preserved along the coast to ensure that the fish can migrate between locations. The more important species for fisheries, the White Spotted Rabbitfish, had comparatively fewer stocks per site. This is worrisome in those areas where fishermen target spawning aggregations.  Many of the sites were dominated by a single genetic stock, and increased fishing at spawning sites will likely lead to local extinction of this species.

In addition to carrying out the research described above, the WIO-magnet has been instrumental in developing infrastructure for future work on the genetics of marine flora and fauna in the region. Laboratories for molecular studies have been constructed at the Institute of Marine Sciences (IMS) in Zanzibar, the Mauritius Oceanography Institute (MOI) in Mauritius and Coastal Oceans Research and Development in the Indian Ocean (CORDIO) East Africa in Kenya. These facilities are invaluable for capacity building and exposing young scientists from the region to modern genetic tools for addressing the important marine conservation and management questions of the region.