Genetic connectivity of the roundjaw bonefish (Albula glossodonta) in the Southwest Indian Ocean

Abstract

The Southwest Indian Ocean (SWIO) islands of Mauritius and Seychelles are both highly dependent on tourism and fisheries for their economies. One of the growing ecotourism sectors is saltwater fly fishing, an industry based on catch-and-release fishing for a host of species, including bonefishes. Bonefishes (Albula spp.) have received significant research attention in the Pacific and Atlantic Oceans, with only sporadic research conducted in the Indian Ocean. My project aimed to investigate the genetic connectivity of the roundjaw bonefish (Albula glossodonta) in two island states (Seychelles and Mauritius) within the SWIO using a mitochondrial genetic marker (cyt-b) and next generation sequencing (ddRADseq). Samples collected were grouped based on their spatial distribution. The Seychelles consisted of four island groups (Inner Island Group, Aldabra Group, Amirantes and Alphonse Group, and Farquhar Group) whereas Mauritius was represented by one island group (Saint Brandon). Genetic analyses were undertaken between and within each of these groups. Mitochondrial cytochrome-b identified two species of bonefish: Albula glossodonta and Albula oligolepis; the latter was only genetically identified from the Inner Island Group. I hypothesise that this is due to habitat partitioning, with A. oligolepis being a deeper dwelling bonefish species compared to A. glossodonta, which occupies shallow water habitats such as sand flats, atoll lagoons and reef flats. Neutral SNP loci revealed a panmictic pattern of distribution for A. glossodonta throughout the Seychelles Island groups but showed a pattern of weak structure between Seychelles and Mauritius. Genetic diversity indices such as allelic richness, showed low diversity across the sampling sites (AR range: 1.761-1.889). Population structure tests such as pairwise FST showed low but significant population structure. The highest FST indices were recorded between the Aldabra and Farquhar Groups, as well as the Aldabra and Saint Brandon Groups (0.044 ± 0.000 and 0.040 ± 0.000, respectively). Descriptive tests such as PCA and DAPC showed similar trends, whereby Saint Brandon clustered separately from the other samples from the Seychelles Island groups. However, these trends were Abstract seen at very low variations (PCA axes 1 and 2 accounted for only 2.0 and 1.9 % of the total variation, respectively). A population assignment test grouped the individuals as one ancestral population. A spatial principal component analysis showed that Saint Brandon was dissimilar to the Seychelles Island groups. Like other Elopomorph species, bonefishes have leptocephalus larvae capable of long-distance dispersal which may explain the well-mixed genetic population observed within the Seychelles islands. Although currents within the Indian Ocean, especially on a mesoscale, are not well understood, the South Equatorial Current likely facilitates connectivity between the Seychelles islands while also limiting gene flow between Seychelles and Mauritius. Understanding population structure is important for informing the appropriate management and conservation strategies, especially in oceanic nations where data informing important industries like tourism and fisheries are often limited. The bonefish fly fishing industry is well-known to be a lucrative sector, generating, for example US$ 1.4 million a year in the Bahamas. This study recognised that there are numerous knowledge gaps relevant to the bonefish industry that need to be addressed, including: 1) understanding the socio-economic importance of fly fishing to island states like Seychelles, 2) estimating the abundance and species distribution of bonefishes within Seychelles, 3) understanding effectiveness of MPAs for recreational fishery species like bonefish and, lastly, 4) generating more fishery-relevant biological information on the heavily targeted fly fishing species within Seychelles. These needs must be met to inform management plans and to better manage the fly fishing ventures that target species like bonefish.