The distribution of South African loggerhead sea turtles (Caretta caretta) as indicated by epibionts and stable isotopes

Many marine species undertake long-distance migrations as part of their life history strategies, and so form an important part of marine ecosystems performing a range of functions, across many habitats. However, these migratory species, including sea turtles, face multiple threats and anthropogenic impacts across their ranges and knowing their movement and distribution patterns enables more effective and appropriate conservation strategies to be devised. Satellite telemetry has provided invaluable information on spatial distribution of marine migrants, but applying this approach to a large proportion of a population is often unfeasible and costly. This study aimed to identify alternative, more cost effective methods that could assist with tracking animal movements across a larger proportion of a population of marine focal species, such as sea turtles. This study used nesting loggerhead sea turtles (Caretta caretta) from the iSimangaliso Wetland Park, South Africa as a model species to test these alternative methods and subsequently combine body condition, habitat use, and distribution range in the South West Indian Ocean. First, epibiont community assemblages were investigated as a proxy to determine sea turtle body condition. A body condition index was created using plastron shape, injuries and skin deformities. Sixty turtles were classified into four body condition categories ranging from poor to very good and this was reflected in their epibiont communities as both species abundance and richness increased with a decline in body condition. A total of twenty-eight epibiont taxa were identified from a range of systematic groups including, but not limited to, Amphipoda, Cirripedia, Brachyura and Polychaeta. The barnacle Chelonibia testudinaria showed the greatest variation among different body conditions with an increase in abundance as turtle body condition deteriorated. These results suggest that epibiont load can be used as an indicator of body condition that is easy to implement in the field. Second, a combination of organic δ13C and δ15N isotopic signatures of turtle epidermis and epibiont communities was used to infer foraging habitat. One hundred and seventy turtles were sampled for stable isotope analysis. These turtles were clustered into two groups based on δ13C at -13.61 ‰ with relative depletion or enrichment indicating foraging in oceanic or neritic environments, respectively. The epibiont communities of IV 80 turtles closely followed this cluster grouping; turtles with depleted δ13C had a higher abundance and frequency of oceanic epibiont species, such Lepas spp. Similarly, three neritic epibionts (Hyale grandicornis, Hyachelia tortugae and Podocerus africanus) were the other habitat-specific species driving community assemblages, with higher occurrence and abundance on turtles in the enriched δ13C cluster. Additionally, the size of the dietary niche was determined by a Bayesian analysis of δ13C and δ15N for 46 turtles in different body condition categories. Although there was overlap among categories, individuals in very good body condition had the smallest dietary niche. These results show the complementarity of using epibionts and stable isotope analysis in determining foraging area. Third, Chelonibia testudinaria barnacles on sea turtles were analysed for δ18O and inorganic δ13C. The δ18O of expected calcite fractionation was mapped for the known migration routes of eight turtles in the South West Indian Ocean. The inorganic carbon values were not very informative on movement, however, the δ18O analysis of the barnacle showed the range of the turtle host moving through the isoscape. Most of the turtles migrated from the north in the Mozambique Channel, to the southern rookery in South Africa, which is in accordance with reports from tag recoveries and satellite telemetry studies. Using this approach to track migratory species that have epibiotic barnacles can provide complimentary approach to satellite tracking that can be used on more individuals within a population. This study aids in providing alternative methods to study body condition, habitat use and regional movement of loggerhead sea turtles. These approaches can be applied to other sea turtle species and migratory marine fauna to help better understand their movement patterns thereby promoting more effective conservation strategies. Future work should consider incorporating different cohorts, examining other epibionts such as meiofauna and diatoms, including additional isotope and trace elements for analysis on habitat and improving the resolution of the isoscape data for δ18O of seawater in the SWIO