Predicting connectivity of green turtles at Palmyra Atoll, central Pacific: a focus on mtDNA and dispersal modelling

Abstract

Population connectivity and spatial distribution are fundamentally related to ecology, evolution and behaviour. Here, we combined powerful genetic analysis with simulations of particle dispersal in a high-resolution ocean circulation model to investigate the distribution of green turtles foraging at the remote Palmyra Atoll National Wildlife Refuge, central Pacific. We analysed mitochondrial sequences from turtles (n = 349) collected there over 5 years (2008-2012). Genetic analysis assigned natal origins almost exclusively (approx. 97%) to the West Central and South Central Pacific combined Regional Management Units. Further, our modelling results indicated that turtles could potentially drift from rookeries to Palmyra Atoll via surface currents along a near-Equatorial swathe traversing the Pacific. Comparing findings from genetics and modelling highlighted the complex impacts of ocean currents and behaviour on natal origins. Although the Palmyra feeding ground was highly differentiated genetically from others in the Indo-Pacific, there was no significant differentiation among years, sexes or stage-classes at the Refuge. Understanding the distribution of this foraging population advances knowledge of green turtles and contributes to effective conservation planning for this threatened species.

Keywords: Chelonia mydas; control region; feeding ground; marine turtle; mixed stock analysis; ocean currents.

Figure 1.

Location of the PANWR (star) with respect to other C. mydas rookeries (white squares), RMUs (references in table 1) and FGs (black dots) previously subject to genetic analysis. References and/or abbreviations for FGs are as follows—Hawaii [30]; Australasia [33], CK, Cocos Keeling; FB, Fog Bay; FI, Field Island; CP, Cobourg Peninsula; SEP, Sir Edward Pellews Island; GOC, Gulf of Carpentaria; Gorgona, Colombia: [12]; Japan [35].

Figure 2.

Neighbour-joining tree of subhaplotypes (approx. 857 bp) found at the PANWR, with respect to rookery clades. Branch lengths are proportional to sequence divergence, and Atlantic haplotypes CMA3.1 and 5.1 were used as outgroups.

Figure 3.

Distribution of 73 000 particles tracked in reverse for 3 years from Palmyra Atoll (green circle) relative to green turtle nesting sites, including those analysed genetically (white squares) as well as those not yet subject to genetic analysis (grey squares). (a) Shading indicates the number of particles at a particular location throughout the 3 year simulations (counted at 5 day intervals). Thus, this map identifies connectivity ‘hot spots’ between oceanic locations and Palmyra Atoll. Note the logarithmic scale. (b) Shading indicates the average number of years a particle would have to drift before reaching Palmyra Atoll from a particular location. (Online version in colour.)