Anna L. Bass

Adult habitat preferences, larval dispersal, and the comparative phylogeography of three Atlantic surgeonfishes (Teleostei: Acanthuridae).

Although many reef fishes of the tropical Atlantic are widely distributed, there are large discontinuities that may strongly influence phylogeographical patterns. The freshwater outflow of the Amazon basin is recognized as a major barrier that produces a break between Brazilian and Caribbean faunas. The vast oceanic distances between Brazil and the mid‐Atlantic ridge islands represent another formidable barrier. To assess the relative importance of these barriers, we compared a fragment of the mitochondrial DNA (mtDNA) cytochrome b gene among populations of three species of Atlantic surgeonfishes: Acanthurus bahianus, A. chirurgus and A. coeruleus. These species have similar life histories but different adult habitat preferences. The mtDNA data show no population structure between Brazil and the mid‐Atlantic islands, indicating that this oceanic barrier is readily traversed by the pelagic larval stage of all three surgeonfishes, which spend ~45–70 days in the pelagic environment. The Amazon is a strong barrier to dispersal of A. bahianus (d = 0.024, ΦST = 0.724), a modest barrier for A. coeruleus (ΦST = 0.356), and has no discernible effect as a barrier for A. chirurgus. The later species has been collected on soft bottoms with sponge habitats under the Amazon outflow, indicating that relaxed adult habitat requirements enable it to readily cross that barrier. A limited ability to use soft bottom habitats may also explain the low (but significant) population structure in A. coeruleus. In contrast, A. bahianus has not been collected over deep sponge bottoms, and rarely settles outside shallow reefs. Overall, adult habitat preferences seem to be the factor that differentiates phylogeographical patterns in these reef‐associated species.

Conservation implications of complex population structure: lessons from the loggerhead turtle ( Caretta caretta )

Complex population structure can result from either sex‐biased gene flow or population overlap during migrations. Loggerhead turtles (Caretta caretta) have both traits, providing an instructive case history for wildlife management. Based on surveys of maternally inherited mtDNA, pelagic post‐hatchlings show no population structure across the northern Atlantic (φST < 0.001, P = 0.919), subadults in coastal habitat show low structure among locations (φST = 0.01, P < 0.005), and nesting colonies along the southeastern coast of the United States have strong structure (φST = 0.42, P < 0.001). Thus the level of population structure increases through progressive life history stages. In contrast, a survey of biparentally inherited microsatellite DNA shows no significant population structure: RST < 0.001; FST = 0.002 (P > 0.05) across the same nesting colonies. These results indicate that loggerhead females home faithfully to their natal nesting colony, but males provide an avenue of gene flow between regional nesting colonies, probably via opportunistic mating in migratory corridors. As a result, all breeding populations in the southeastern United States have similar levels of microsatellite diversity (HE = 0.70–0.89), whereas mtDNA haplotype diversity varies dramatically (h = 0.00–0.66). Under a conventional interpretation of the nuclear DNA data, the entire southeastern United States would be regarded as a single management unit, yet the mtDNA data indicate multiple isolated populations. This complex population structure mandates a different management strategy at each life stage. Perturbations to pelagic juveniles will have a diffuse impact on Atlantic nesting colonies, mortality of subadults will have a more focused impact on nearby breeding populations, and disturbances to adults will have pinpoint impact on corresponding breeding populations. These findings demonstrate that surveys of multiple life stages are desirable to resolve management units in migratory marine species.

Natal homing in juvenile loggerhead turtles (Caretta caretta)

Juvenile loggerhead turtles (Caretta caretta) from West Atlantic nesting beaches occupy oceanic (pelagic) habitats in the eastern Atlantic and Mediterranean, whereas larger juvenile turtles occupy shallow (neritic) habitats along the continental coastline of North America. Hence the switch from oceanic to neritic stage can involve a trans‐oceanic migration. Several researchers have suggested that at the end of the oceanic phase, juveniles are homing to feeding habitats in the vicinity of their natal rookery. To test the hypothesis of juvenile homing behaviour, we surveyed 10 juvenile feeding zones across the eastern USA with mitochondrial DNA control region sequences (N = 1437) and compared these samples to potential source (nesting) populations in the Atlantic Ocean and Mediterranean Sea (N = 465). The results indicated a shallow, but significant, population structure of neritic juveniles (ΦST = 0.0088, P = 0.016), and haplotype frequency differences were significantly correlated between coastal feeding populations and adjacent nesting populations (Mantel test R2 = 0.52, P = 0.001). Mixed stock analyses (using a Bayesian algorithm) indicated that juveniles occurred at elevated frequency in the vicinity of their natal rookery. Hence, all lines of evidence supported the hypothesis of juvenile homing in loggerhead turtles. While not as precise as the homing of breeding adults, this behaviour nonetheless places juvenile turtles in the vicinity of their natal nesting colonies. Some of the coastal hazards that affect declining nesting populations may also affect the next generation of turtles feeding in nearby habitats.

Mixed‐stock analysis reveals the migrations of juvenile hawksbill turtles (Eretmochelys imbricata) in the Caribbean Sea

Hawksbill turtles (Eretmochelys imbricata) migrate between nesting beaches and feeding habitats that are often associated with tropical reefs, but it is uncertain which nesting colonies supply which feeding habitats. To address this gap in hawksbill biology, we compile previously published and new mitochondrial DNA (mtDNA) haplotype data for 10 nesting colonies (N = 347) in the western Atlantic and compare these profiles to four feeding populations and four previously published feeding samples (N = 626). Nesting colonies differ significantly in mtDNA haplotype frequencies (ΦST = 0.588, P < 0.001), corroborating earlier conclusions of nesting site fidelity and setting the stage for mixed‐stock analysis. Feeding aggregations show lower but significant structure (ΦST = 0.089, P < 0.001), indicating that foraging populations are not homogenous across the Caribbean Sea. Bayesian mixed‐stock estimates of the origins of juveniles in foraging areas show a highly significant, but shallow, correlation with nesting population size (r = 0.378, P = 0.004), supporting the premise that larger rookeries contribute more juveniles to feeding areas. A significant correlation between the estimated contribution and geographical distance from nesting areas (r = −0.394, P = 0.003) demonstrates the influence of proximity on recruitment to feeding areas. The influence of oceanic currents is illustrated by pelagic stage juveniles stranded in Texas, which are assigned primarily (93%) to the upstream rookery in Yucatan. One juvenile had a haplotype previously identified only in the eastern Atlantic, invoking rare trans‐oceanic migrations. The mixed‐stock analysis demonstrates that harvests in feeding habitats will impact nesting colonies throughout the region, with the greatest detriment to nearby nesting populations.

Origin of Hawksbill Turtles in a Caribbean Feeding Area as Indicated by Genetic Markers

Hawksbill turtles move between nesting colonies and feeding grounds, but in most cases it is not known which reproductive populations occupy a particular feeding habitat. In this study, genetic markers derived from mitochondrial DNA sequences are used to estimate the contribution of Caribbean nesting colonies to a feeding ground at Mona Island, Puerto Rico (n = 41). Maximum likelihood analysis indicates that this feeding population is not composed primarily of turtles from the neighboring nesting colony (also on Mona Island), but is drawn from nesting populations throughout the Caribbean region. A sampled nesting colony in the southern hemisphere (Bahia, Brazil) did not contribute, at detectable levels, to the Mona Island feeding ground. From this evidence, we concluded that hawksbill turtles recruitment to feeding grounds over a scale of hundreds of kilometres, but not over the scale of 7000 km that separate Mona Island from Bahia, Brazil. These data indicate that a hawskbill turtle harvest on feeding grounds will reduce nesting populations throughout the Caribbean region.

Multi-year analysis of stock composition of a loggerhead turtle (Caretta caretta) foraging habitat using maximum likelihood and Bayesian methods

Genetic markers have proven useful for determining which sea turtle rookeries contribute to a particular feeding ground. This information is especially relevant when management concerns include anthropogenic mortality of feeding cohorts, and the suspected presence of endangered populations. One such feeding habitat is the Pamlico–Albemarle Estuarine Complex in North Carolina, which was established as an index area in 1995 to monitor population-specific recovery of sea turtles. Pound nets in the study area were surveyed at random from September–December (1995–1997) to enumerate incidental captures of sea turtles as an index of sea turtle abundance. In this study, we estimated the rookery origins of this feeding cohort using both maximum likelihood and Bayesian based stock analysis programs and compare and contrast these different methodologies. The Bayesian methods appear to yield more realistic estimates of percent contribution to the feeding cohort when information regarding relative population sizes was used. Subsequently, we tested for temporal variation in the frequency of mitochondrial DNA haplotypes and resulting estimates of contribution over a 3-year time span. Mixed stock analysis of the combined data indicated that 80 of the individuals originated from the south Florida nesting population, 12 were from the northeast Florida to North Carolina nesting population, 6 from Yucatan, Mexico, and 2 from other rookeries. Although statistically significant shifts in haplotype frequencies were not observed between the three annual sampling periods, estimates of composition indicated subtle differences in the contributions to this foraging area over the sampling period.

Origin of Green Turtles, Chelonia mydas, at "Sleeping Rocks" off the Northeast Coast of Nicaragua

The largest foraging population of the Atlantic green turtle, Chelonia mydas, occurs in offshore waters of Caribbean Nicaragua (Carr et al., 1978). Green turtles are primarily herbivorous, and the extensive sea grass beds in Nicaraguan waters are utilized both by adults and juveniles (Nietschmann, 1972; Mortimer, 1981; C. Campbell, unpubl. data). Based on tags recovered from adult females, at least two nesting populations contribute individuals to this feeding aggregate: Tortuguero, Costa Rica; and Aves Island, Venezuela (Carr et al., 1978; Sole, 1994). The largest nesting population of Atlantic green turtles is at Tortuguero, which supports an estimated annual nesting of approximately 14,000 females. The second largest nesting colonies in the Caribbean are the Surinam and