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Tidal and Lunar Correlates on Sea Turtle Emergence Patterns in Ada Foah, Ghana

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Date Issued:
2014
Abstract:
Environmental cues that trigger beach emergences by female sea turtles are still not fully understood. It is important to understand these cues because they can influence nest success and hatchling phenotypes. Previously, researchers have relied on local knowledge to aid their understanding of sea turtle biology. In this paper, we use four years of nesting data in Ghana to assess the accuracy of local knowledge regarding patterns of sea turtle nesting emergences in Ada Foah, Ghana. The Ada area contains approximately 15 km of beach used by nesting leatherback (Dermochelys coriacea) and olive ridley (Lepidochelys olivacea) sea turtles. Conversations with area residents indicated a commonly held view that sea turtles emerge to lay eggs when the moon is closer to full and is positioned in the western sky. Residents also indicated sea turtles require a high tide to emerge onto the beach. To determine the accuracy of their assertions, we conducted nesting surveys from August 2006 through February 2010 to examine if moon and tidal variables are related to sea turtle emergence patterns. Of the 1,509 emergences recorded in Ada Foah, 610 sea turtles were directly observed and used for analyses. All data were converted to a more appropriate circular format and each dataset was tested for significance using either a Rayleigh test or a Hodges-Ajne test. The analyses show a preference for low tide emergence in both D. coriacea and L. olivacea. Looking at the lunar cycle, D. coriacea show a cyclical pattern with a drop in emergences near the full moon, while L. olivacea emergences are skewed toward the waning half of the cycle as well as the darkest 10 days. Neither species under study had a preference for a particular moon altitude, but for moon azimuth L. olivacea are highly concentrated in the eastern quadrant of the sky. A goodness-of-fit test showed that D. coriacea emergence patterns are distributed based on the moon's normal circuit, but the patterns for L. olivacea are not. While the majority of specific predictions by local Ghanians were not accurate, these predictions are likely based only on anecdotal encounters with sea turtles. However, the general predictions of a preference for environmental variables were correct. Because of this, especially for the moon azimuth variable, which has not been studied previously, this study validates the importance of using traditional ecological knowledge in formulating research questions.
Title: Tidal and Lunar Correlates on Sea Turtle Emergence Patterns in Ada Foah, Ghana.
Name(s): de Witt, Derek Ward
Type of Resource: text
Issuance: single unit
Date Issued: 2014
Physical Form: Dissertation
Extent: 57 pgs.
Language(s): English
Abstract: Environmental cues that trigger beach emergences by female sea turtles are still not fully understood. It is important to understand these cues because they can influence nest success and hatchling phenotypes. Previously, researchers have relied on local knowledge to aid their understanding of sea turtle biology. In this paper, we use four years of nesting data in Ghana to assess the accuracy of local knowledge regarding patterns of sea turtle nesting emergences in Ada Foah, Ghana. The Ada area contains approximately 15 km of beach used by nesting leatherback (Dermochelys coriacea) and olive ridley (Lepidochelys olivacea) sea turtles. Conversations with area residents indicated a commonly held view that sea turtles emerge to lay eggs when the moon is closer to full and is positioned in the western sky. Residents also indicated sea turtles require a high tide to emerge onto the beach. To determine the accuracy of their assertions, we conducted nesting surveys from August 2006 through February 2010 to examine if moon and tidal variables are related to sea turtle emergence patterns. Of the 1,509 emergences recorded in Ada Foah, 610 sea turtles were directly observed and used for analyses. All data were converted to a more appropriate circular format and each dataset was tested for significance using either a Rayleigh test or a Hodges-Ajne test. The analyses show a preference for low tide emergence in both D. coriacea and L. olivacea. Looking at the lunar cycle, D. coriacea show a cyclical pattern with a drop in emergences near the full moon, while L. olivacea emergences are skewed toward the waning half of the cycle as well as the darkest 10 days. Neither species under study had a preference for a particular moon altitude, but for moon azimuth L. olivacea are highly concentrated in the eastern quadrant of the sky. A goodness-of-fit test showed that D. coriacea emergence patterns are distributed based on the moon's normal circuit, but the patterns for L. olivacea are not. While the majority of specific predictions by local Ghanians were not accurate, these predictions are likely based only on anecdotal encounters with sea turtles. However, the general predictions of a preference for environmental variables were correct. Because of this, especially for the moon azimuth variable, which has not been studied previously, this study validates the importance of using traditional ecological knowledge in formulating research questions.
Identifier: de Witt_fgcu_1743_10055 (IID)
Note(s): Degree Awarded: Master of Science
Subject(s): Africa
Ghana
nesting
reproduction
sea turtles
Persistent Link to This Record: http://purl.flvc.org/fgcu/fd/de Witt_fgcu_1743_10055
Owner Institution: FGCU