Master of Science
Frank V. Paladino
Date of Award
The East Pacific green turtle (Chelonia mydas agasizzi) is a sub-population of the widely distributed green turtle (Chelonia mydas). Like all sea turtles, East Pacific green turtles have a type III survivorship curve, which is characterized by long-lived adults that have a low mortality rate and high reproductive output with a low hatchling survival rate. For this to be successful, the adults must live through multiple reproductive seasons, and in the Eastern Pacific, there is high mortality on adult East Pacific green sea turtles. The continued success of this distinct population relies on protection during key in water movements: the nesting season and migrations from foraging grounds to nesting beaches and back. Management techniques need to be developed on a site-specific basis so it is crucial to understand the specific habitat needs for each nesting population as defined by the local oceanography. I used satellite telemetry to map movements of Pacific green turtles nesting on Playa Cabuyal, Costa Rica to understand the temporal and physical distribution of turtles both two and three dimensionally during the inter-nesting period and post-nesting migrations to foraging grounds.
I deployed ten satellite transmitters across two nesting seasons, 2012-2014, six SPOT5 transmitters and four MK10 transmitters. The sample size for this study included 11 inter-nesting turtles and four post-nesting migrations (two post-nesting turtles were also tracked during their nesting season), curved carapace length ranged from 82.2 to 91.6 cm (mean ± SD = 85±2.84 cm) while curved carapace width ranged from 76 to 90 cm (mean ± SD = 79.5±3.80 cm). The observed inter-nesting period was between 7 and 17 days (mean ± SD, 13.1±2.5 days), which is comparable to the mean of 15.4 days observed as an average inter-nesting interval for turtles nesting on this beach. Post-nesting turtles moved over a period of 19 to 189 days (107.25±90.77 days) with one resident of the Gulf of Papagayo and three that migrated an average of 500 km away from the nesting beach.
During the inter-nesting period turtles spread out across the Gulf of Papagayo and, in some cases, migrated out of the gulf and along the coast before returning to nest. The minimum convex polygon (MCP) with percent area use contours indicates that the highest use areas were close to the beach (within 10 km) and a couple isolated areas off the coast in the southern part of the gulf. Overall, this high use inter-nesting area totals 27 km2 and represents the high density twenty-five percent (75% of all positions received) of recorded location data during their movements between nests. Inter-nesting dive behavior indicated that, on average, fifty-five percent of the dives recorded were in the top 15 m of the water column, and sixty-six percent of inter-nesting dives lasted 30 minutes or less. Overall, ninety percent of the time inter-nesting turtles were within 15 m of the surface even though the ocean floor is generally 25 m or deeper throughout the Gulf of Papagayo. The oceanographic characteristic that limited turtles’ dive behavior was the water temperature. The temperatures experienced at varying depths changed as the nesting season progressed showing a significantly shallower thermocline in the spring months when compared to the winter months of this study. In December and January the temperature at the surface temperature was 28 °C and stayed above 25 °C to depths of 25 m. In February and March the surface temperature was 25 °C and at 25 m the temperature had already dropped below 20 °C. Turtle behavior changed to reflect this shift in the water temperature with more time spent in the Surface - 5 m depth bin during February – March as compared to December when the waters at depth were warmer.
Post-nesting turtles took up residence in locations along the Costa Rica, Nicaragua, El Salvadoria and Guatemala coasts; a pattern similar to other Pacific green turtles nesting more to the South along the Pacific coast of Costa Rica. During migration, the turtles remained within 50 km of the coastline, which allowed them to stay in shallow warmer coastal waters. Dive behavior of these post-nesting turtles shows a bimodal distribution in depth use not previously described for this sub-population, with peak dive depths of 11 to 15 m and 46 to 50 m. This could be indicative of foraging while migrating. Currents are one of the most important factors in migration routes because they determine hatchling dispersal and locations of primary productivity. Chlorophyll distribution was correlated to the post-nesting movements of one turtle.
Conservation efforts should focus on regulating the fishing efforts in the area of inter-nesting habitats and migratory corridors because by-catch mortality pressure on adults is currently the biggest threat to the population. By providing the local fisheries with depth integration levels and dates of passage, the set of nets and long lines could be below these normal behaviors and reduced during migration dates to reduce bycatch and fisheries interactions. Fishing regulations need to be enforced and regulated locally on site-by-site bases, eliciting the help of each country and community. Future work with this inter-nesting and post-nesting data will be to analyze the turtle interaction points with local and international fisheries in hopes of generating a management strategy through cooperation along the Eastern Pacific.
Chelsea E. Clyde-Brockway (2014).
Inter-Nesting and Post-Nesting Movements and Behavior of East Pacific Green Turtles (Chelonia mydas agassizii) from Playa Cabuyal, Guanacaste, Costa Rica.