Assessments of surface-pelagic drift communities and behavior of early juvenile sea turtles in the northern Gulf of Mexico

• Main Author: Hardy, Robert F.
• Format: Others
• Published: Scholar Commons 2014
• Subjects: critical habitat; developmental habitat; Landsat; remote sensing; Sargassum; satellite telemetry; Marine Biology; Oceanography
• Online Access: https://scholarcommons.usf.edu/etd/5367; https://scholarcommons.usf.edu/cgi/viewcontent.cgi?article=6561&context=etd

Knowledge of species distribution and habitat associations are essential for conservation measures. Such information is lacking for many marine species due to their occupancy of broad and ephemeral habitats that are difficult to access for study. Sea turtles, specifically the surface-pelagic juvenile stage of some species, are a group for which significant knowledge gaps remain surrounding their distribution and habitat use. Recent research has confirmed the long-standing hypothesis that the surface-pelagic juvenile stage occurs within surface-pelagic drift communities (SPDC). Within the North Atlantic and surrounding basins, the holopelagic macroalgae Sargassum spp. dominates SPDC and serves as a remotely-detectable indicator of SPDC. The present study focuses on surface-pelagic habitats of four sea turtle species and addresses knowledge gaps using two approaches: habitat mapping and behavioral examination. Remote sensing techniques were used to identify SPDC, and satellite telemetry to examine behavior. This work was conducted in three parts and is presented in three chapters. Imagery collected from the Landsat satellites (5 and 7) was used to quantify the area of SPDC (km2). Approximately 1,800 Landsat images collected from 2003–2011 were examined for SPDC. The first chapter discusses the abundance, seasonality, and distribution of SPDC within the eastern Gulf of Mexico waters where surface-pelagic green, hawksbill, Kemp’s ridley, and loggerhead turtles are known to occur. SPDC was found year-round within the eastern Gulf of Mexico, and the amount of habitat peaked during summer months. The amount of SPDC within the eastern Gulf of Mexico varied annually with peaks in 2005, 2009, and 2011. High concentrations of SPDC were discovered within offshore waters of the northeastern Gulf of Mexico and southern West Florida Shelf. Within the second chapter, the behavior of 10 surface-pelagic juvenile Kemp’s ridleys was examined using satellite telemetry. Using remotely-sensed imagery, the sea surface habitats used by tracked turtles were examined. Surface-pelagic juveniles are hypothesized to be principally passive drifters. The behavior of tracked turtles was examined to determine if they exhibited periods of active and passive behavior, which may indicate periods of swim and drift. The proximity of tracked turtles to remotely-detected SPDC was examined when coincident Landsat imagery was available (within one day of the turtle’s position). Turtles were tracked for 36.5 days (mean) and exhibited primarily passive behavior during the tracking period. The satellite transmitters messaged frequently and reported temperatures significantly higher than sea surface temperatures. Landsat imagery was available coincident to the tracks of nine individuals. SPDC was present within 74% of images, and the mean distance between tracked turtles and SPDC was 54 km. Close associations between tracked turtles and SPDC were documented for four individuals. Results suggest that the tracked turtles spent a majority of the time drifting within SPDC. The final chapter discusses the density of SPDC within northern and western Gulf of Mexico waters from 2009–2011. Seasonal abundance peaks occurred throughout the study area, but the timing varied. SPDC peaked earlier (late spring) within the northwestern Gulf of Mexico. Moving eastward, the timing of seasonal peaks shifted progressively later during the year. Within the western portions of the study area, SPDC was found to be significantly higher than in the eastern Gulf of Mexico. The eastern Gulf of Mexico may provide critical developmental habitats for several North Atlantic sea turtle species. Additional study is necessary to determine if portions of the western Gulf of Mexico could serve in a similar capacity. SPDC is extremely vulnerable to anthropogenic impacts, specifically oil spills and the occurrence of persistent marine debris. Conservation of SPDC may be challenged by its ephemeral nature; however, the results presented herein could advise conservation efforts (e.g., delineation of critical habitat). The present study described spatial patterns of SPDC occurrence, regions of high abundance, and seasonality. The description of the behavior surface-pelagic sea turtles offers refinements to the spatial distribution of this life stage. These results, coupled with information on circulation patterns and the distribution of sea turtle nesting beaches, can be used to better predict when and where sea turtles and SPDC may be found. For example, the year-round persistence of SPDC within the eastern Gulf of Mexico and the location of major nesting beaches located upstream support the area’s designation as critical habitat for surface-pelagic green, hawksbill, Kemp’s ridley, and loggerhead turtles.