Sea Turtle Movement and Habitat Use in the Northern Gulf of Mexico
Science Center Objects
The Bureau of Ocean Energy Management (BOEM) uses trawling to capture and relocate live sea turtles away from dredging locations to minimize the risk of turtle entrainment. These incidental turtle captures provide a unique opportunity to fill critical data gaps for difficult to capture life-stages of marine turtles.
The Science Issue and Relevance: The Bureau of Ocean Energy Management (BOEM) uses trawling to capture and relocate live sea turtles away from dredging locations to minimize the risk of turtle entrainment. These incidental turtle captures provide a unique opportunity to fill critical data gaps for difficult to capture life-stages of marine turtles. Determining distribution, seasonal movements, vital rates and habitat use for all life-stages of marine turtles has been identified by the U.S. Fish and Wildlife Service (USFWS) and the National Marine Fisheries Service (NMFS) as a major action required to achieve recovery for these imperiled species (NMFS and USFWS 1991). The green turtle recovery plan (NMFS and USFWS 1991) also states that “to adequately protect and enhance survival of sea turtles, we must know where they occur, in what numbers, at what times, and what factors contribute to mortality.” Because turtles come ashore to nest, the adult (specifically, female) life-stage is most easily studied; however, much less information is available about habitat use and movements of sub-adults and juveniles. Both recovery plans for the endangered Kemp’s ridley and threatened loggerhead sea turtles call for the establishment of index in-water sites to monitor population abundance, trends, and demographic parameters of these life stages (NMFS and USFWS 2008, NMFS et al. 2011). Only recently has information become available about sea turtle high-use areas along the Gulf of Mexico (GOM) coast (Hart et al. 2013, Shaver et al. 2013, Foley et al. 2014, Hart et al. 2014). However, fine-scale information on dive profiles is still lacking for sea turtles in the GOM; such information can provide key data on an individual’s time spent in various portions of the water column. This project directly addresses those recovery and protection goals and provides information on in-water aggregations of sub-adult, juvenile, and adult marine turtles in the GOM.
Methodology for Addressing the Issue: We use trawling operations to opportunistically tag sea turtles and collect and analyze biological samples to inform management decisions related to trawling and dredge operations. Tracking sea turtle movements can provide data needed to validate the distance required for turtle relocation which could help reduce project costs and prevent multiple captures of the same turtle. This information can also inform other program areas such as decommissioning of oil platforms (by evaluating dive times) and optimization of current sea turtle visual and aerial survey efforts by using data gathered on time spent in the upper two meters (m) of the water column.
The data generated from this study are expected to provide information on sea turtle use of discrete foraging areas, genetic origin of sea turtles sampled at foraging areas, general sea turtle distribution patterns across space and time, and sea turtle dive profiles and patterns.
Future Steps: To date we have captured, marked, and satellite tagged 26 individual marine turtles in the Northern Gulf of Mexico (10 Kemps, 16 loggerheads, see http://www.seaturtle.org/tracking/index.shtml?project_id=1205). We continue to collect data to characterize dive profiles and areas used by sea turtles throughout the year. We are identifying and assessing physical and biological features of specific high-use habitats, especially those that overlap with proposed BOEM dredging sites.
Determining the extent of turtle movements and seasonal site fidelity among habitats is a long-term project goal. We are assessing the population structure of sea turtles through genetic analyses, and assessing resource use by evaluating the isotopic signatures of blood and tissues collected from tagged individuals. This work lays the foundation for a long-term capture-mark-recapture study to determine turtle abundance and distribution in critical segments of the GOM study area. Dive profiles will be made available to link to the GOMMAPPS project to assist with determining aerial correction factors (ACFs), or the proportion of time each individual spends in the upper 2 m of the water column.