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STUDY TITLE: Sea Turtle Tag Study

REPORT TITLE: The movement and habitat associations of sea turtles in the Northern Gulf of 

Mexico

CONTRACT NUMBER: M10PG00088

SPONSORING OCS REGION: Gulf of Mexico

APPLICABLE PLANNING AREA(S): Western Gulf of Mexico, Central Gulf of Mexico, Eastern 

Gulf of Mexico

FISCAL YEAR(S) OF PROJECT FUNDING: 2010, 2011

COMPLETION DATE OF REPORT: August 2015

COSTS: FY 2010: $245,000; FY 2011: $252,000

PROJECT MANAGER(S): Dr. Lance P. Garrison

AFFILIATION of Project manager: National Marine Fisheries Service, Southeast Fisheries 

Science Center

ADDRESS: 75 Virginia Beach Dr., Miami, FL 33149

PRINCIPAL INVESTIGATOR(S): Dr. Lance P. Garrison

KEY WORDS: loggerhead turtle, Caretta caretta, Kemp’s ridley turtle, Lepidochelys kempii, 

movement, habitat use, dive-surface behavior, satellite telemetry tag

BRIEF ABSTRACT: Spatial and seasonal information on dive-surface behaviors of sea turtles 

is critical for deriving spatially explicit models of turtle density from visual survey data to avoid 

biases in presumed habitat relationships that are driven by behavioral differences. Little data 

exists on the dive-surface behaviors, spatial distribution, habitat use, and abundance of 

loggerhead sea turtles (Caretta Caretta) and Kemp’s ridley sea turtles (Lepidochelys kempii), 

both federally listed species under the Endangered Species Act (ESA), in the Gulf of Mexico 

(Gulf). Loggerhead and Kemp’s ridley turtles were tagged at various sites in the Gulf to gather 

data on dive-surface behavior, movement, and habitat use. Data derived from this study will 

inform spatial models and in-water abundance estimates of loggerhead and Kemp’s ridley 

turtles in the Gulf.

BACKGROUND: The loggerhead sea turtle and Kemp’s ridley sea turtle are both federally 

listed under the ESA and found in the Gulf. There are numerous anthropogenic threats to these 

turtles in the Gulf, such as fishery bycatch from commercial fisheries, and activities associated 

with energy exploration and extraction. Lack of information on loggerhead and Kemp’s ridley 

spatial distribution, habitat use, and abundance restricts the ability to assess the effects of these 

threats. 

Visual surveys of turtles can be impaired by perception bias and several negative biases. 

Satellite telemetry tags that report dive-surface behavior are the primary tool for addressing 

negative biases associated with the surface availability of turtles, and can yield insight into 
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movement patterns and habitat use. However, there is few data on turtle dive-surface behaviors 

in the Gulf. To develop accurate abundance estimates of turtle abundance from visual surveys 

in the Gulf, improved information on dive-surface behaviors is needed.

OBJECTIVES: 1) Collect data on dive-surface behaviors of loggerhead and Kemp’s ridley 

turtles in continental shelf waters of the Gulf through satellite telemetry tagging; 2) Obtain 

information on movement and habitat use, particularly spatial and temporal changes in dive 

behavior.

DESCRIPTION: These study goals were accomplished using satellite tag telemetry on 

loggerhead and Kemp’s ridley turtles. Tags were deployed on individuals captured using a 

variety of methods associated with sampling efforts by collaborating researchers at various sites 

in the Gulf. Late juvenile and adult sea turtles were focused on. In-water captures of sub-adults 

were conducted to provide information about the behaviors and movement patterns of these life 

history stages. Data collected during this study will add to the knowledge of movement patterns 

and important habitats for sea turtles in the Gulf, and will be used to support accurate and 

precise abundance estimates from visual line transect surveys.

SIGNIFICANT CONCLUSIONS: Loggerhead turtles in this study were typically found in shallow 

water in late spring-early summer, then migrated into deeper water for fall/winter months, 

contrary to Kemp’s ridley turtles. In combination with prior studies, data collected during this 

study suggest that a broad range of habitats over the continental shelf serve as important 

foraging habitats for loggerhead turtles which are resident in limited areas for periods of several 

months, primarily during cooler water periods from fall through winter. The dive-surface 

behaviors for both loggerhead and Kemp’s ridley turtles indicated important seasonal, diurnal, 

and spatial effects on the time available at the surface. In loggerhead turtles, the spatial and 

seasonal variation represents the shift in habitats and behavioral modes across seasons with 

animals moving into deeper waters and spending progressively less time at the surface during 

cooler months. For Kemp’s ridley turtles, there was a significant interaction between season and 

day indicating that the diurnal effects were different among the different seasons.

STUDY RESULTS: During the active periods of the tags, loggerhead turtles generally moved 

from nearshore waters to offshore waters and occupied waters over the inner continental shelf. 

Average duration for loggerhead turtle deployments was 210.9 days (SD = 165.7) with a range 

of 51 to 520 days. The mean total distance travelled was 2,350 km with an average speed of 0.9 

km/hr. Loggerhead turtles did not move into the western Gulf during the tag deployments, nor 

did any animals leave the Gulf or move into deeper, offshore waters. 

The movements of Kemp’s ridley turtles were typically more limited than those of loggerhead 

turtles, though that may be a function of overall shorter tag durations. The average duration for 

Kemp’s ridley tags was 63.3 days (sd = 57.0) with a range of 8 to 189 days. The mean total 

distance travelled was 619.5 km with an average speed of 1.0 km/hr. Animals tagged in the 

eastern Gulf largely remained in nearshore waters relatively close to their release site. In the 

western Gulf, animals likewise stayed in nearshore coastal waters and moved along the Texas 

coast.

There appear to be two aggregation regions for loggerhead turtles in the eastern Gulf. The 

position counts within 5x5 km grid cells indicate discrete areas on the outer portion of the central 

west Florida continental shelf and in shallower waters just south of the Florida panhandle with 

high counts of turtle locations. These areas reflect the areas that turtles migrated to and then 

stayed within a localized area for long potions of their tag deployments. In addition, the region 
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south of Mississippi Sound in the north-central Gulf also seemed to be an aggregation area with 

high numbers. In contrast to the tagged loggerhead turtles, Kemp’s ridley turtles had more 

restricted use areas, particularly in the eastern Gulf. Counts of locations in grid cells show that 

animals had very limited movement ranges and were restricted to nearshore waters close to the 

tagging locations.

Examination of the data from loggerhead turtles indicated that there were three groups of 

animals based upon their average dive behavior patterns termed, “shallow”, “intermediate” and 

“deep” animals. The “shallow” animals were generally restricted to dive depths of less than 20m. 

“Intermediate” depth animals tended to have a more bimodal average dive behavior with peak 

rates in the 20–30m depth range and in the 40–50m depth range. “Deep” animals had dive 

records dominated by depths of 40m or greater. The depth behavior groupings are related to the 

spatial distribution and habitat use of the turtles during their tag deployments. The “shallow” 

group turtles included one animal with a restricted range off of the Florida panhandle and two 

animals from the north-central Gulf that remained in shallow waters south of Mississippi Sound 

throughout their deployment period. The “intermediate” depth turtles included animals that had 

some portion of their distribution in nearshore water and then moved to establish restricted area 

movements in intermediate depths. The “deep” loggerhead turtles, in contrast, spent relatively 

little of their deployment period in nearshore shallow waters and instead moved onto the outer 

continental shelf.

For those turtles that had long deployments and had intermediate depth behaviors, there was a 

common seasonality to their movement patterns and depth behaviors. In general, animals 

remained in nearshore waters during the warmest months of the year (June–July), and had high 

percentages of their time in depths of 20m or less. Typically, during August these animals 

transitioned from nearshore habitats to habitats further offshore. During this transition period 

when the turtle is travelling, the proportion of time spent in relatively shallow depths remained 

high. During the fall and winter months, turtles typically occupied deep water habitats and spent 

a large portion of their time (>80%) at depth.

Unlike loggerhead turtles, the Kemp’s ridley turtles tagged in this study had less variability in 

habitats and movement patterns, and therefore less variation in depth behaviors. The turtles 

remained in shallow waters and spent the majority of their time in depths less than 20m. The 

Kemp’s ridley turtles occurring in waters off Mississippi Sound and in Southwest Florida spent 

the majority (typically >70%) of their time in water depths from 0–10m, while those in Texas and 

the Florida Panhandle more evenly split their time between the 0–10m and 10–20m depth bins. 

Turtles occupying waters in coastal waters of Texas occurred more frequently in deeper bins 

than those within estuarine habitats. Along the Texas coast, where some individuals undertook 

larger scale movements, there was evidence for seasonal migration from several individuals. In 

general, animals moved further south to waters off of Southern Texas and Mexico during late 

winter (January–March) and remained further north during the spring-fall.

The overall average percentage of time spent at the surface for loggerhead turtles was 11.45% 

(95% confidence interval: 8.59–14.31). However, there was significant variation in the 

percentage time at the surface for each of the factors tested in this analysis. Most notably, there 

were important seasonal effects with a lower time at the surface during winter months compared 

to the other seasons. In addition, day compared to night effects were important in some 

seasons, with higher average time spent at the surface during night. Finally, the average time at 

the surface was generally higher in shallow waters compared to deep waters. There was limited 

ability to evaluate regional differences due to small sample sizes. There were insufficient 

samples to estimate means and variances particularly in the central region, and few records in 
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the “deep” stratum for the central area. Therefore, data were collapsed across regions for 

subsequent analyses.

The overall average percentage time at the surface for Kemp’s ridley turtles was 15.14% (95% 

confidence limit: 10.44–19.84%). Initial analyses indicated generally higher availability at the 

surface during daylight hours compared to night (average 17.00% compared to 10.54%). The 

seasonal pattern was weaker than that observed in loggerhead turtles, and there was little 

overall difference in the mean time at surface between the western and eastern regions. There 

were limited numbers of samples in winter months, particularly in the western region. Data were 

collapsed across regions in subsequent analyses.

STUDY PRODUCT(S):

Garrison LP, Glenn III DW, Karrigan H. 2020. The movement and habitat associations of sea turtles in 

the Northern Gulf of Mexico. New Orleans (LA): Bureau of Ocean Energy Management. Contract 

No.: M10PG0008. OCS Study BOEM 2020-010. 71 p.