There is a growing need by marine scientists and managers for acquiring direct evidence of how various environmental conditions affect the behavior of marine life.
Tracking Sea Turtles: New Data-Collection Techniques Provide Fine-Scale Data
This article is part of the June-July 2020 issue of the Sound Waves newsletter.
As global sea conditions change and the potential for human-caused environmental crises increases, there is a growing need by marine scientists and managers for acquiring direct evidence of how various environmental conditions affect the behavior of marine life. Although there are multiple approaches to collecting oceanographic data, animal-borne data loggers allow for monitoring immediate behavior and physiology of marine animals. Hard-shelled turtles are ideal subjects for carrying such devices due to their large size, extensive migratory movements, and need to regularly surface for air. Their carapace also provides an ideal substrate for affixing long-term tags with epoxy, as opposed to suction cup or intramuscular attachment methods used on marine mammals and fishes, respectively. One complication is that these high-resolution data loggers must be recovered from the animals.
A “popoff” package that includes a float, a high-resolution Acceleration-Depth-temperature data Loggers (ADL), and a radio tag was deployed and retrieved on threatened and endangered sea turtles as part of a cooperative study between USGS and the Bureau of Ocean Energy Management (BOEM). Researchers are using advanced datalogging techniques to acquire novel data on dive profiles and activity patterns of sea turtles to gain detailed insights into turtle physiology and behavior. Some of the turtles were also equipped with satellite tags for accurate tracking over time after release of the popoff package. This additional location information is useful for discerning whether a turtle is a “resident” at the site where behavior was logged at very fine scales or if, instead, it was migrating through the area. Together, this detailed information enhances our overall understanding of sea turtle behavior, migration, and habitat use. This detailed information about sea turtles has been elusive and difficult to obtain until now.
The Popoff Accelerometer Datalogger (ADL)
The device is a custom-designed tagging package that is affixed to the turtle for a specific period and then “pops” off and floats at the surface for retrieval by scientists.
The “popoff package” consists of a microsphere and polyurethane epoxy float with a satellite/VHF transmitter and datalogger encased along with a base plate. The team designed various package options (for a range of sea turtle sizes) with the goals of reducing size and drag while maintaining buoyancy and float angle to allow for tag relocation and recovery. The design of the tag package is tailored specifically for use on hard-shelled turtles. While affixed to the turtle, the tag collects second-by-second data, giving scientists detailed information on turtle activity.
“We can now know—second by second—what marine animals are doing in specific locations,” said USGS Research Ecologist Kristen Hart.
“It’s exciting to be able to glimpse into the daily life of these animals and see how they respond to human activities,” said USGS Research Biologist Margaret Lamont. Lamont has been tracking turtles through important life stages; and due to the tags and collaborative work with colleagues, the team has made extraordinary discoveries about turtle migratory patterns.
USGS scientists and partners at the Anderson Cabot Center for Ocean Life at the New England Aquarium conceived of, designed, tested, deployed, and successfully retrieved two popoff ADL packages on sea turtles in the Gulf of Mexico: one in the Dry Tortugas and one in the northern Gulf of Mexico. The second-by-second data stream logged in the accelerometers revealed patterns of diving, surfacing, and resting for 48 and 12 hours, respectively, for two imperiled turtles. The information can transform our understanding of sea turtle diving and activity patterns with implications for management of human activities affecting these imperiled species.
“Just a few years ago, the idea that we’d be able to tell how many seconds an animal spends on the bottom during a dive, how many flipperbeats it took while it was there, and what its body angle was seemed impossible. We can now answer all of those questions and more with this technology,” said Biologist Nick Whitney of the New England Aquarium.
In June 2018, the first popoff ADL package was tested on a loggerhead sea turtle (Caretta caretta) in South Florida. The ADL was on the turtle for 32 hours in Dry Tortugas National Park, FL, with Nick Whitney (New England Aquarium), Andrew Crowder, and David Roche (USGS). On June 28th, the ADL package was deployed (with a 2-day corrosive link release) on a subadult loggerhead sea turtle. Two days later, Whitney led the recovery of the ADL package via VHF receiver, antenna, and headphones in just under 48 hours after release of the turtle.
In September 2019, the second deployment took place in the Gulf of Mexico. It was shorter, and the ADL package was applied instead on “Finley” a female endangered Kemp’s ridley sea turtle (Lepidochelys kempii)/ The ADL stayed on the turtle for 12 hours and 6 minutes. The turtle displayed strong and consistent flipper beats and spent the first 45 minutes after release actively swimming. After this period, the individual displayed consistent resting “U-Dives” to 6 meters of depth. Each dive lasted about seven minutes in duration followed by about 45 seconds at the surface. As the night progressed, the turtle displayed active and resting dives. The longest dive that the team observed was a resting dive at 10 a.m. (EST) that lasted 25 minutes. The detailed data from the ADL combined with satellite tagging, provides clues that will aid in managing for her species and others.
Scientists can use the acquired data to assess fine-scale behaviors of tagged turtles to assess their use of the water habitat around dredge sites that are active, inactive, or previously dredged. Sea turtles are at highest risk when they are close to active dredging operations. By knowing when and where turtles spend more time near the bottom, especially as related to the location of dredging operation intakes, the dredging operators can implement best practices to avoid the intake of turtles. All sea turtles are imperiled species, so reducing injuries and fatalities by dredging operations is a joint stewardship improvement goal for multiple agencies. The information on sea turtle behavior in the water column is particularly important for BOEM officials tasked with permitting offshore dredge operations. Additionally, the acquired data will be useful for informing BOEM management decisions associated with Protected Species monitoring, decommissioning activities, and mitigation best practices.
Looking ahead, Hart and USGS colleagues plan to deploy popoff packages on sea turtles in the northern Gulf of Mexico in summer 2020 to collect fine-scale dive and activity data for up to four days on each turtle. Information collected at 1 Hz (second by second) frequency over four days is a lot of data, but this detailed information gives scientists a much better idea about turtle activity patterns throughout the course of a day, which can be translated to when they are more likely to be on the bottom foraging or other activity. This kind of information will be useful to improving dredge site operations and permitting strategies.
All work was conducted under NMFS permits 17381 and 17304-3, MTP-17-176, and DRTO-2018-SCI-0007. The ongoing study was funded in part by the U.S. Department of the Interior and BOEM through Interagency Agreement M19PG00003 with the USGS.