Identifying brumating Northern Diamondback Terrapins (Malaclemys terrapin terrapin) by incorporating environmental sensor and drone technology

The Northern Diamondback Terrapin (Malacleyms terrapin terrapin) (Schoepff 1793) is a temperate turtle species that is found along the Atlantic coasts in estuarine systems. (Coker and Ashe 1906; Palmer and Cordes 1988; Brennessel 2006). Terrapins are semi-aquatic and rely on a healthy marsh and a nearby brackish water ecosystem for most of their life cycle for foraging, basking, nesting, and brumation (Burger 1975; Hart and Lee 2006; Tulipani 2013). Juveniles (0 – 3 years) mature while hidden within the high marsh grasses (Burger 1976; Coleman et al. 2014) and forage on small invertebrates within shallow waters (Selman 2018). Most of the adult terrapin activity data along the Atlantic Coast has been collected from coastline population assessments during the nesting season, including mark and recapture studies, trapping, and monitoring surveys (Burger and Montevecchi 1975; Montevecchi and Burger 1975; Brennessel 2006, Selman and Baccigalopi 2012; Budd et al. 2024). Knowledge gaps for this species still remain, especially regarding the locations and activities for the juvenile stages (Lovich et al. 2001) and also the winter brumation locations for adult terrapins (Brennessel 2006).

Brumation is a period of dormancy that is initiated by extended cold water temperatures and allows for the conservation of energy resources by slowing down reptilian metabolism. (Coker and Ashe 1906; Coker 1920; Yearicks et al. 1981; Brennessel 2006). Terrapins begin brumation in early winter depending on water temperature, which can vary latitudinally (Coker and Ashe 1906; Hildebrand 1929; Carr 1952; Brennessel 2006). In northern latitudes, brumation lasts from October until late April (Brennessel 2006). In New Jersey, Yearicks (1981) reported that terrapins will undergo brumation from mid-to-late November until April or May. Farther south, terrapins experience a shorter brumation period, which begins in late October but ends in April in Maryland, March in North Carolina, and in February in Florida (Coker and Ashe 1906; Brennessel 2006).

Terrapins entering brumation will bury themselves into or atop of embankment surfaces and undercuts, or beneath the water (Yearicks et al. 1981). They can be found in clusters or singularly within shallow waters of (1.5 – 2.5 m) (Yearicks et al. 1981), below the lower tidal zone (Brennessel 2006), or in the deep waters of creeks (Hart and Lee 2006), where they burrow into depressions along the floor of the mud substrate (Yearicks et al. 1981; Brennessel 2006). Terrapins will remain inactive until the water temperature warms again (Yearicks et al. 1981) at around 13°C, (Brennessel 2006). Little is known about specific brumation conditions regionally because the terrapins are difficult to detect while they are buried in mud. Their immobility and aggregating during the brumation period makes them vulnerable to poachers (Haramis et al. 2011), or disturbance by dredging (Castro-DeSantos et al. 2019; Dickerson et al. 2004, Brennessel 2006).

The detection and protection of terrapin hibernacula is often challenging and labor intensive, however, because they are either inaccessible or located generally farther from the terrapin’s summer home range (Hart and Lee 2006; Castro-Desantos et al. 2019, Lamont et al. 2021). Techniques for capturing terrapins during the brumation period, such as scrape fishing, snorkeling, SCUBA retrieval and cast or dip nets are also tedious, require skill, or are not efficacious (Haramis et al. 2011; Roosenburg and Burke 2018). Consequently, brumation areas are not well known in most terrapin populations across their range.

Unmanned aerial vehicles (UAV) or drones, are used extensively worldwide as a tool for obtaining species behavior and population data (Harden et al. 2009; Elsey 2016). When coupled with artificial intelligence, UAV can now assist in quickly detecting and tracking at-risk species and increase conservation efforts (Gonzalez et al. 2016). In the current study, our purpose was to increase our understanding about terrapin movement and habitat surroundings during brumation by using environmental sensor and drone technologies. We first attempted to establish a range of conditions that terrapins experience during brumation by adhering temperature-depth environmental sensors on female terrapins with a home range within Barnegat Bay, New Jersey, USA. Under similar temperature-depth brumation conditions that were recorded by our sensor, we then determined environmental conditions for identifying brumation habitats by navigating a drone from a land based position to find brumating terrapins in a barrier island’s cove within the Barnegat Bay - Little Egg Harbor Estuary (BBay-LEH), New Jersey, USA. By incorporating specific water temperature-depth information from the sensor with the environmental data from drone surveys, we hoped to identify conditions and habitats for terrapin brumation within the estuaries of Barnegat Bay, New Jersey.

Increasingly, researchers rely more frequently on new and advanced technologies to study and protect at-risk species. Our research allowed for the surveillance of brumating diamondbacks during a vulnerable period and often unknown habitats of their life cycle. By using an unmanned aerial vehicle (drone), we were able to easily, quickly and effectively identify brumating terrapins in a location that showed a high density of terrapin activity during the post-brumation period. This methodology avoids the difficulties that Harden et al. (2007) experienced with transmission and signal strengths of the radio tracking sensors that were used to monitor terrapin movement and habitat temperatures. Our methods are also less labor intensive than conducting visual population head-count surveys (Harden et al. 2009; Levasseur et al. 2019) or randomly probing the bottom of creeks to find brumating terrapins (Yearicks et al. 1981). Knowing the exact location of a terrapin brumation area can be particularly important, especially when estimating the threat and disturbance to a habitat due to restoration or dredging projects. In Massachusetts, Castro-DeSantos et al. (2019) was able to assess associated risks to terrapins from harbor dredging by detecting their movement to and from their summer sub-drainage habitats as they passed through fixed-receiver arrays. Although this process identified harbor dredge zones that were less likely to contain brumating terrapins, the exact brumation locations could not be determined by using this methodology.

We were also able to identify corresponding external conditions of water depth and temperature during brumation from an environmental sensor that had been affixed to the carapace of an adult female terrapin. We found that the temperatures that were collected by our environmental sensor during brumation aligned with the bay water temperatures for our drone surveys and also support a non-active brumation state when they are compared to the water temperatures for active feeding to occur in other turtle species. Ernst (1976) reported a minimum foraging water temperature of 14°C for spotted turtles (Clemmys guttata) in Pennsylvania. However, the average water feeding temperature for males was 18.47°C and 19.93°C for females. A 15°C minimum feeding water temperature was determined by Avery et. al (1993) for juvenile slider turtles (Trachemys scripta) in captivity. Future studies should focus on latitudinal variations in the brumation period.

Our environmental sensor temperatures also conform to the minimal water temperature of 13.90°C that we set for our methodology parameters. Our findings are in agreement with water temperatures reported for other brumation studies (Yearicks et al. 1981; Williard and Harden 2011; Akins et al. 2014), as well as the minimal brumation 13.00°C temperature that was cited by Brennessel (2006) for the Massachusetts area. The 13.78°C temperature reading that was recorded during one of our drone surveys was most likely due to slightly warmer temperatures during the previous day, which was then followed by the return of colder temperatures the next day (USGS, 2023a). Since we identified terrapin on the bay floor in the same location as the previous survey, and because the cove’s water did not experience an extended warming, this provided us with the evidence that brumation conditions still existed. According to the United States Geological Survey (USGS) for the Little Egg Harbor Inlet (USGS 2023b), there was a shift toward gradually warmer temperatures that began five days after our last drone survey. Water temperatures then ranged from a low of 11.00°C to a high of 17.50°C and with each day’s temperatures well above 13.00°C, which may have roused the terrapins from their brumation state.

Our drone monitoring also allowed us to find terrapins nestled in the shallow water (< 1 m) at the bottom of the cove’s floor, which was supported by the water depth data that was collected from our environmental sensor. These findings are in agreement with the brumation water depths reported by Yearicks et al. (1981) for creek bottoms at low tide along the Atlantic Ocean side of the Cape May Peninsula, New Jersey (1.5 – 2.5 m), and also Akins et al. (2014) reporting for South Carolina (15 cm) (Table 2). Our brumation mud depths (< 12 cm) conform to the findings of Akins et al. (2014), and Williard and Harden (2011) and also the creek bank brumation burial mud depths reported by Yearicks et al. (1981) (Table 2). In terms of location, we identified brumating terrapins beyond the sloped embankments and buried underwater on the bottom of the cove’s bay floor, which is similar to locations reported by Yearicks et al. (1981).

Despite differing regional locations and habitats, we found that similar water temperatures, water depth and burial mud depth exist during terrapin brumation. Our cove site in BBay-LEH provides many of the same ecological elements that are essential for a safe terrapin winter habitat. The uniqueness of this cove, however, is that brumation is occurring in the same area as their other life cycle functions and that a precise location could be determined with drone technology. Based on our environmental sensor’s brumation water depth readings and Yearicks et al. (1981) findings that brumating terrapins covered in a thin layer of mud could be easily seen when the water was extremely shallow, we conducted our drone surveys specifically during a low tide with low tidal height, so that optimal viewing could be achieved. Further, we have found that when these brumation and tidal conditions are present, that drone surveys can be effectively conducted to allow for non-invasive viewing of terrapin brumating habitats.

Our environmental sensor data is unique in that it depicts the actual conditions of a brumating adult female diamondback terrapin in Barnegat Bay, New Jersey. Other brumation studies included temperature observations recorded at the onset of brumation, or during periodic field assessments (Yearicks et al., 1981; Hart and Lee 2006; Capella, 2021). Our environmental sensor data recorded temperature and depth four times daily, which indicates that terrapins undergo fluctuations in temperature consistent with tidal fluctuations and low water events during the winter low tide cycles (Harden et. al, 2007). From mid-December 2019 through early February 2020, BHIO experienced a high frequency of water depths lower than 0.5 m than during the first month and last two months of brumation (Figure 3). Terrapins can become active during brumation if water temperatures increase to active temperatures that are over 13°C as reported in Brennessel 2006; however, water temperatures remained below those values throughout a continuous brumation period from November 2019 through early April 2020, with temperatures consistent with the brumation temperatures reported by Yearicks et al. (1981) in New Jersey. BHIO showed activity on 8 April 2020, when the temperature ranged from 12.76°C – 14.52°C, which is consistent with Brennessel (2006) and our terrapin activity at the BBay-LEH cove. During low tides and a strong west wind in the winter, water depths become noticeably lower throughout Barnegat Bay, which is no more than 1.5 m deep on average (Kennish 2001). Therefore, diamondback terrapin brumation areas at Barnegat Bay experience specific conditions for both depth and temperature that are influenced by ocean water influx. Although these conditions align with existing terrapin knowledge, this study utilized this information to identify brumation and post-brumation behaviors in an area of high terrapin densities.

As researchers and lawmakers seek new ways to preserve and restore essential marsh habitats, which are compromised due to global warming and sea level rise, special attention must be directed toward the impact that conservation projects will have on at-risk species. Consequently, new approaches for best management practices at coastal restoration projects are being designed to protect turtle species, especially for nesting habitats, which reduce take during projects (Evans 2015; Guilfoyles et al. 2019; Clausner et al. 2004). Beach replenishment and dredging operations by the U.S. Army Corps of Engineers, at potential endangered sea turtle locations, include species identification, impact assessment and monitoring of habitats (Dickerson et al. 2004). Changes to dredging equipment, such as turtle tickler chain mats that are dragged along the seafloor, encourage sea turtles to swim away from the project area and potential harm (Clausner et al. 2004; Welp et al. 2024). Biological opinions issued by state and federal agencies are also responsible for enforcing seasonal work restrictions, thus providing further protection, especially during the nesting season (Evans 2019). Although some of these best management practices may have an overlapping benefit for terrapins, few are specifically intended to minimize their risk during brumation (Castro-Santos et al. 2019). For declining diamondback terrapins, it is hoped that drone site surveys will become a standard requirement that is conducted prior to scheduled winter restoration or dredging projects, thereby affording protection to brumating terrapins during this vulnerable period of their lifecycle. This study has contributed to our understanding of movement and environmental conditions that a terrapin experiences, however, further research will help to fill remaining brumation knowledge gaps so that generalizations can be determined about the overwintering process across the range of terrapins in the temperate latitudes.