The diamondback terrapin (Malaclemys terrapin) is the only species of turtle in North America that exclusively occupies brackish water. There are 7 subspecies of diamondback terrapin, and the species has a continuous range throughout tidal marshes of the Atlantic and Gulf coasts of the United States (Brennessel 2006). Diamondback terrapins are a key part of these marsh ecosystems, as their top-down control of marsh periwinkles (Littoraria irrorata) has direct implications for plant biomass and production (Silliman and Bertness 2002). The most recent International Union for Conservation of Nature assessment has listed the diamondback terrapin as a vulnerable species due to 25%–30% range-wide population declines over the last 30 yrs resulting from estuarine saltmarsh loss, bycatch in commercial blue crab (Callinectes sapidus) fisheries, and the loss of limited nesting habitat due to development (Roosenburg et al. 2019).

In Mississippi, depredated nests have historically been used as means of determining which areas are used by diamondback terrapins for nesting (Mann 1995). It is known that most nests are destroyed by predators within 48 hrs of oviposition, and as eggshells are persistent on nesting beaches this provides a means to easily detect nesting activity at an area, albeit with a conservative minimum estimate (Butler et al. 2004; Munscher et al. 2012). At the Grand Bay National Estuarine Research Reserve (GNDNERR) on the Mississippi Gulf Coast, nest monitoring efforts for the Mississippi diamondback terrapin (M. t. pileata) have occurred over the past 2 decades (Mann 1995; Watters 2007; Cooley 2009; Tappa 2014). Terrapins are known to nest at Point Aux Chenes and Grand Battures within the reserve, and monitoring efforts have been predominantly conducted at the location with the most ongoing habitat loss, Grand Battures. In 1994, a coastwide survey of Mississippi salt marshes was undertaken to obtain baseline data on which areas were used by diamondback terrapins in the state (Mann 1995). It was determined that within the GNDNERR, 2 areas of nesting habitat were present—Point Aux Chenes and Grand Battures. Mann (1995) conducted 2 surveys at each location found to be diamondback terrapin nesting habitat, and over those 2 surveys, 39 depredated nests were counted at Grand Battures and 95 nests at Point Aux Chenes. In 2007, 3 transects were established and surveyed at the east end (300 m), center (200 m), and west end (100 m) of the Grand Battures coast for depredated terrapin-nest monitoring (Watters 2007). From May to August, 180 depredated nests were counted. The same 3 transects were monitored in 2008 from late April to mid-August, and 208 depredated nests were counted (Cooley 2009). The majority of nests (76%) were detected on the 300-m transect at the east end of Grand Battures.

In 2014, 8 nest surveys were conducted at Grand Battures by walking the entirety of its southern coastline as well as a small island just off the west end, which was connected to the Grand Battures coast in the 1990s but had since become separated (Tappa 2014) (Fig. 1). Together, the southern coastline and disjoined island provided approximately 2.34 km of survey area. These surveys found 167 nests, 158 depredated and 9 successfully hatched, detecting 1 nest per 112 m walked. Ten predated adult terrapins were also found at this site during surveys. Only 2 nest surveys occurred at Point Aux Chenes during 2014 (36 nests were found and 31 were depredated), so we did not attempt to draw any comparisons with our 2021 surveys of this area. Most nests found at Grand Battures in 2014 were in locations that were underwater in 2021 (Fig. 2). Tappa (2014) described raccoons (Procyon lotor) as the predominant predator during these surveys, recommending their removal from nesting habitats at the GNDNERR to offset anthropogenic impacts and increase nesting success.

View Figure

• Download as Powerpoint
• Download Figure

View Figure

• Download as Powerpoint
• Download Figure

While it now connects with the Pascagoula River as a tributary, the Escatawpa River historically emptied into the Mississippi Sound in the Point Aux Chenes–Grand Bay area (Eleuterius and Criss 1991). Before changing course, the fluvial sediments deposited by the Escatawpa River offset sediments lost to erosion in our study area. Since the river's change in course, Grand Battures has been eroding continuously (Peterson et al. 2007). An analysis of shoreline change from 1848 to 2017 has shown that the Grand Battures shoreline has the highest rate of retreat at the GNDNERR, with Grand Battures and Point Aux Chenes experiencing retreat at a rate of 2.0–6.55 and 0.50–1.99 m/yr, respectively (Terrano et al. 2019). Based on aerial imagery, we estimated that 40%–50% of available beach area was lost at Grand Battures in the 6-yr period from 2013 to 2019 (Fig. 1).

Throughout their range, diamondback terrapins face threats from habitat loss due to development and estuarine salt marsh loss, commercial harvesting, and entrapment in crab pots (Seigel and Gibbons 1995). While terrapin populations face a certain amount of predation from native species through their range, they can also face additional impacts to their habitats and nesting success from invasive species such as feral hogs (Sus scrofa) (Roosenburg et al. 2019). Within the GNDNERR, the primary threats facing Mississippi diamondback terrapins appear to be habitat loss due to erosion and incidental catch in crab pots. In 2021, diamondback terrapin depredated nest surveys were conducted at Point Aux Chenes and Grand Battures to examine how current nesting activity compares with previous survey efforts in the face of current shoreline and habitat changes.

Methods. — During the terrapin nesting season (May–August), beaches were surveyed in mornings between 0800 and 1200 hrs for potential nesting activity (Grosse et al. 2011). Surveys occurred weekly unless precluded by weather. Two observers walked the entirety of available nesting beach from the high tide line at the southern shore to the northern beach edge where elevation decreased into frequently flooded areas dominated by black needlerush (Juncus roemerianus). Most diamondback terrapin nests are destroyed by predators within 48 hrs of oviposition, so depredated nests were used as a proxy for overall nesting activity since eggshells are persistent on the nesting beach and are more easily detected than intact nests (Butler et al. 2004; Cooley 2009; Munscher et al. 2012). Upon detection of a depredated nest (indicated by turned sand and eggshell fragments at the surface), a global positioning system point was taken, eggshells were collected, and the nest cavity was refilled with sand to avoid double counting. The area surveyed in 2021 included the entirety of Grand Battures' southern coast (also known as South Rigolets Island), approximately 1.45 km, and an approximately 2.9-km-long stretch of sand beach adjacent to Point Aux Chenes Bay (Fig. 2).

Results. — During the 2021 nesting season, 65 depredated nests were detected from 15 nest surveys at Grand Battures (1 nest per 334 m walked) and 695 depredated nests were detected from 13 nest surveys at Point Aux Chenes (1 nest per 53 m walked). Predator tracks observed on Point Aux Chenes nesting beaches in 2021 included coyotes (Canis latrans) and bobcats (Lynx rufus). No tracks were observed on beaches at Grand Battures in 2021 as open sandy areas have become limited and are often filled with shells, although coyote scat containing terrapin eggshells was observed at both sites. Coyotes are a known predator of sea turtle nests, although our observations are possibly the first recorded instances of coyotes predating diamondback terrapin nests (Lovemore et al. 2020). Despite raccoons seeming to be the foremost predator of terrapin nests in 2014, signs of raccoon predation were not directly observed in 2021. At Point Aux Chenes, 14 female terrapins were observed on nesting beaches, of which 12 were actively nesting (Fig. 3). One dead female terrapin was also found at this site (presumed killed by predator), and 1 male terrapin was rescued from a derelict crab trap that had washed ashore. Only 1 live terrapin was observed at Grand Battures—a female that was not actively nesting.

View Figure

• Download as Powerpoint
• Download Figure

Over multiple visits to the island off the west end of Grand Battures coast, where nests were previously found in 2014, it was determined that the island no longer supports terrapin nesting as it now sees complete overwash of water during a tide cycle (Figs. 1 and 2). Without this island, and with ongoing erosion to Grand Battures' southern shoreline, approximately 1.45 km of coast were monitored (0.89 km less survey area than 2014). Most nesting locations from previous nesting surveys at Grand Battures are now underwater and the shoreline changes observed through aerial imagery at Grand Battures are also corroborated by changes in vegetation. Nests are frequently found at higher-elevation locations on the beach often within saltmeadow cordgrass (Spartina patens) and other high marsh vegetation. Photos taken from the same locations at Grand Battures in 2011 and 2021 show that areas previously dominated by S. patens are now occupied by saltmarsh cordgrass (Spartina alterniflora), which grows in low marsh areas, which are areas frequently inundated by tide (Fig. 4).

View Figure

• Download as Powerpoint
• Download Figure

Discussion. — Nest survey effort at Grand Battures in 2021 was the highest to date, and nest detection was the lowest recorded since the first reported nesting surveys by Mann in 1994. It is important to address the caveat that depredated nests provide a minimum estimate of nesting activity, although the decreased detection of nests through this method alongside the observed habitat changes are still noteworthy trends. Terrapins are known to exhibit nest site fidelity, and it is likely that the lower numbers of nests and adult terrapins seen at Grand Battures over time is related to the shoreline retreat and loss of nesting habitat observed at this area (Hurd et al. 1979; Lovich and Gibbons 1990) (Fig. 1). Over a third of the available shoreline nesting habitat present during 2014 surveys had disappeared by the time 2021 depredated nest surveys occurred. The vegetation community along Grand Battures' southern coast also saw changes indicative of a shift to low marsh habitat, such as the greater occurrence of S. alterniflora in areas which were previously nesting habitat dominated by S. patens. Additional work (e.g., nest elevation ranges, vegetation types and percentages of cover, and home ranges of nesting females), is needed to quantify the loss and quality of nesting habitats at Grand Battures and Point Aux Chenes, and this work may have implications for future conservation and restoration planning efforts at the reserve.