MTN 159:1-4 Beach Crabbing as a Possible Hindrance to Loggerhead Marine Turtle Nesting Success

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Beach Crabbing as a Possible Hindrance to Loggerhead Marine Turtle Nesting Success

Emily M. Drobes^1,2, Matthew Ware¹, Valencia K. Beckwith¹ & Mariana M. P. B. Fuentes¹
¹Department of Earth, Ocean, and Atmospheric Science, Florida State University, Tallahassee, FL 32306, USA (E-mail: emd14@my.fsu.edu, mw15w@my.fsu.edu, vkb14@my.fsu.edu, mfuentes@fsu.edu);
²Florida Fish and Wildlife Conservation Commission, Tallahassee, FL 32399, USA

Marine turtles are impacted by a multitude of threats at both their terrestrial and oceanic life stages (Wallace et al. 2011; Fuentes et al. 2015). At nesting grounds, impacts are generally from artificial light (Witherington 1992; Silva et al. 2017), coastal development (Fuentes et al. 2016; Nelson Sella & Fuentes 2019), coastal armoring (Rizkalla & Savage 2011), beach nourishment (Brock et al. 2009), poaching (Koch et al. 2006), predation (Lutcavage et al. 1996), marine debris (Duronslet et al. 1991; Beckwith & Fuentes 2018; Garrison & Fuentes 2019), and climate change (Janzen 1994; Hawkes et al. 2009; Fuentes et al. 2011; Hamann et al. 2013). The presence of humans during marine turtle nesting can also disturb nesting, causing marine turtles to return to the water or even stop them from emerging onto land (Witherington 1992; Jacobson & Lopez 1994). Although less documented, recreational catch-and- release of ghost crabs, Ocypode quadrata, (i.e., “beach crabbing”) in coastal areas is an activity commonly conducted on beaches in the northern Gulf of Mexico which may influence marine turtle emergence and nesting decisions. Beach crabbing usually consists of one person or a group of several people with flashlights walking up and down the beach chasing ghost crabs with nets, usually accompanied by noise. To explore whether marine turtle nesting is affected by beach crabbing, we investigated the spatial extent of beach crabbing and marine turtle activities on St. George Island, Florida, one of the most important nesting sites for the Northern Gulf of Mexico Loggerhead (Caretta caretta) Recovery Unit.

Tourism is an important industry on St. George Island: tourist numbers average over 136,000 total visitors per year, and an estimated 113,400 of those visitors stay overnight on the island (Blair pers. comm. 2017). During the summer, 81.8% of the homes are seasonally occupied with visitors rotating on a weekly basis (Fuentes et al. 2016). From 2014 to 2017, the study region has had, on average, 312 ±171 SD nests per year across the nesting season with peak nesting occurring in mid-June (Becker pers. comm. 2018). To determine the exposure of nesting turtles to beach crabbing and potential implications, we conducted nighttime surveys along 17.5 km of the nesting beach at St. George Island (Fig. 1) on foot between 2100 hrs and 0400 hrs during the peak of the nesting season in 2017: 10 June to 19 June. The study area included the southwestern and middle sections of the island. The southwestern end contains a gated community with access restricted to residents and guests (Fig. 1). The middle of the island is accessible to all visitors with the main access point located at the lighthouse close to the bridge from the mainland. There are 17 additional smaller access points concentrated around the middle of the island (Fig. 1, Beach Access St. George Island, Florida 2018). The gated community on the southwestern end of the island enforces rules against noise and lights at night to mitigate human impacts on the beach ecosystem (<http://www. stgeorgeplantation.com/page/37437~701534/Rules-Regulations>). The survey area excluded the state park at the north end of the island as this area is not publicly accessible at night.

During our surveys, the time and GPS location of any loggerhead activities (nests and false crawls) were recorded in addition to any beach crabbing activities and the number of people involved, as well as the number of flashlights used and the color of the light. Based on the time of the observation, marine turtle and human activities were grouped into one of two operational time categories: 2100 to 2359 hrs and 0000 to 0400 hrs, if an event overlapped between both time periods, it was categorized based on the time the event was first observed. If a turtle was encountered during the surveys, researchers minimized their potential disturbance to the turtle by maintaining a safe distance from the turtle during her inbound crawl and nesting attempt, minimizing noise, and using red light only as needed, as per permit regulations. Differences in nesting success by time category were assessed by binomial generalized linear models. Kernel density estimates of turtle and beach crabbing activities were used to visualize distribution patterns. All analyses were run in R version 3.5.0 or ArcMap 10.3 (ESRI 2014).

Figure 1. St. George Island, Florida, United States. This study included the gated community (southwest) and public access (middle) sections of the island. The State Park was not included in this study as it is not publicly accessible at night.

Figure 2. Kernel density estimates of all loggerhead turtle activities (A) and beach crabbing events (B) from 10 to 19 June 2017 at St. George Island, Florida.

During the surveys, there were a total of 117 loggerhead turtle activities (50 nests and 67 false crawls) in the study area with an overall nesting success of 42.7%. Most of the recorded turtle activities occurred after 0000 hrs (65.0%), with no significant difference in nest success (p = 0.840) between times. By comparison, 94 beach crabbing events were documented before 0000 hrs with an average of 6.0 ±4.0 SD people using 4.6 ±4.1 SD white lights and 0.1 ±0.7 SD red lights per event. Only six beach crabbing events were recorded after 0000 hrs with an average of 2.7 ±1.4 SD people using 2.7 ±1.4 SD white lights and no red lights per event. Kernel density maps indicate that during the survey period, the greatest concentration of marine turtle activities were located just southwest of the state park and the lowest densities were near the lighthouse (mid-island, public access area) and southwest end of the island within the gated community (Fig. 2A). The greatest density of crabbing activity was located near the lighthouse while the lowest density was adjacent to the airport (Fig. 2B). This spatiotemporal pattern of turtle emergence may indicate that marine turtles at the study site may be avoiding times and locations of increased human nocturnal activity such as beach crabbing. Indeed, we did not observe any interaction between crabbers and turtles.

Avoidance of human activities is not the only reason why a female turtle may emerge later in the night to nest. Nesting may occur any time of night (Caldwell 1959) and a turtle may choose to abandon a nesting attempt for many reasons including distance from the nearest human settlement (Kikukawa et al. 1998, 1999), artificial lighting (Raymond 1984; Proffitt et al. 1986; Witherington 1992), sand temperature (Stoneburner & Richardson 1981), improper beach substrate (Mortimer 1990), and poor weather conditions (Hughes et al. 1967), in addition to human disturbance (Dodd 1988). Although no formal lighting or topographical surveys were conducted as part of this study, artificial lights on St. George Island during the survey period could be seen from far distances as the beach’s wide, flat berm and short dunes do not restrict residential or handheld lights. No major changes in sand grain size, scarp formation, or other geological characteristics along the length of the study site exist which could explain the distribution of turtle activities noted during the nighttime surveys. Marine turtles may time their emergence with the high tide to decrease the risk of nest inundation during incubation and to minimize energy expenditure (Frazer 1983, Wilson 2018). Though high tide did occur later in the night during the survey period, the small tidal range at St. George Island (0.37 m) means that turtles in the study area are unlikely to use high tide timing as a significant emergence cue (Frazer 1983).

Further research across the full nesting season and at additional beaches should be conducted to evaluate whether a causal relationship exists between beach crabbing and marine turtle nesting activities. Bustard et al. (1975) and Limpus (1985) showed that loggerheads approach and observe the beach from shallow water before coming onshore. This is the time that they are most sensitive to disturbance and they will retreat if they observe perceived danger. The movement of people and the use of bright white lights near the high water mark associated with beach crabbing activities could potentially be discouraging turtles from emerging from the water (avoidance behavior) or cause turtles to return to the water without completing a nest (Jacobson & Lopez 1994; Silva et al. 2017). Although prevalent on beaches along the Gulf coast of Florida, the exposure and impact of beach crabbing on turtles has not been previously quantified nor identified as a major issue.

Human leisure activities on beaches are increasing, intensifying pressures on these environments and the species that use them (Schlacher et al. 2008). As St. George Island is a marine turtle nesting site where houses are seasonally occupied, with a turnaround of tourists every week during the nesting season, it may require outreach and education efforts to be structured specifically to engage with seasonal residents and elicit their support for conservation actions (Fuentes et al. 2016). One approach to minimizing the potential impacts of beach crabbing on marine turtles is to develop and advertise the use of turtle-friendly crabbing kits. These could contain red lights and a pamphlet explaining tips to reduce impacts to nesting marine turtles such as use of red lights instead of white lights, the need to minimize noise, and instructions to minimize interactions with a nesting turtle. Additional education efforts could include visible, short guides in rental homes and improved signage at beach entrances on how to minimize disturbances to nesting female turtles.

Acknowledgments. We express our appreciation to the following who contributed to this project: the Marine Turtle Research, Ecology, and Conservation Group at Florida State University, St. George Island Sea Turtle Program, additional volunteers that assisted with beach monitoring, Curt Blair of the Franklin County Tourist Development Council, and Kelly Grove of Florida State University Libraries. Research was conducted under FWC MTP-17-239 and FSU Animal Care Committee 1534.

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