Conservation: Emergence from the Nest: Shrinkage Stoping
The eggs buried in the beach should hatch after about 52-56 days. An egg tooth is used to cut through the egg's flexible, tough eggshell as Hatching is initialized. This activity is coordinated in the clutch because most hatchlings exit their eggs at about the same time. The embryos, enrolled within the spherical eggshells during their development, straighten as their carapaces and plastrons harden. Reduction of egg turgor and size during hatching opens an air space above the clutch (Kraemer and Richardson, 1979) that averages 4.0 ± .26 cm. The mass of hatchlings begins crawling over one another in an emergence frenzy that dislodges sand from the roof of the egg chamber and tramples the underlying egg shells from which they emerged. Sand dislodged from the roof falls through the mass of hatchlings and is trampled onto the top of the hatched eggshells maintaining the thin air space above the hatchlings.
This process of mining themselves out of the beach is called shrinkage stoping by miners who use the same technique to mine ore in underground mines. The process of shrinkage stoping allows the hatchlings to mine their way to the surface as sand falls through the mass of hatchlings and is trampled onto the stope floor beneath them. Shrinkage stoping in a sea turtle nest continues for one to two days as a frenzied activity until the hatchlings near the surface (Spotila, 2004: 17).
If the sand near the surface is warm, having been heated by the sun, the hatchlings will become lethargic and stop moving until the sand cools in the evening and early night, which once again triggers the stoping frenzy, allowing the turtles to reach the surface as a more or less coherent mass. The turtles usually emerge as a large group and scamper across the dark beach toward the ocean, keying on slope, bright starlight sky backed by island silhouette, reflection from breaking waves, and perhaps wave sound or salt smell.
Evidence of emergence is often recorded in the presence of an emergence crater, a small "hole" or dimple in the backbeach from which the turtles exited and a fan-shaped series of hatchling crawlways fanning out from the emergence crater toward the ocean.
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The sea turtle conservationist should see, note, and document the evidence of emergence during daily nest inspections when monitoring, unless the weather has erased the evidence with blowing sand or rain-wash. By day 57 - 60, if there has not been an emergence the intern should become concerned and "mark" the nest for close scrutiny and carefully check the nest by digging down to the clutch vertically by hand to see what the nest's status is.
In the case of big rain events, the surface sand will often be dampened by rainwater percolation and have its cohesion significantly enhanced. In such cases, the stoping by the hatchlings will become difficult as sand is not easily bumped off the roof while compaction of the stope floor is enhanced, opening an enlarged air chamber above the hatchlings, sometimes becoming so large that the turtles can no longer bump the ceiling … stopping the stoping process by formation of an "air dam." When this happens turtles will expend inordinate energy in completing the emergence and become vulnerable to desiccation over time or to drowning by flooding and fluidization of the overlying sand in case of a subsequent large rain event. If the nest has been checked daily and dug into to ascertain its status, hatchlings in air-dammed stopes will be discovered and may be released by collapsing the domed roof by digging into it from the top and recovering the nest for a natural emergence, or by removal of the hatchlings to a cooler to assure their complete egress and release into the ocean at night. This latter protocol is suggested in case of imminent rain or obstructed pathways to the beach.
Case History: Nest 07-042a, North Beach, St. Catherines Island
Nest 07-042 was deposited on the north end of St. Catherines on the south margin of St. Catherines Sound on July 17, 2007. The nest was a simple, unobstructed, backbeach nest in danger of being inundated by spring and/or storm high tides and possibly eroded by The Storms of September. The clutch was documented, validated, and 142 eggs relocated onto the front of a nearby dune by the 2007 Cohort of Teacher-Interns. The Labor Day Nor'easter of 2007 delivered over 13" of rain to the Island on September 2-3, a rain that drowned four nests and was thought to have induced cohesive effects in the sand overlying active nests. On nest development day 52, the "late" nest 07-042a was carefully opened from the top to ascertain the status of the clutch and the possible presence of an air-dammed shrinkage stope. This was indeed the case and the nest was excavated from the front to document the presence of an air-dammed shrinkage stope.
The crawlway of Nest 07-042 was discovered by students in the 2007 Cohort of Teacher-Interns on the morning of July 17, 2007 as they monitored North Beach, St. Catherines Island. Once all Interns were assembled from their monitoring tasks on St. Catherines' beaches, we predicted the position of the egg chamber using a "mock turtle" blow-up float. The nest was then dug by one of our Teacher-Interns, who successfully located the clutch of eggs. The Interns correctly assessed the nest as a "doomed nest" at high risk of being washed out by storms or erosion. A nearby, natural site for relocation was selected by them and the clutch of 142 eggs was successfully relocated in a rigid bucket into the front of a nearby dune. The nest was monitored daily and rained upon heavily (33.20 cm, ~13.0") during an early September Nor'easter of September 2-3, leading to the fear of drowning by flooding or formation of an air-dammed stope. These possibilities were assed by careful excavation on September 8, 2007. One hundred thirty eight live mature, hatchlings and 4 unhatched eggs were removed from the air-dammed shrinkage stope and released that night on North Beach.
Video documentation of the excavation of nest 07-042a shows Bishop removing the plastic screen protecting the clutch from predators and then he shaving the area above the egg chamber downward very carefully. As the egg chamber discontinuity becomes apparent he uses, a smaller, more precise tool, a trowel. As the bottom of the trech is shaved off, he gently tapped the horizontal surface to test its integrity, and probed the egg chamber discontinuity with an index finger, which was seen penetrating, then suddenly "dropping," into the stope above the egg chamber. Enticed by the prospect of finally being able to document an air-dammed shrinkage stope, the documentation group became excited as he began to define a trench seaward of the egg chamber by digging into the face of the dune, then cutting the face of the trench back toward the egg chamber using a shovel and trowel. Finally Bishop cut into the air chamber in the stope exposing its collapsed ceiling and the mass of lethargic hatchling trapped below the high, domed air-dam. Continuing the careful excavation, the hatchlings were exposed as is the mass of hatched eggshells underlying them at the bottom of the egg chamber. The hatchlings then became active as they began to wiggle their flippers and crawl about. One hundred thirty eight healthy hatchlings were removed and placed in a cooler to be released in the dark of night. Four unhatched, undeveloped eggs were also removed from the egg chamber for a total of 142 eggs. The hatch success of 07-042a was 138/142 x 100 = 97.18%. If the nest had not been opened, it is likely that the 138 hatchings would have died by desiccation or drowned by flooding.
Case History: Nest 08-097a, North Beach, St. Catherines Island
Nest 08-097a was deposited on July 8, 2008 on South Beach (31.60590 N; 81.14489 W), validated and relocated at 07:16 to the front of a dune ridge (31.62327 N; 81.13537 W) on McQueen Dune Field (Shadroui, 1990). It was monitored daily for 57 days and was watched carefully as Tropical Storm Kay passed by and deposited 6.71 cm of rain in the nearby McQueen Rain Gage (31.63391 N; 81.13000 W) over a three day interval. Because of the occurrence of air-dammed stoped nests after that storm event, and the digging of four other stoped nests earlier in the day in preparation for Tropical Storm Hanna, an air-dammed, stoped nest was suspected. Excavation was done on nest-day 57 during the afternoon low tide on 9/03/08 by GAB, assisted by Zak and Aly Huberty. An air-dammed, stoped nest was encountered and documented.
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Because four other air-dammed, stoped nests had been encountered on 9/03/08 before excavating 08-097a, this nest was systematically excavated using a stainless Bon Tools 70-159, 5" x 1.5" Stainless Steel Margin Trowel with a sharpened leading edge. The nest was excavated from above, taking sediment off a centimeter at a time, forming a horizontal trench bottom. The cross-section of the egg chamber discontinuity was encountered approximately 10 cm below the dune surface and followed downward by scraping. At a depth of approximately 24 or 25 cm careful probing with an index finger caused the collapse of the top of the domed air-chamber, proving the existence of an air-dammed stoped nest. The top of the air-chamber was exposed by very careful excavation until a hand-sized opening was formed within the bullseye of the egg chamber discontinuity. Collapsed sediment was then removed with fingers until hatchlings were encountered. The hatchlings were photographed, then gently swept off with a photographic brush to enhance their visibility and rephotographed. Hatchlings were then removed one or two at a time and counted into a sand-floored container until all were out of the air-chamber, which was again photographed. Egg shells beneath the sand-covered stope floor were then removed and counted. Eighty-six hatched eggs and 32 unhatched, undeveloped eggs were counted (72.88% hatch success), and 86 hatchlings removed form the egg chamber neck of Nest 08-097a.