MTN 76:8-11 Galvanized Wire Cages Can Prevent Nest Depredation

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GALVANIZED WIRE CAGES CAN PREVENT NEST DEPREDATION

Introduction: A variety of methods have been used to protect sea turtle nests from destruction by non-human predators (see Stancyk, 1995 for review). In the U. S., raccoons (Procyon lotor) are a major cause of nest destruction. This is particularly true on Key Island, Florida. Since predator control (e.g., traps, poison) is not an option on this beach, other methods must be utilized to prevent egg depredation. These methods have varied over the years and have included a head-start program (to enhance hatchling production), a self-releasing hatchery (to confine and safeguard nests) and, more recently, a program to protect nests in situ.

Initially, flat screens (4 ft², or 61 cm x 61 cm) of chain-link fencing were used to protect nests in situ. The screens were fixed in place with tent stakes (1 ft, or 30 cm long) and covered with about 2 in (5 cm) of sand to obscure visual cues. However, raccoons still exhumed significant numbers of nests by digging in from the side of the egg chamber. Shallow nests were also depredated from the top. Raccoons were able to dig into egg chambers by sticking their paws through the top of the screens. This behavior, coupled with their weight on the screen, allowed them to dig to surprising depths. In 1992, a pilot study was conducted to compare the effectiveness of wire mesh cages with that of flat screens in preventing nest destruction. The cages proved to be far more effective than the screens (Addison and Henricy, 1994). Since 1993, the cages have been used exclusively and with excellent results against indigenous raccoons. The cages may prove useful in other parts of the world in deterring other medium-sized predators (e.g., coatis, Nasua narica).

Materials and Assembly: Cages are fabricated from 3 ft x 100 ft (91 x 3048 cm) rolls of galvanized wire mesh. In order to be self-releasing, mesh size must be 2 in x 4 in (5 cm x 10 cm). The wire can be either 12 or 14 gauge; however, the thicker and more rigid 12 gauge wire lasts longer in the beach environment, resisting corrosion, natural forces, and the activity of predators. One hundred foot rolls of 12 and 14 gauge mesh cost approximately US$ 53.00 and $36.00 each, respectively, in Florida.

One roll of wire makes seven cages with enough left over to cut two additional 2.5 ft (76 cm) sides. The cages are constructed by first cutting an 8 ft (244 cm) length of wire mesh from the roll (linesmen pliers are recommended). When bent, this forms the 3 ft x 3 ft (91 cm x 91 cm) top and two of the 2.5 ft x 3 ft (76 cm x 91 cm) sides (Figure 1). The other two sides are made by cutting two 2.5 ft (76 cm) sections from the roll (Figure 1). For transport or storage, the 8 ft (244 cm) section can be collapsed (the sides folded against the top) with the two smaller side sections nested inside. Nylon "wire ties" or rope can be used to bind the folded cages completely flat.

Figure 1. Exploded view of cage used to prevent sea turtle nest depredation by raccoons on Key Island, Florida. Dimensions are given in inches.

The cages are easily assembled in the field. Prior to assembly, bend a 0.5 ft (15 cm) flap along the base of all sides. This holds the cage firmly in the sand and keeps predators from digging in from underneath. Although thin steel wire can be used to fasten the cage together, 3.6 in (9 cm) nylon wire ties are much faster and more convenient. A package of 100 wire ties costs about US$ 3.00; 25-30 ties are required to assemble a cage. [ N.B. When disassembling cages, carry small side-cutting pliers to snip off the wire ties so that cages can be folded flat for transport. ] Total cost of materials is about US$ 8.50 (12 gauge wire) or US$ 5.25 (14 gauge wire) per cage. About 200 cages are used each year on Key Island, evenly divided between 12 and 14 gauge wire. The total cost in materials is about US$ 1,525.00. If the cages are stored inside, they should last through four or five nesting seasons. Some cages (made from heavier gauge wire) from the 1992 season are in good enough condition to be used again in 1997. Storm tides and waves have caused the greatest loss of cages to date.

Installation and Removal: For proper installation, the egg chamber must be located so the cage can be centered exactly over the nest. If egg-laying was not observed, then the upper-most eggs must be found by gentle excavation. This is done slowly by hand to minimize the chance of damaging eggs. Probing with a rod, etc. is not recommended. If nesting is observed, three lines leading from the nest cavity are drawn in the sand at right angles to each other. Once the turtle has left the nest, extending the lines to where they intersect accurately locates the eggs. Experience has shown that, in some cases, gentle excavation may still be required.

After the nest is located, center the cage over the nest and mark off the perimeter. Remove the cage and dig a trench 12 in (30 cm) deep and 6 in (15 cm) wide around the inside of the perimeter line. A square-ended garden spade with a standard 6.5 in x 11.5 in (ca. 15 cm x 30 cm) blade works best. Make sure the finished trench is 12 in (30 cm) deep before setting the cage. Cages that are not set deep enough in the sand are more vulnerable to predators, tidal flooding, etc. Conversely, about 12 in (30 cm) of mesh should be exposed above the sand so predators cannot reach the eggs from the top. For added support on top, a wooden brace can be affixed diagonally across the roof of the cage. Once the trench is refilled, tamp the sand down. Remember, the most critical steps are centering the cage and achieving adequate trench depth.

Both installation and removal involve considerable physical effort. If the egg chamber is easily located, it requires about 30 minutes for one person to assemble and install a cage. It takes about the same amount of time to remove one. On Key Island, installation involves two people each night; removal is handled by one person. The expense, time and labor involved in caging probably limits their use to projects where annual nest numbers do not exceed the low hundreds. They may also be useful on beaches where specific areas experience high rates of depredation. To date, the greatest number of nests caged on Key Island in one season has been 197.

Results: Prior to the implementation of caging, the mean gross depredation (partial and total) rate of flat-screened nests was 26.2%. Caging has reduced this to 12.5%. Exclusive of one year when 21% of caged nests were depredated (attributable to faulty cage installation and overwash by storm events), the mean drops to 3.5%. This also emphasizes the importance of proper cage installation. The reduction in partial nest depredation resulting from caging has probably contributed to an overall improvement in nest hatching percentage.

Cages also accurately mark nest locations which facilitates data collection on hatching success, etc. If nests are monitored throughout incubation and are routinely exhumed post-hatching, a length of surveyor's tape marked with pertinent data (e.g., nest number, date laid) can be tied to the base of the cage (before burial) as a reference if above-ground markers fade or are lost. Once in place, cages can be pulled out by hand only with great difficulty. To date, no cages have been lost to unauthorized human intervention.

In closing, it is worth noting that cages set near the high tide line are likely to be periodically washed over by spring tides or storm waves. As a result, these cages may be either buried or uncovered. In the latter case, cages can be reset without too much effort; however, those in areas where storms result in beach accretion are sometimes nearly completely covered by sand. Whether this accretion is exacerbated by the placement of the cage(s) has not been determined. Sand depths at caged nests located higher on the beach are much more stable. This may be attributable in part to the composition (fine quartz sand and shell hash) of the beach material in this bioregion.

Acknowledgements: My sincere thanks to the many interns who have made this project possible by spending many nights looking for turtles and bending over shovels to install cages.

Addison, D. S. and S. Henricy. 1994. A comparison of galvanized wire mesh cages vs. flat chain-link screen in preventing Procyon lotor depredation of Caretta caretta nests, p.174. In: K. A. Bjorndal, A. B. Bolten, D. A. Johnson and P. J. Eliazar (Compilers), Proc. Fourteenth Annual Symposium on Sea Turtle Biology and Conservation. NOAA Tech. Memo. NMFS-SEFC-351. U. S. Dept. Commerce.

Stancyk, S. E. 1995. Non-human predators of sea turtles and their control, p.139-152. In: K. A. Bjorndal (Editor), Biology and Conservation of Sea Turtles, Revised Ed. Reprinted by Smithsonian Institution Press. Washington, D. C.

DAVID S. ADDISON, The Conservancy Inc., 1450 Merrihue Dr., Naples, Florida 34102 USA.