Saturday, January 21, 2012

Trapping Turtles in Costa Rica: Week in Review

A respectable haul

With my first week of trapping turtles in Costa Rica coming to a close, I think it’s an appropriate time to take stock of the results.

I set my traps on Monday and left them there all day and night before checking them on Tuesday morning. Although I had high hopes for five traps all full of turtles, the reality was a little less rewarding. Nothing. In the language of turtle trappers everywhere, I got skunked.

For me, this was some cause for concern. The first night of trapping, when the bait is most fresh, is often the most productive. To not catch a single turtle on what was supposed to be the best of nights led me to question whether I had made an major miscalculation. In looking through some of the old data from previous trapping efforts, I saw that most of the trapping had taken place in June and July, months that see considerably more rain. This rain results in full swamps. I was hoping that my project would add some new information by trapping in the dry season, when water levels were low. Specifically, I wanted to know what turtles did as the habitat dried up. But, the first step was to catch some turtles. As I looked at my empty traps, I wondered if turtles were less susceptible to being trapped when the water was low. Maybe they had already left the swamp to find a place in the surrounding forest to spend the dry season.

The snapping turtle is in the background and there
are two mud turtles in the foreground

Much soul-searching and hand-wringing occurred until Wednesday morning, when the turtle gods saw fit to have me check four empty turtle traps before coming up on the fifth and final trap, which was brimming with some familiar rustling. I immediately made out the form of a snapping turtle, but it was only until I had taken the trap out of the water that I noticed there were also three White-lipped Mud Turtles inside, the target species.

The South American Snapping Turtle
The turtle on the right probably looks very familiar but it is actually considered a different species than the animal we know from North America. That species is Chelydra serpentina, known simply as the Snapping Turtle. However, this turtle, which is found from Honduras south to Columbia and Ecuador, is known as the South American Snapping Turtle, Chelydra acutirostris. In looking through the data from past trapping efforts in this swamp, which date back to 1991, only two of these animals have been identified here.

Of the White-lipped Mud Turtles, all appeared to be adults. Two were females and one was a male. The male appeared to have been captured before. By coding each scute (scale) on a turtle shell with a different number, we can make marks in the shell to give each turtle an individual number. By marking turtles individually, we can begin to estimate how many turtles are in the population. In any turtle study, if most of the captured turtles have been marked before, we can have high confidence that we have captured most of the turtles in the population. If only a small percentage of the turtles we catch are recaptures, then we know our trapping effort is not intensive enough to make accurate estimates. This could be because the animals are hard to trap, it could mean that animals simply have large home ranges and we’re only sampling a small fraction of all the animals out there, or it could just mean that animals don’t live long enough to be captured in multiple efforts.

A White-lipped Mud Turtle burrows into the
mud after being released
In any case, I believe the male had been marked with number four, which would mean he had originally been captured in June of 1991, the year the Soviet Union was dissolved and the year Bill Clinton announced he would run for President of the United States. Presumably, this turtle, which was already an adult when it was first captured, has been crawling around the swamp since then.

The next two days were not as productive, as I only caught one turtle each day. Interestingly, another turtle was a recapture, this time a female. She had been originally captured in June of 2003, when 50 Cent’s “In da Club” was the top single in the United States. On Wednesday, I caught sight of a Gray-necked Wood-rail investigating one of the traps. I imagine it was interested in the fish that were swimming around the turtle bait. On Thursday, as I was walking around the swamp looking for some more pools to set a trap (White-lipped Mud Turtles are unlikely to be evenly distributed across the whole swamp and I did not want to overlook a spot with more animals), I found a turtle walking in some dry vegetation. Perhaps she was also looking for some water.

The final tally for the week was one South American Snapping Turtle and and Six White-lipped Mud Turtles. It's something, but not yet enough turtles to say much about the population. Today I was once again skunked. I have been watching with some dismay as the shallow pools in the swamp quickly dried up. Several of my traps are now sitting on top of the mud. If it does not rain soon, my trapping will be over sooner than I had anticipated.

The study site is looking less and less like a swamp each day

Much of what I write is based on my experience in the field. However, I also rely on the research of others. Citations to relevant scientific articles are included below.

Morales-Verdeja, S., & Vogt, R. (1997). Terrestrial Movements in Relation to Aestivation and the Annual Reproductive Cycle of Kinosternon leucostomum Copeia, 1997 (1) DOI: 10.2307/1447847

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