Encouraging Trend Found at Tortuguero, Costa Rica

Spring 1998 Issue Articles:

* International Agreement for the Conservation of Caribbean Sea Turtles Signed
* Snapper Sighted!
* Encouraging Trend Found at Tortuguero, Costa Rica

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Encouraging Trend Found at Tortuguero, Costa Rica

By Dr. Karen Bjorndal

In 1971, CCC Scientific Director Archie Carr extended the systematic survey of green turtle nesting activity at Tortuguero, Costa Rica, beyond the standard patrol area of the northernmost five miles. He asked Durham Rankin, long-time Tortuguero resident and steadfast friend of the CCC, to walk the northern 11 miles of the 22-mile beach in the early morning and count the number of nests that had been deposited the previous night (see story about the Rankin family on page 4). Durham would then have to reverse the trip and walk back home. Sometimes, when boats were available to pick up Durham at the southern end of the beach, he would walk the entire 22 miles. These surveys were continued from 1971 up to the present; since 1986, almost all surveys have covered the entire 22 miles. Durham also trained his sons (Alonso, Eddy, Roberto, Danny and Willis) to conduct the surveys, and they have carried on this work for CCC. Although logistic difficulties sometimes intervened, surveys were generally conducted at weekly intervals throughout the major part of the nesting seasons.

A green sea turtle nests among the vegetation on Tortuguero beach.

Using elegant statistical models designed by Jerry Wetherall of the National Marine Fisheries Service, Honolulu Lab, we have been able to estimate the numbers of nests deposited in the northern 11 miles each year from 1971 through 1996 (Bjorndal, K.A., J.A. Wetherall, A.B. Bolten and J.A. Mortimer. In press. Twenty-six years of green turtle nesting at Tortuguero, Costa Rica. Conservation Biology). When these annual nest estimates are plotted against year, the most striking aspect is the tremendous variation in the number of nests from year to year. The numbers can change by a factor of four or five between successive years. (Editor’s Note: More than 100,000 nests are deposited during high nesting seasons.)

Evaluation of trends in sea turtle nesting populations requires many years of data because of the great degree of annual variation in nesting numbers. Most encouraging, however, is that the trend line plotted through the annual estimates points upward. In other words, an increasing number of nests are being deposited at Tortuguero. Evaluation of the trend indicates a relatively consistent increase from 1971 to the mid-1980s, constant or perhaps a slight decrease in nesting during the late 1980s, and then resumption of an upward trend in the 1990s. The annual number of nests at Tortuguero has approximately doubled from 1971 through 1996.

This encouraging trend must be evaluated with caution for several reasons. First, if the mean number of nests deposited by each female each year (clutch frequency) varies significantly among years, changes in numbers of nests among years could reflect changes in number of nesting females, clutch frequency, or both. Second, the trend represents only one segment of the pop-ulation (mature females), which may or may not represent the trend of the entire green turtle population. Because green turtles require many years to reach sexual maturity, trends in the numbers of nesting females may not reflect changes in juvenile mortality for many years. Third, survey frequency, and thus confidence in annual estimates, varied among years. Most importantly, this upward trend must be assessed in the perspective of the catastrophic decline that the Caribbean green turtle populations have experienced since the arrival of Europeans.

But despite these caveats, it seems safe to conclude that the stewardship that the Costa Rican government and CCC have provided for the Tortuguero colony has had positive results. We must maintain and increase these efforts to ensure the positive trend continues.

Dr. Karen Bjorndal is the Director of the Archie Carr Center for Sea Turtle Research at the University of Florida, Gainesville and a member of CCC’s Scientific Advisory Committee.