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Marine Turtle Newsletter 76:6-8, © 1997

ON THE IMPORTANCE OF EGGS

1) Many life stages for sea turtles include a large proportion of the population, as they are aggregates of several year classes. For example, the large juvenile stage specified by Crouse et al. (1987) from Frazer's life table (1983) spanned ages 8 - 15. Depending on mortality rates in the pelagic phase, this could include a great many turtles.

2) As an individual approaches sexual maturity, its "reproductive value" increases. This is an individual's contribution to future reproduction, and is based on (i) the probability that it will live long enough to reproduce and (ii) the number of offspring it will generate.

In an age- or stage-based model, the proportional sensitivity of population growth to changes in annual survival is dependent on the number of individuals affected and their reproductive value. Because the reproductive value of eggs and hatchlings is generally much lower than that of large juveniles, subadults or adults, an increase in the annual survival in the first year of life will always have a comparatively small impact on these long lived, late maturing animals.

The results from life table-type models appear insensitive to changes in egg production because they are not really simulation models; rather than predicting fluctuations in a population through time, these models are best for comparing trends in a qualitative manner, such as comparing the overall impact of head-starting programs with the predicted impacts of turtle excluder devices (Heppell et al., 1996a). Because there is a long time lag between birth and reproduction in sea turtles and adults may be long~ lived, large variations in hatchling production may be dampened over time, producing relatively little fluctuation in the number of adults. Notwithstanding, a species must demonstrate successful reproduction in order to survive. Crouse et al. (1987) clearly state that while beach protection alone could not reverse the declining trend in loggerhead sea turtles nesting in the southeastern United States, egg and hatchling production must be maintained as a management tool. Likewise, Heppell et al. (1996a) showed that if survival rates of older turtles can be maintained or returned to high levels, enhancing first year survival can increase population growth. Once harvest (intentional or incidental) of older turtles is reduced, nest protection programs may give populations an added "boost" by increasing the number of surviving eggs each year. Heppell et al. (1996a) also point out that head-starting programs, which may enhance survival of only a small proportion of eggs laid, are unlikely to greatly enhance population growth. In situ or hatchery programs that increase egg survival may be far more beneficial for recovering populations, and do not remove hatchlings from the environment.

Egg harvest is almost certainly less detrimental to a sea turtle population than adult harvest at the same level. Even the loss of 100% of eggs laid in a single year is far less critical to the population at large than the death of all gravid females arriving to nest in a single year. But clearly no population can sustain high levels of egg harvest. Unfortunately, the repercussions of egg over-harvesting may not be evident for several decades, due to the many years it takes a sea turtle to reach maturity and long adult lifespan (Mortimer, 1995).

Changes in the Kemp's ridley population at Rancho Nuevo, Tamaulipas, México illustrate the importance of protecting eggs. Massive egg harvest took place during the late 1940's and 1950's (Márquez, 1994). Conservation efforts began in 1966, when it became clear that the Kemp's population was declining rapidly (rough estimates of nest counts suggest a decline of at least 10-15% per year, based on a minimum of 40,000 nests in 1947). Although the number of nests each year continued to decline, a steady stream of 10,000 - 30,000 hatchlings was released each year from protected corrals through the 1970's (Márquez, 1994). Instead of near certain extinction, the population decline slowed to only 3-4% during the 1980's.

Today nearly all of the nests laid at Rancho Nuevo are moved to corrals, and the use of turtle excluder devices (TEDs) and reduced shrimping effort in México are presumably reducing mortality in the benthic feeding juvenile and adult stages. The number of nests laid at Rancho Nuevo is increasing (Márquez et al., 1996), probably due to a combination of these effects on survival (but see Ross, 1996). Large cohorts produced in the 1980's should now be reaching maturity, boosting the nesting female population substantially. Although egg protection alone may not have saved the Kemp's ridley, it almost certainly slowed the population's decline, giving biologists and conservationists time to devise new solutions to promote population recovery.

In populations known to be stable or increasing, it may be possible to sustainably harvest sea turtle eggs if a biologically significant proportion of the unharvested eggs are protected from poaching, predation, and disease. However, heavy egg harvest, even when subadults and adults are protected, can reduce the recruitment of new nesting females below a viable level. It is also important to remember that populations subject to human-induced mortality in the subadult and adult stages, as well as populations that are at extremely low levels, should be afforded full protection in all life stages. In particular, a continued harvest (at any life stage) in populations known to be declining cannot be condoned.

In conclusion, eggs are important and cannot be ignored in recovery plans. Threats to nesting beaches may become even more critical as coastal human populations grow, bringing increased development, erosion, pollution, and artificial lighting. We must ensure high adult and subadult survival rates characteristic of sea turtle life histories, while not losing sight of the importance of hatchling production critical for recruitment to these later stages. Crouse, D. T., L. B. Crowder and H. Caswell. 1987. A stage-based population model for loggerhead sea turtles and implications for conservation. Ecology 68:1412-1423.

Crowder, L. B., D. T. Crouse, S. S. Heppell and T. H. Martin. 1994. Predicting the impact of Turtle Excluder Devices on loggerhead sea turtle populations. Ecological Applications 4: 437-445.

Frazer, N. B. 1983. Demography and life history evolution of the Atlantic loggerhead sea turtle, Caretta caretta, nesting on Little Cumberland Island, Georgia. Ph.D. Dissertation, University of Georgia, Athens, Georgia, USA.

Heppell, S. S., D. T. Crouse and L. B. Crowder. 1996a. A model evaluation of headstarting as a management tool for long-lived turtles. Ecological Applications 6:556-565.

Heppell S. S., C. J. Limpus, D. T. Crouse, N. B. Frazer and L. B. Crowder. 1996b. Population model analysis for the loggerhead sea turtle, Caretta caretta, in Queensland. Wildlife Research (Australia) 23:143-159.

Márquez M., R. 1994. Synopsis of Biological Data on the Kemp's Ridley Turtle, Lepidochelys kempi (German, 1880). NOAA Tech. Memo. NMFS-SEFSC-343. U. S. Dept. Commerce. 91 pp.

Márquez M., R., R. A. Byles, P. Burchfield, M. Sanchez, J. Diaz, M. A. Carrasco, A. S. Leo and M. C. Jimenez. 1996. Good news! Rising numbers of Kemp's ridleys nest at Rancho Nuevo, Tamaulipas, México. Marine Turtle Newsletter 73:2-5.

Mortimer, J. A. 1995. Teaching critical concepts for the conservation of sea turtles. Marine Turtle Newsletter 71:1-4.

Ross, J. P. 1996. Caution urged in the interpretation of trends at nesting beaches. Marine Turtle Newsletter 74:9-10.

SELINA S. HEPPELL, Duke University Marine Laboratory, 135 Duke Marine Lab Road, Beaufort, North Carolina 28516 USA; E-mail: ssh4@acpub.duke.edu