MTN 76:17-18 Further Scrutiny of Mexican Ridley Population Trends

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FURTHER SCRUTINY OF MEXICAN RIDLEY POPULATION TRENDS

Márquez et al. (1996a,b) reported that the nesting populations of ridley turtles on the Mexican beaches of Rancho Nuevo and Escobilla appear to be increasing. According to those authors, both populations have undergone declines due to human exploitation, but with the increased protection that was implemented beginning in the late 1960's and early 1970's, the populations are now increasing in size. The evidence for this is the apparent increase in numbers of nesting females seen on the beaches during recent seasons. In response, Ross (1996) has suggested that there is no reason to infer from the data presented by Márquez et al. that the populations are increasing. He has cautioned that the best that can be said about these populations is that they are not decreasing. However, there are some apparent inconsistencies with the logic given for these cautions, which I believe deserve scrutiny.

The first caution offered by Ross is his suggestion that the number of nesting females is not a reliable indication of population size. An increase in numbers of nesting females may not necessarily be related to an increase in population size. In general, this observation should be kept in mind by all. However, given the difficulties in monitoring sea turtles in their marine habitat, at the current time the best estimate of population size is and must be based on numbers of females nesting. Indeed, Ross (1982) himself has used data on changes in numbers of nesting females to infer declines in sea turtle populations. It seems inconsistent to use changes in numbers of nesting females as evidence for declining sea turtle populations, but not as evidence for increasing ones.

The second caution relates to the assertion that the yearly numbers of nesting females fluctuates greatly over time, and the Mexican data from the last few years fall within this range of variation. Although it is clear that there is fluctuation in numbers of nests across years on both beaches, the recent data from both rookeries suggest that there are positive increases. I have estimated the numbers of nests as accurately as possible from the graphs given in Márquez et al. (1996a,b). In the case of La Escobilla, the 718,800 nests of the 1994/95 season exceed the 95% confidence interval (151,130 - 270,690) generated from the data given from 1973 and 1994. This strongly suggests that this recent increase in nests does not fall within the normal range of variation, in contrast to what Ross contends. As well, Márquez et al. (1996a) reported a staggering 670,480 nests for the first 75% of the 1995/96 season, which reinforces the interpretation that the population is indeed increasing. In the case of Rancho Nuevo, Márquez et al. (1996b) reported that in the last eight years there has been a steady increase in the numbers of nesting females. I have fitted a regression line to the numbers of nests from 1988 to 1995. From the regression analysis, the slope was 59.7, and is significantly different from 0 (p=0.014), which strongly suggests an increase in numbers of nesting females over the past eight years.

The third caution given by Ross is in the form of analogy, where he compares the numbers of nesting females in a given season to playing dice or cards. Surely the analogy does not fit, for it suggests that the number of nests laid each year on a particular beach is an entirely random event. Instead, the number of nests to be laid in one year does have some relationship to howmany were laid in the previous years, and this is why it is possible to detect long term patterns of nesting females. If we record numbers that are different (increasing or decreasing), then we can analyze them for indications of increases or decreases. In the case of La Escobilla and Rancho Nuevo, it appears that recent numbers point to increases in nesting females, and this is currently the best index of increasing populations.

Márquez M., R., C. Peñaflores and J. Vasconcelos. 1996a. Olive ridley turtles (Lepidochelys olivacea) show signs of recovery at La Escobilla, Oaxaca. Marine Turtle Newsletter 73: 5-7.

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

Ross, J. P. 1982. Historical decline of loggerhead, ridley, and leatherback sea turtles, p.189-195. In: K. A. Bjorndal (Editor), Biology and Conservation of Sea Turtles. Smithsonian Institution Press. Washington, D. C. 583 pp.

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

MATTHEW H. GODFREY, Department of Zoology, University of Toronto, 25 Harbord St., Toronto, Ontario, M5S 3G5 CANADA.