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Marine Turtle Newsletter 76:14-17, © 1997

A NEW INTERPRETATION OF MEXICAN RIDLEY POPULATION TRENDS

Turtle conservationists have often been accused of refusing to accept good news, and the accusations have often been true. The motive is clear enough; "good news" might be dangerously equated to "recovered populations," leading inexorably to "delisting," "renewed harvest," "rapid return to endangered status," and similar problems. But the pessimistic approach can lead to its own difficulties. If public resources are expended for many years on projects for which even those receiving the funds and conducting the projects never seem to admit any success, the funds are in growing danger of being cut off. Furthermore, the "scientifically rigorous data analysis" urged by Ross actually supports optimism rather than skepticism in the evaluation of turtle nesting trend data. "Bad news" -- a poor nesting season -- may not be bad news at all, it may simply mean that few turtles were on-cycle to nest that year. But "good news" is less equivocal -- it is impossible for a lot of turtles to nest if there are not a lot of turtles out there.

I would further take issue with Ross (1996) in his gloomy suggestion that non-reproducing females and males may not be following these upward trends. Not only would Occam's Razor recommend against such purely theoretical arguments, but a cynic would simply argue "who cares?" If there are enough males to keep the eggs fertilized, that should be sufficient; and if non-breeding females are getting scarcer (either by some improbable differential mortality or by recruitment into the breeding female population), I shall lose no sleep. It has been observed from time to time that a count of nesting turtles is a poor index to population size because it reflects recruitment a generation ago rather than gives information about the current vigor of the population. But if, while counting those females, we also paid some attention to the numbers of hatchlings emerging and entering the sea, we would have a useful index to the potential population size a generation into the future in addition to our index to breeding success a generation ago. Where many managers (e.g. in the Talang Turtle Islands, or in Terengganu) have miscalculated is that they limited themselves to counting adults, becoming complacent if the numbers were good, and failing to protect eggs or take other means to ensure adequate recruitment into the population.

Ross's parallel with gamblers who foolishly interpret random runs of aces, sevens, or anything else as genuine trends, worth betting on, is flawed. The numbers of turtles nesting on a given beach in a given year constitute real data, not random numbers drawn from a hat or a roll of a dice. The trend lines may have to be interpreted carefully to allow for the alternation of "good" and "bad" years often seen in turtle populations (although this effect is much smaller in the case of the two ridley species, which often nest in successive years), but unlike the parallel with the gambler drawing all the aces from the pack, figures rising steadily over the years do reflect some form of reality. Even the most pessimistic scenario -- that the number of turtles is not increasing, but instead a higher percentage of the population is nesting each year -- is still good news; as far as the future is concerned, the numbers of eggs and hatchlings produced in the current season is more important than the absolute size of the adult population that produced them. And it is nonsense to suggest that there is no way that, say, 1000 turtles will nest this year just because that number came up before.

Furthermore, the 1994 Escobilla nesting numbers manifestly do not fall "well within normal annual variation." Rather, they represent about five times the typical level of annual nesting, and about double that of the best seasons observed in the preceding 20 years! Note too that the two best years during the preceding 20 were 1992 and 1993! In that the population, massively harvested for decades, was abruptly protected in 1990, the observed figures make eminent sense. The very roughly stable (actually slightly dropping) nesting numbers from 1973 to 1990 correspond to a population that was sufficiently stressed by harvest to be well below carrying capacity, but that with good recruitment brought about by nesting beach protection was able, more or less, to replace the very large numbers of females slaughtered each year for industrial purposes. Following legal protection and closure of the slaughterhouse in 1990, neophyte breeding females now had a reasonable chance of surviving into subsequent nesting seasons, and the histogram shows precisely the expected result -- a rather rapid and progressive accumulation of these survivors into the observed nesting population, which showed an increase from about 150,000 animals in 1990 to 700,000 in 1994. This increase is probably typical of protected sea turtle populations with a recent history of heavy harvest, but which had good hatchling production and recruitment throughout.

The histogram for numbers of nesting Kemp's ridleys for the 30-year period 1966 to 1995 (Márquez et al., 1996a) is very different, but again is similar to that which would have been predicted from the known history of this population. As is well known, an arribada of about 40,000 females was seen and filmed at Rancho Nuevo in 1947 (Carr, 1963), yet by the time beach protection started in 1966, total nesting was down to 2000-3000 per year. During the following decade, despite good beach protection and hatchling production, nesting numbers failed to recover, and indeed dropped to a low plateau of about 500 nests per year. This almost certainly reflects very low hatchling production for many years up to 1966; Hildebrand (1963) obtained some word-of-mouth reports of highly organized, industrial-scale egg collection during these years, and even for many years after the instigation of beach protection, competition for eggs (but not for turtles) from local residents was acute.

Caillouet et al. (1995) have argued that the maturation time of Kemp's ridley is approximately 10 years, and indeed, a little more than a decade after the first season of beach protection at Rancho Nuevo, the number of nests recorded annually doubled quite abruptly, to around 1000 nests per year. This presumably represents the first year of significant recruitment to the adult population for decades. The increase was not sustained. For many years after 1978, the histogram shows similarities to that for the olive ridleys at Escobilla between 1973-1990; that is, nearly stable, but progressively slightly declining numbers, at a level far below the carrying capacity of the system. There was little directed slaughter of adult ridleys during those years, but the loss of mainly subadult and juvenile turtles to shrimp trawlers was so intense that it was sufficient to inhibit recovery. A crucial difference in the demographies of Kemp's and olive ridleys is that the former are benthic, coastal feeders and are very susceptible to trawler mortality as juveniles as well as as adults, whereas the latter are pelagic as juveniles and generally it is only the adults that are caught or drowned in trawls.

Nonetheless, the declining curve at Rancho Nuevo leveled out in 1985-1987, and after that showed a reasonably steady increase, with the population undergoing an apparent doubling by 1995. The new variable, introduced in 1978, was the head-starting program, whereby the offspring from about 2,000 eggs each year were raised in captivity for nine to eleven months and "imprinted" on the shore of Padre Island, Texas before release (Shaver, 1989). The interval between initiation of head-starting and the first signs of apparent recovery of the nesting population was a year or so less than ten years, but it is not unlikely that the abundant feeding opportunity enjoyed by the turtles during their time in captivity caused them to reach a size more typical of two-year-olds by the time of release. Shaver (1996) reports the nesting of a head-started ridley on the Texas coast ten years after hatching.

In conclusion, there is no mystery about the form of the nesting histograms given by Márquez et al. (1996a,b). In both cases, the population in question responded in a reasonably predictable fashion, and after an appropriate response time, to the various manipulations to which it was subjected -- industrial slaughter, intensive egg exploitation, trawler mortality of subadults, egg protection, head-starting, and legal protection of adults. Should we then tolerate a recrudescence of industrial slaughter of olive ridleys, and ongoing trawler mortality of Kemp's ridleys? The answer to both is "no", for the following reasons:

i) Years ago (Pritchard, 1969), I argued that the socially appropriate pattern of turtle exploitation in coastal México is a controlled take of eggs, by local people, from those beaches where the turtles are at high and stable levels, combined with steps to ensure that the vast majority of eggs are protected and allowed to hatch. This philosophy is currently followed at Ostional, Costa Rica. Slaughter of turtles for the luxury leather market does very little for local people.

ii) Head-starting is an expensive procedure, and although it has recently achieved encouraging vindication (Shaver, 1996), it still qualifies as "halfway technology" (Frazer, 1992) and it does not make economic sense to head-start a Kemp's ridley at public expense only to have it lose its life in a trawl net.

iii) While "eco-tourism" has been oversold as an economic justification for protecting even unremarkable natural resources, there is no question that a large arribada of ridleys is a phenomenon so spectacular that many people would pay good money to see it. But a dispersed arribada, such as may result from heavy slaughter of the females as they gather offshore to nest, is not unusually spectacular, and has much less eco-tourism potential.

iv) Olive ridley skins may no longer be sold on international markets. In the absence of such a market, a large-scale commercial harvest would be pointless.

v) Turtle excluder devices (TEDs) work!

Caillouet, C. W., C. T. Fontaine, S. A. Manzella-Tirpak and D. J. Shaver. 1995. Survival of head-started Kemp's ridley sea turtles (Lepidochelys kempii) released into the Gulf of Mexico or adjacent bays. Chelonian Conservation and Biology 1(4):285-292.

Carr, A. F. 1963. Panspecific reproductive convergence in Lepidochelys kempi. Ernebn. Biol. 26:298-303.

Frazer, N. B. 1992. Sea turtle conservation and half-way technology. Cons. Biol. 6(2):169-184.

Hildebrand, H. H. 1963. Hallazgo del rea de anidación de la tortuga "lora" Lepidochelys kempii (German) en la costa occidental del Golfo de Mexico (Rept., Chel.). Ciencia, Mex. 22(4):105-112. Márquez M., R., R. A. Byles, P. Burchfield, M. Sanchez, J. Diaz, M. A. Carrasco, A. S. Leo and M. C. Jimenez. 1996a. Good news! Rising numbers of Kemp's ridleys nest at Rancho Nuevo, Tamaulipas, México. Marine Turtle Newsletter 73:2-5.

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

Pritchard, P. C. H. 1969. El programa Mexicano de tortugas marinas visto desde el exterior. Tecnica Pesquera, México 23:38-40.

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

Shaver, D. J. 1989. Results from eleven years of incubating Kemp's ridley sea turtle eggs at Padre Island National Seashore, p.163-165. In: S. A. Eckert, K. L. Eckert and T. H. Richardson (Compilers), Proc. Ninth Annual Workshop on Sea Turtle Conservation and Biology. NOAA Tech. Memo. NMFS-SEFC-232. U. S. Dept. of Commerce.

Shaver, D. J. 1996. Head-started Kemp's ridley turtles nest in Texas. Marine Turtle Newsletter 74:5-6.

PETER C. H. PRITCHARD, Florida Audubon Society, 1331 Palmetto Avenue, Suite 110, Winter Park, Florida 32789 USA.