Organochlorine contaminants in complete clutches of Morelet's crocodile (Crocodylus moreletii) eggs from Belize
Introduction
Studies investigating the reproductive effects of organochlorine (OC) contaminants on oviparous wildlife routinely involve analysis of chemical concentrations in eggs collected from the field. The literature is replete with reports of OCs in bird eggs, and reports also exist for fish and reptile eggs (primarily turtle and crocodilian) (Johnson and Morris, 1974, Portelli and Bishop, 2000). Only a small number of these studies, most pertaining to birds, have analyzed complete clutches for OCs (see Reynolds et al., 2004 and references therein); no complete reptile clutches have been examined. Instead, the most common approach for determining OC burdens in wild clutches is the egg sample technique (Blus, 1982), in which a sub-sample of eggs from a given clutch is examined for OCs and the contaminant profile of the entire clutch estimated from these samples. The basic assumption of this technique is that the OC residues in sampled eggs accurately reflect the OC profiles of the remaining eggs in the respective clutch (Blus, 1982, Custer et al., 1990). The advantage of this sampling technique is that it reduces the number of eggs taken from the wild as well as the costs associated with analyzing large numbers of eggs. However, for the technique to be useful, the optimal number or sequence of eggs to be collected from a clutch must first be determined. Multiple studies have addressed this topic, using both complete clutches and the egg sample technique, with results varying based on the animal species examined and the technique employed (for example, Blus, 1982, Custer et al., 1990, Ormerod and Tyler, 1990, Bishop et al., 1995, Reynolds et al., 2004).
Research on exposure and response of crocodilians to environmental contaminants has increased substantially over the last decade, due largely to studies showing population declines and reproductive impairment in American alligators (Alligator mississippiensis) inhabiting contaminated lakes in Florida, USA (see Guillette et al., 2000). To date, OCs have been detected in eggs of four of the 23 extant species of crocodilians (Ross, 1998): American alligators, American crocodiles (Crocodylus acutus), Morelet's crocodiles (C. moreletii), and Nile crocodiles (C. niloticus) (Campbell, 2003 and references therein; Pepper et al., 2004, Rauschenberger et al., 2004a, Sepúlveda et al., 2004). Based on sub-samples of eggs from multiple clutches, Hall et al. (1979) reported low intra-clutch variability in the types and levels of OCs in American crocodile eggs and attributed this consistency to the reptilian reproductive pattern of yolking and depositing an entire clutch simultaneously. Similarly, Heinz et al. (1991) observed close agreement in OC residues between pairs of alligator eggs from the same nests, and suggested that one egg per nest is likely sufficient for contaminant analysis. However, no study has examined OC profiles within and among complete clutches of crocodilian eggs.
The objective of this study was to examine the distribution and variation of OCs within and among complete clutches of Morelet's crocodile eggs from Belize. From this information, we then determined a minimum number of eggs needed from a clutch to precisely determine the mean concentration of a selected OC (p,p-DDE) representative of that clutch.
Section snippets
Egg collection and handling
Seven complete clutches of Morelet's crocodile eggs (one non-viable, two flooded, four which failed to hatch for unknown reasons) were collected from three freshwater wetlands in northern Belize during ongoing studies of the status, reproductive ecology, and ecotoxicology of this species (Platt, 1996, Platt and Thorbjarnarson, 2000, Wu et al., 2000a, Wu et al., 2000b, Rainwater et al., 2002, Rainwater, 2003, Pepper et al., 2004). Five clutches were collected from Gold Button Lagoon (GB), one
Results
A total of 175 Morelet's crocodile eggs from seven complete clutches was analyzed for OCs. Clutch sizes ranged from 17 to 35 eggs (Table 1). The major OC detected was p,p-DDE, occurring in 100% of the eggs. The parent compound, p,p-DDT, was the next most frequently detected OC, occurring in 57% of the eggs. Methoxychlor, aldrin, p,p-DDD, and endosulfan I were found less frequently (30%, 22%, 1%, and 1% of eggs, respectively). The highest concentration of OCs (sum of all OCs) in an individual
Discussion
Organochlorine pesticides were found in each of the seven complete clutches analyzed in this study. All eggs examined (n = 175) were found to contain p,p-DDE, which agrees with previous data on OCs in incomplete clutches collected from the same areas (Wu et al., 2000a, Wu et al., 2000b) and may reflect greater persistence and/or historical use of p,p-DDE over other OCs present in these wetlands. The presence of DDT metabolites in eggs reflects a more ubiquitous distribution of exposure throughout
Acknowledgments
We are grateful to Mark and Monique Howells for logistical support and accommodation in Belize. The necessary research and collection permits were issued by Rafael Manzanero and Emil Cano, Forest Department, Conservation Division, Ministry of Natural Resources, Belmopan, Belize. Support for this research was provided by Lamanai Field Research Center, Indian Church, Belize and the US EPA (Grant no. R826310 to STM). Support for SGP was provided by the Wildlife Conservation Society.
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