Inter- and Intraclutch Variability in Heavy Metals and Selenium Levels in Audouin’s Gull Eggs from the Ebro Delta, Spain M. Morera 1 , C. Sanpera 1 , S. Crespo 1 , L. Jover 2 , X. Ruiz 3 1 Departament de BiologiaAnimal, Biologia Vegetal i d’Ecologia, Facultat de Veterina `ria, Universitat Auto ` noma de Barcelona, 08193 Bellaterra, Spain 2 Departament de Salut Pu ´blica i L.S. (Bioestadistica), Facultat de Medicina, Universitat de Barcelona, Avda. Diagonal s/n, 08028 Barcelona, Spain 3 Departament de Biologia Animal (Vertebrats), Facultat de Biologia, Universitat de Barcelona, Avda. Diagonal 645, 08028 Barcelona, Spain Received: 10 June 1996/Revised: 28 October 1996 Abstract. Heavy metal (Zn, Cu, Mn, Cd, Pb, Hg) and Se concentrations were analyzed in 57 Audouin’s gull (Larus audouinii ) eggs belonging to different clutch sizes. Inter- and intraclutch variability in metal concentrations was investigated as a potential source of bias in the assessment of pollution levels. Moreover, we analyzed the relationship between metal levels in the shell and in the contents, to evaluate the reliability of museum eggshells as indicators of historical changes of these pollutants. An outstanding female effect and/or a laying order effect underlies egg Hg levels; the fact that eggs in a clutch are not independent observations needs to be taken into account both when designing sampling strategies and when performing any analysis or interpretation of the results. The relationship between Hg in shells and contents is not sufficiently accurate to allow the use of egg-shell concentration as a reliable predictor of egg-contents concentration. However, if changes in the ecosystems are large enough it could be used to trace gross historical trends of these pollutants. The Audouin’s Gull (Larus audouinii ) is one of the rarest gull species in the world, and one of the few SPEC species (Species of European Conservation Concern) belonging to category 1 (species of global conservation concern) in the Mediterranean (Tucker and Heath 1995). The main potential threat for this species comes from the concentration of 84% of the world population in only two breeding colonies, those of the Ebro Delta and the Chafarinas Islands (Pedrocchi and Ruiz 1995). This situation is particularly sensitive to epidemiological and ecotoxicological risks that may affect such populations during the breeding season (Mace and Collar 1994). In previous studies both, the incidence of organochlorine compounds at the Ebro Delta colony (Pastor et al. 1995a, 1995b) and potential risks derived from blood parasites (Ruiz et al. 1995) on these populations during reproduction, have been reported. However, nothing is known about other potentially hazardous elements, such as heavy metals. Mercury pollution is particularly relevant to Audouin’s gull, because it is a pelagic predator endemic to the Mediterranean (Ruiz et al. 1996), a sea characterized by comparatively high Hg levels (Lambertini and Leonzio 1986; Renzoni et al. 1986). Winterquarters of this species are located in the Atlantic coasts of Northern Africa (Oro and Martı ´nez-Vilalta 1994). Because eggs can adequately reflect the environmental levels of some metals (Hg or Se) they have been used to monitor the exposure of wild bird populations (Furness 1993). However, although eggs have several advantages over tissue samples (Becker 1989; Burger and Gochfeld 1995) they also have some drawbacks, especially when the potential bias associated with inter- and intraclutch variability factors is unknown (Furness 1993). Therefore, the study of these sources of variability is a key factor in the design of sampling procedures to assess the incidence of such pollutants (Becker 1989, 1992). In this study, inter- and intraclutch variability in concentra- tion of heavy metals (Zn, Cu, Cd, Mn, Pb, Hg) and Se borne by shells and contents of Audouin’s gulls eggs at the Ebro Delta was analyzed. Moreover, the relationship between levels of heavy metals in egg contents and in egg shell was examined, in order to assess the reliability of museum eggshells as surrogates of egg-contents, to trace historical changes of those pollutants in ecosystems (Furness 1993). Material and Methods Sampling During the 1992 breeding season, 6,714 breeding pairs nested at the Ebro Delta colonies. Fifty-nine nests (14 one-egg, 27 two-egg, 16 three-egg, and 2 four-egg clutches) of Larus audouinii were monitored daily to determine egg characteristics according to laying order. Once discovered, eggs were marked with a felt tip pen on the blunt end. A total of 57 freshly laid eggs were collected (under license): 6 one-egg, 11 two-egg, 7 three-egg, and 2 four-egg clutches, being replaced in the nest by surrogate eggs to avoid interference with the laying process. Eggs were transported refrigerated to the laboratory for further analyses. Correspondence to: C. Sanpera Arch. Environ. Contam. Toxicol. 33, 71–75 (1997) ARCHIVESOF E nvironment al Cont aminat ion and T oxicology r 1997 Springer-Verlag NewYork Inc.