Monitoring organochlorine pollution in Audouinʹs Gull eggs: the relevance of sampling procedures

Levels of PCBs, DDT and HCB were determined in 56 eggs belonging to 26 complete clutches of Audouinʹs Gull (Larus audouinii) breeding in the Ebro Delta (Northeast Spain). DDT levels were comparable to those found in other areas of the region, while those of PCBs were far more variable being more site dependent. Variability in pollutant load was analysed for both intra- and inter-clutch effects. A case clutch approach was used to assess pollutant differences among clutch sizes and whether a laying order effect on egg xenobiotic contents exists. Two-egg clutches presented a significant female effect, whilst in three-egg clutches this was only significant for the second and third eggs. The environmental relevance of these results for monitoring programs and the usefulness of seabird eggs as biological indicators are discussed.