Vulnerability of loggerhead turtle eggs to the presence of clay and silt on nesting beaches

Sea turtle nests usually suffer a high mortality on important nesting grounds. Understanding the main factors that influence hatching success and productivity on important rookeries of endangered populations is essential to properly manage and protect them. The amount of clay can be high on some nesting beaches and could affect egg incubation. In the main loggerhead rookery in the Eastern Atlantic (Boa Vista, Cape Verde), clays and silts are very common on near 10% of the main nesting beaches, and turtles do not avoid clay substrates when they lay their eggs. Nests incubated on sandy substrates rich on clay and silt had a very high mortality. The same occurs on nests with the eggs stained with clay and incubated in sand free of clay. The eggs experimentally incubated with a covering layer of clay (30%, 50% and 80% of the eggshell) suffered an important loss of water, which in the extreme cases was irreversible, causing the death of the embryos. 75% of the eggs with 80% of their eggshells covered with clay died, while the mortality rate of the control eggs was only 25%. The salt content of the clay/silt seems not to be responsible for the egg dehydration and death. The hatchlings born from eggs with only 30% of the eggshell covered by clay were slower that those born from control eggs. Substrates with a significant presence of clay/silt can seriously disrupt embryonic development, reducing the emergence success of nests and should be avoided on nest relocation programs.

We would like to thank the Ministry of the Environment of the Regional Government of Andalusia, the Regional Government of the Canary Islands and the Fuerteventura Inter-Island Council (Cabildo de Fuerteventura) for funding this research. We thank the General Directorate of Environment of Cape Verde (Direcção Geral do Ambiente), the Boa Vista municipal chamber (Câmara Municipal da Boavista), the Canary Institute of Marine Sciences and the University of Las Palmas de Gran Canaria for their help. We are grateful to Fernando Madrid for the soil sample analysis in the Institute of Natural Resources and Agrobiology of Seville (IRNAS), to the Cape Verdean NGO Cabo Verde Natura 2000 and to its staff for their logistical support, to all volunteers who took part in the collection of nests and the preparation of the experiments and, especially, to E.