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Grey-headed Gull and Royal Tern Eggs Contained Trace Metals and PAHs in Senegal

Grey-headed Gulls in Saloum Delta National Park, Senegal
Conservation · Threats

Lead, cadmium, mercury and polycyclic aromatic hydrocarbons (PAHs) were detected in eggs of grey-headed gull (Chroicocephalus cirrocephalus) and royal tern (Thalasseus maximus) from two protected coastal areas in Senegal. The 40 abandoned or non-viable eggs were collected in June 2019 — ten of each species in both Langue de Barbarie National Park and Saloum Delta National Park — and were intended to provide a chemical baseline for later monitoring.

Eggshells and egg contents were analysed separately. Lead, cadmium and mercury were measured as trace metals, while PAHs — organic contaminants associated with sources such as incomplete combustion and petroleum-related pollution — were analysed as a separate pollutant group.

Royal tern eggs generally contained more trace metals

Lead was the dominant trace metal in most samples and reached its highest measured concentrations in royal tern eggs from Saloum Delta. Royal terns generally had higher trace-metal concentrations than grey-headed gulls, especially in egg contents. Mercury was comparatively low and occurred mainly in the egg contents; at Saloum Delta it could not be quantified in the eggshells of either species.

The statistical comparisons are important. The study did not find a clear overall difference in trace-metal contamination between the two parks. Species differences were more evident in egg contents than in shells, so the results do not support a simple conclusion that one park was uniformly more contaminated than the other.

PAHs were distributed differently between shells and egg contents

PAH patterns varied among species, parks and egg compartments. Royal tern eggs from Langue de Barbarie had relatively high total PAH concentrations in both shells and contents. At Saloum Delta, grey-headed gull egg contents also contained high PAH concentrations, with benzo(a)pyrene making up most of the measured total there.

Benzo(a)pyrene was the dominant compound in many egg-content samples. The apparent differences in total PAHs between species and parks were not statistically clear, but shells and egg contents differed significantly within each species. Lower-molecular-weight PAHs tended to occur more in shells, whereas heavier compounds such as benzo(a)pyrene were more prominent in the egg contents.

The 2019 samples are a baseline, not evidence of a pollution source

The authors discuss several plausible local inputs, including shipping and fishing activity, fuel and oil discharges, wastewater, runoff and combustion of biomass or waste. However, the study did not analyse water, sediment, prey or adult birds, so it cannot identify where the contaminants measured in the eggs originated.

The samples were collected before Senegal’s later major offshore hydrocarbon production and are therefore useful as a pre-development reference for future comparisons. They cannot be used to attribute the 2019 contamination to oil and gas projects that began later.

The study measured exposure, not reproductive harm

Only ten eggs per species per park were analysed during one breeding season, and the study did not measure embryo viability, eggshell thickness, breeding success or other toxicological endpoints. Mercury was also measured as total mercury rather than separated into chemical forms.

The results therefore document chemical exposure in two protected coastal bird populations, not demonstrated toxic effects. Their main value is that future measurements can be compared with this 2019 baseline to test whether contaminant patterns change as coastal and offshore development increases.

Sources and image credit

  1. Diankha, O., & Gueye, I. (2026). Assessment of trace metal and polycyclic aromatic hydrocarbon concentrations in seabird eggs and eggshells from two national parks in Senegal. Journal of Applied Chemical Science International, 17(2), 59–72. https://doi.org/10.56557/jacsi/2026/v17i210753Primary open-access research article published 25 June 2026. Ten Grey-headed Gull eggs and ten Royal Tern eggs were sampled in each of Langue de Barbarie and Saloum Delta national parks. Lead, cadmium, mercury and PAHs were analysed separately in eggshells and egg contents.

Image: By cheikh pic / Wikimedia Commons — Grey-headed Gulls in Saloum Delta National Park, Senegal; photographed in one of the two study parks · CC BY-SA 4.0 · Source

About this content: This story was produced with AI assistance within an editorial workflow developed by Wildlife Vagabond. Editorial responsibility remains with Wildlife Vagabond.How AI is used

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