Diet of Breeding Caspian Terns Hydroprogne caspia at Banc d'Arguin, Mauritania, is Diverse and Differs Between an Offshore and an Inshore Site

Studying the breeding patterns and diet of conspicuous predators is a useful way to understand the structure of marine coastal food webs. The Banc d'Arguin, Mauritania, hosts a significant portion of the West African population of a widespread and conspicuous fish predator, the Caspian Tern Hydroprogne caspia. At two small islands, the relatively offshore Arel and the inshore Zira, in the Parc National du Banc d'Arguin, the size of the colonies and the timing of the first wave of breeding were measured during the breeding seasons 2015–2018. At Arel, the summed number of nests counted varied between 128 in 2015 to 306 in 2018 and at Zira between 375 in 2015 to 279 in 2018. Breeding at Arel commenced about a month later than at Zira. During the breeding season of 2016, the diet of Caspian Terns was assessed by the identification of hard items found in pellets collected near nests. All of these appeared to be parts of fish (e.g., otoliths, bones, vertebrae, scales, bullae and atlas). Despite the effort, for Arel 47% and for Zira 45% of the fish parts remained unidentified. Nevertheless, diet composition differed in interesting ways between the two colonies. At offshore Arel, the identifiable part of the diet consisted primarily of pelagic species, namely Hemiramphidae, while at inshore Zira, pellets consisted primarily of bentho-pelagic prey, namely Cichlids. Repeated sampling across years may show whether the evidence for earlier inshore breeding holds up and is consistently correlated with dietary differences.