Long-term dietary segregation of common dolphins Delphinus delphis in the Bay of Biscay, determined using cadmium as an ecological tracer

Dietary studies in marine mammals are traditionally performed by stomach contents analyses, which may be insufficient to determine long-term dietary preferences of these upper level predators. Our primary objective was to test the efficiency of trace metal measurements as complementary tools in dietary studies. Variations in cadmium (Cd) exposure through the diet and its effective renal levels in the short-beaked common dolphin Delphinus delphis were investigated to study the long-term feeding ecology of this predator in the neritic and oceanic waters of the Bay of Biscay. Based upon previous stomach contents analyses, the main prey occurring in the diet of common dolphins were analysed for their Cd contents. Results showed that cephalopods, and especially oceanic Cranchids and Histioteuthids, constituted a major source of Cd for common dolphins. Estimated Cd intake would therefore be 12 times higher in oceanic common dolphins (1400 ± 65 μg d–1) compared to neritic ones (120 ± 30 μg d–1). Accumulation of renal Cd concentrations with age was 5 times higher in by-caught oceanic dolphins than neritic ones (p < 0.0001). Within the neritic area, renal Cd accumulation rate was 2 times higher in by-caught individuals compared to stranded ones (p = 0.002). Thus, the use of Cd concentrations in by-caught dolphins proved efficient for assessing the existence of dietary segregation between neritic and oceanic common dolphins from the Bay of Biscay. However, using Cd data to make inferences about the feeding ecology of stranded animals should be considered cautiously.