Foraging ecology of five toothed whale species in the Northwest Iberian Peninsula, inferred using carbon and nitrogen isotope ratios

The feeding ecology and habitat use of the most frequently sighted and/or regularly reported stranded or by-caught toothed whale species of the North Western Iberian Peninsula (NWIP) were examined, with a special focus on their trophic position (TP) and relationships with their prey. With this aim, the stable isotope ratios of carbon (d13C) and nitrogen (d15N) of common dolphin (Delphinus delphis), bottlenose dolphin (Tursiops truncatus), harbour porpoise (Phocoena phocoena), striped dolphin (Stenella coeruleolba) and long-finned
pilot whale (Globicepahala melas) were analyzed in muscle samples taken from stranded and by-caught animals between 2004 and 2008. Stable isotopes were also measured in 17 species of fish and cephalopods previously identified as prey species, based on stomach content analyses, and in plankton. The trophic enrichment factors (TEF) were calculated for all five species and in addition, isotopic mixing models were applied to estimate the proportional contribution of each prey source to the diet of the common dolphin, which was the toothed whale species best sampled in our study. Plankton, fish and cephalopods exhibited an increasing trend in their d13C values (from-19.6‰to-15.3‰) along the offshore-inshore axis, with a less clear spatial pattern observed for d15N values. Striped dolphins exhibited the lowest mean d13C, d15N and TP values
(-17.6‰, 10.8‰ and 4.3, respectively), which confirms the oceanic character of this species and its lower trophic position when compared to the other toothed whales analyzed. The common dolphin exhibited mean d13C, d15N and TP values that were at an intermediate level (-17.0‰, 11.7‰ and 4.7, respectively)
and results of the mixing model indicated that blue whiting (Micromesistius poutassou) was the main component of the diet. The harbour porpoise, bottlenose dolphin and pilot whale exhibited higher and very similar isotopic compositions and TPs. The mean TEF obtained between predators and their main prey were 1.4‰ for d15N and 0.8‰ for d13C. These results provide information on stable isotope incorporation data for toothed whales, which are essential if conclusions are to be drawn in issues concerning trophic structures and habitat
use in the NWIP.