Use of stable isotope ratio analysis to investigate the biology and clinical significance of seal parasites.

Stranded seals are often infected with a broad range of parasites, although whether they are the cause of significant morbidity or an incidental finding is usually unclear. In this study we used stable isotope ratio analysis, a method frequently used to investigate food webs, to explore the extent to which common seal parasites feed directly on host tissue and fluids or absorb host-derived metabolites, which in turn may give an indication of their potential impact on the host's health. The trophic discrimination factor ΔN for the nasal mite, , was mostly positive, ranging between −0.015 and 3.2‰ ( = 6), while for the Acanthocephalan worm, and the anisakid nematode, , ΔN ranged between −4.2 and −2.0‰ ( = 7), and between −6.7 and −0.8‰ ( = 5) respectively. In the case of the lungworm, , ΔN measured between −5.6 and 0‰ for worms collected in the stomach ( = 5), between −1.1 and 0.2‰ for worms collected from the heart ( = 3), between −0.7 and 1.9‰ for worms situated in the lungs ( = 4). Based on ΔN, parasites could be clearly divided into those that were on a higher trophic level than their host suggesting a predator–prey-like relationship, and those that were not. It is hypothesized that ΔN may be indicative of the clinical significance of parasite–host associations.