Stable isotopes reveal contrasting trophic dynamics between host-parasite relationships: A case study of Atlantic salmon (Salmo salar) and parasitic lice (Lepeophtheirus salmonis and Argulus foliaceus).

Across existing fish host-parasite literature, endoparasites were depleted in δ N compared to their hosts, while ectoparasitic values demonstrated enrichment, depletion and equivalence relative to their hosts. δ C enrichment varied extensively for both endo- and ectoparasites across taxa and host tissues. In our case study, sea lice (Lepeophtheirus salmonis) were enriched in δ N relative to their farmed Atlantic salmon (Salmo salar) hosts, although the value contradicted the average that is currently assumed across the animal kingdom. Common fish lice (Argulus foliaceus) did not show a consistent trend in δ N compared to their wild S. salar hosts. Both parasitic species had a range of δ C enrichment patterns relative to their hosts. Farmed and wild S. salar had contrasting δ C and δ N, and signals varied across muscle, fin and skin within both groups. L. salmonis and A. foliaceus subsequently had unique δ C and δ N, and L. salmonis from opposite US coasts differed in δ N. Given the range of enrichment patterns that were exhibited across the literature and in our study system, trophic dynamics from host to parasite do not conform to traditional prey to predator standards. Furthermore, there does not appear to be a universal enrichment pathway for δ C nor δ N in parasitic relationships, which emphasizes the need to investigate host-parasite linkages across species.