Reconstructing trophic position over the past century for five Puget Sound fish species

The comparison of historical and modern food web dynamics allows ecologists to test whether the trophic connectivity we observe today is 'normal' in its historical context. Fish densities and abundances have changed across time, making it likely that fish trophic interactions and their trophic positions have also changed. Historical trophic data of marine fishes can now be extracted from the tissues of fluid-preserved specimens held in natural history collections via compound-specific stable isotope analysis of amino acids (CSIA-AA) of nitrogen. We conducted CSIA-AA to quantify trophic position change over the past century in 5 ecologically important fishes of Puget Sound, Washington, USA: Pacific hake Merluccius productus, walleye pollock Gadus chalcogrammus, copper rockfish Sebastes caurinus, English sole Parophrys vetulus, and Pacific herring Clupea pallasii, and examined the canonical trophic (glutamic acid) and source (phenylalanine) amino acids. For all fishes except copper rockfish, trophic position, glutamic acid, and phenylalanine values remained similar across time. For copper rockfish, glutamic acid but not phenylalanine values increased over time, indicating an increase in this species' trophic position. The observed increase in copper rockfish trophic position may be a function of diet switching and declining prey quality rather than a consequence of rockfish consuming higher trophic level prey. This study leverages more than 100 yr of trophic data of fishes representing various feeding guilds and demonstrates that some fish species may be more trophically resilient to major environmental change than expected. Efforts should be made to identify and conserve the trophic interactions of species experiencing change.