There are few resources available for assessing historical change in fish trophic dynamics, but specimens held in natural history collections could serve as this resource. In contemporary trophic ecology studies, trophic and source information can be obtained from compound-specific stable nitrogen isotope analysis (CSIA-AA-N). Can this method yield reliable trophic information from chemically-altered fish specimens held in natural history collections? We tested whether CSIA-AA-N can be used on fluid-preserved specimens by experimentally subjecting specimens to the most common natural history collections preservation protocol, and determining the effects of preservation on trophic level estimates and whether these effects stabilize over time.

We subjected whole Sebastes ruberrimus and Clupea pallasii to formalin fixation and 70% ethanol preservation. We extracted tissue samples from each fish pre-fixation, after each chemical change, and then in doubling time for 32–64 days once placed in the final preservative. All samples were subjected to CSIA-AA-N, and their glutamic acid and phenylalanine profiles and associated trophic position were examined for differences over time by species.

Glutamic acid and phenylalanine values were inconsistent in direction and magnitude, particularly during formalin fixation, but stabilized similarly (in 70% ethanol) among conspecifics. In some cases, the amino acid values of our final samples were significantly different than our initial pre-preservation samples. Nonetheless, significant differences in glutamic acid, phenylalanine, and estimated trophic position were not detected among samples that were in 70% ethanol for >24h.

Our results suggest that the relative trophic position of fluid-preserved specimens can be estimated using CSIA-AA-N, and CSIA-AA-N estimates for fluid-preserved specimens should only be reported as relative differences. Timelines of trophic position change can be developed by comparing specimens collected at different points in time, revealing trophic information of the past and cryptic ecosystem responses.

Please see main publication and appendices for detailed methods.