The effects of chemical preservation on bulk isotope ratios in Atlantic bluefin tuna and Atlantic bonito muscle tissue*

Biological samples collected during oceanographic research are often chemically preserved to maintain tissue integrity prior to analysis. However, chemical preservation can produce changes in isotopic signatures and elemental compositions of the preserved samples. These changes typically adhere to predictable ranges, but effects vary by species. The impacts of two commonly used chemical preservatives, formalin and ethanol, were tested on tissue samples from Atlantic bluefin tuna (Thunnus thynnus) and Atlantic bonito (Sarda sarda). Tissue samples underwent bulk isotope signature and elemental analysis for δ15N, δ13C, %N, %C, and C:N before chemical preservation and again after 1, 3, and 12 months. Significant increase in δ15N occurred after preservation in both formalin and ethanol (12-month preservation: +0.95 ‰ ± 0.2 formalin, +0.83 ‰ ± 0.3 ethanol T. thynnus; +0.9 ‰ ± 0.2 formalin,+0.86 ‰ ± 0.2 ethanol S. sarda). In most cases, a significant decrease in δ13C after preservation was observed, but the effect from formalin was most extreme (12-month preservation: -2.93 ‰ ± 0.2 formalin, -0.34 ‰ ± 0.4 ethanol T. thynnus; -2.86 ‰ ±0.2 formalin,-0.33 ‰ ±0.1 ethanol S. sarda). Changes to tissue C:N ratio were significant after preservation in formalin (+0.18 ± 0.1 T. thynnus; + 0.27 ± 0.1 S. sarda), but not after preservation in ethanol. Similarities in changes of each parameter were observed between both Scombrid species. The observed changes in δ15N (∼1 ‰) were minor relative to expected differences between trophic levels (3-5 ‰). However, decrease in δ13C by formalin (∼3 ‰) may result in misinterpretation of primary producer communities if corrections for preservation effect are not done. Changes in elemental composition (%N, %C, and C:N) were more variable. The mechanisms by which chemical preservatives interact with tissue carbon and nitrogen require further study to explain the relative changes in elemental composition over time.