Effects of fixatives on stable isotopes of fish muscle tissue: implications for trophic studies on preserved specimens.

Abstract

Isotopic ecology has been widely used to understand spatial connectivity and trophic interactions in marine systems. However, its potential for monitoring an ecosystem's health and function has been hampered by the lack of consistent sample storage and long-term studies. Preserved specimens from museum collections are a valuable source of tissue for analyses from ancient and pre-modern times, but isotopic signatures are known to be affected by commonly used fixatives. The aim of the present study was to understand the effects of fixatives on isotopic signatures of bulk tissue (δ13 Cm and δ15 Nm ) and amino acids (δ13 CAA and δ15 NAA ) of fish muscle and to provide correction equations for the isotopic shifts. Two specimens of each: blue cod (Parapercis colias), blue warehou (Seriolella brama), and king salmon (Oncorhynchus tschawytscha) were sampled at five locations along their dorsal musculature, at four time periods: (1) fresh, (2) after 1 month preserved in formalin, and after (3) 3 and (4) 12 months fixed in either ethanol or isopropanol. Lipid content was positively correlated with C:N ratio (r²=0.83) and had a significant effect on δ13 C after treatments, but not on δ15 N. C:N ratio (for δ13 Cm ) and percent N (for δ15 Nm ) from preserved specimens contributed to the most parsimonious mixed models, which explained 79% of the variation due to fixation and preservation for δ13 C and 81% for δ15 N. δ13 CAA were generally not affected by fixatives and preservatives, while most δ15 NAA showed different signatures between treatments. δ15 NAA variations did not affect the magnitude of differences between amino acids, allowing scientists to retrieve ecological information (e.g., trophic level) independently of time under preservation. Corrections were applied to the raw data of the experiment, highlighting the importance of δ13 Cm and δ15 Nm correction when fish muscle tissues from wet collections are compared to fresh samples. Our results make it possible to retrieve δ13 Cm , δ15 Nm , δ13 CAA , and δ15 NAA from museum specimens and can be applied to some of the fundamental questions in ecology, such as trophic baseline shifts and changes in community's food web structure through time.