Assessment of Compound-Specific Fatty Acid δ¹³C and δ²H Values to Track Fish Mobility in a Small Sub-alpine Catchment

Abstract

Methods for identifying origin, movement, and foraging areas of animals are essential for understanding ecosystem connectivity, nutrient flows, and other ecological processes. Telemetric methods can provide detailed spatial coverage but are limited to a minimum body size of specimen for tagging. In recent years, stable isotopes have been increasingly used to track animal migration by linking landscape isotope patterns into movement (isoscapes). However, compared to telemetric methods, the spatial resolution of bulk stable isotopes is low. Here, we examined a novel approach by evaluating the use of compound-specific hydrogen and carbon stable isotopes of fatty acids (δ 2 H FA and δ 13 C FA ) from fish liver, muscle, brain, and eye tissues for identifying site specificity in a 254 km 2 sub-alpine river catchment. We analyzed 208 fish (European bullhead, rainbow trout, and brown trout) collected in 2016 and 2018 at 15 different sites. δ 13 C FA values of these fish tissues correlated more among each other than those of δ 2 H FA values. Both δ 2 H FA and δ 13 C FA values showed tissue-dependent isotopic fractionation, while fish taxa had only small effects. The highest site specificity was for δ 13 C DHA values, while the δ 2 H isotopic difference between linoleic acid and alpha-linolenic acid resulted in the highest site specificity. Using linear discrimination analysis of FA isotope values, over 90% of fish could be assigned to their location of origin; however, the accuracy dropped to about 56% when isotope data from 2016 were used to predict the sites for samples collected in 2018, suggesting temporal shifts in site specificity of δ 2 H FA and δ 13 C FA . However, the predictive power of δ 2 H FA and δ 13 C FA over this time interval was still higher than site specificity of bulk tissue isotopes for a single time point. In summary, compound-specific isotope analysis of fatty acids may become a highly effective tool for assessing fine and large-scale movement and foraging areas of animals.

Analysis of omega-3 and omega-6 polyunsaturated fatty acid metabolism by compound-specific isotope analysis in humans

“…Deuterium is fractioned similarly to 13 C but within the water cycle through different processes such as condensation across different latitudes and evaporation ( 54 ). 2 H isotope composition has been used to track migration using precipitation gradients across geographical regions ( 55 ), though this method is rapidly advancing to assess food sources and metabolism of PUFA ( 56 ). 2 H values may have a larger isotopic separation between diet sources because of larger relative mass differences ( 57 ), and could be use to study PUFA metabolism in humans in a future study.…”

Protocols for sample preparation and compound-specific stable-isotope analyses (δ2H, δ13C) of fatty acids in biological and environmental samples

“…In different study settings, we recently demonstrated that carbon and hydrogen isotope values of fatty acid generally show independent patterns [ 21 , [23] , [24] , [25] , [26] , [27] ]; however, these patterns might influence each other and exhibit negative kinetic inhibition in metabolic processes if one isotope is artificially enriched in an experimental setup [21] . So far, our findings also indicate that δ 13 C values of FA are conservative, meaning they show fewer inter-species and metabolic (i.e., ecophysiological) dynamics within a specific spatial and temporal area and have a high potential for particular site tracing of the origin of a sample.…”

“…In contrast, δ 2 H values of FA are highly sensitive to environmental and physiological changes. They can exhibit a comparatively extensive range of δ -values; thus, H isotopes are preferable when investigating metabolic and ecophysiological processes [ 21 , 25 ].…”

“…When comparing δ 2 H values of fatty acids between diets and consumers, four key processes with high potential for H-isotope fractionation have to be considered: (1) H-isotope fractionation during dietary fatty acid uptake and allocation to individual tissues; (2) the H-isotope composition of cellular water, acetate, and NADPH, which are used for fatty acid synthesis and modification; (3) H-isotope fractionation by enzymes involved in the lipid biosynthetic pathways, which in general utilize lighter substrates faster than their heavier isotopologues, and; (4) H-isotope fractionation due to fatty acid transport and metabolism [31] . As a result, fatty acids that a consumer synthesize are often 2 H-depleted compared to the diet, while fatty acids that are not needed by a consumer and thus catabolized become comparatively enriched in 2 H [ 24 , 25 ].…”