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 (δ²H_FA and δ¹³C_FA) from fish liver, muscle, brain, and eye tissues for identifying site specificity in a 254 km² sub-alpine river catchment. We analyzed 208 fish (European bullhead, rainbow trout, and brown trout) collected in 2016 and 2018 at 15 different sites. δ¹³C_FA values of these fish tissues correlated more among each other than those of δ²H_FA values. Both δ²H_FA and δ¹³C_FA values showed tissue-dependent isotopic fractionation, while fish taxa had only small effects. The highest site specificity was for δ¹³C_DHA values, while the δ²H 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 δ²H_FA and δ¹³C_FA. However, the predictive power of δ²H_FA and δ¹³C_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.

Keywords: carbon stable isotopes; compound-specific isotope analysis; fatty acids; fish migration; hydrogen stable isotopes; site specificity.

Figure 1

Map of the river Ybbs catchment (47° 84′ 50″ N, 15° 81′ 20″ E) with fish sampling sites. Thick lines indicate impassable (high) migration barriers. Dashed lines indicate low impact migration barriers such as fish ladders or weirs that might be seasonally passable.

Figure 2

δ²H and δ¹³C values of FA in fish samples by sampling site altitude. A strong correlation was observed for δ¹³C, which showed all fatty acids getting isotopically lighter with altitude. No significant correlation with altitude was observed for δ²H values of fatty acids, with some fatty acids showing weak positive or negative correlations.

Figure 3

(A) Canonical correlation analysis (CCA) of tissue FA δ²H and δ¹³C data revealed significant isotopic separation of the brain from other tissue types in the first dimension. (B) Additionally, the third dimension revealed some minor differences between salmonids and bullheads.

Figure 4

(A) Site-specific repeatability analysis, controlling for taxa, tissue type, and sampling year, revealed high site specificity for n – 3 LC-PUFA (e.g., DHA and EPA) ¹³C-CSIA, but not for ²H-CSIA, in which ALA and ARA showed higher specificity. (B) Plotting the isotopic difference between δ²H_ALA and δ²H_LIN against δ¹³C_DHA already revealed site-specific clusters in the fish muscle tissue (ellipses show 50% CI). More distinct site-specific separation was achieved when plotting the first three dimensions of a linear discrimination analysis, which can be seen in the interactive view of the online Supporting Information of this article (Figure S3: δ²H and δ¹³C values of 11 FA quantified in all samples; proportions of trace: LD1 = 48.2%, LD2 = 23.6%, LD3 = 7.8%).