Compound-Specific Nitrogen Isotope Analysis of Amino Acids in Eye Lenses as a New Tool to Reconstruct the Geographic and Trophic Histories of Fish

Abstract

Fish migration has always played an essential role in marine conservation and fisheries management. However, migration patterns are changing globally alongside changing ocean conditions. This affects the spatial scale of required governance and, consequently, our food supply. Technological advances in the bio-tracer approach using isotopes would make it possible to monitor fish migration differently from the conventional bio-logging method. Eye lenses of fish are incrementally grown and metabolically inert. Therefore, stable isotope analysis of eye lenses can reconstruct the geographic and trophic histories of fish. However, it is difficult to distinguish between the two pieces of information: migration and changing trophic relationships with the conventional bulk stable isotope approach. In this study, we measured the nitrogen isotope ratios of individual amino acids in eye lenses of chub mackerel (n = 3) from the western North Pacific to test whether the two pieces of information could be distinguished. A maximum of 34 growth layers was obtained from a single eye lens. For assigning the isotopic trends observed in lens tissues to specific life stages or size classes, we established a relationship between lens size and fork length (FL) as a reference framework. The reconstructed nitrogen isotope chorology of chub mackerel showed an increase in trophic position of about 1 unit from juvenile (FL ∼30 mm) to adult stages (FL ∼300 mm). The variation in nitrogen isotope ratios of phenylalanine (a source amino acid) in the eye lenses was –3 to 5‰. This baseline variability could be used for estimating fish movements. This study proposed compound-specific nitrogen isotope analysis of amino acids in eye lenses as a new tool capable of reconstructing the geographic and trophic histories of migratory fish.

A new era of isotope ecology: Nitrogen isotope ratio of amino acids as an approach for unraveling modern and ancient food web

“…from chub mackerel (Scomber japonicus). 137) The amino acid isotopic results of some of these samples showed lifelong changes in the ' 15 N of baseline. In particular, early life histories of the fish is densely recorded, allowing to reconstruct the diet for juvenile, which is often important for conservation and aquaculture of the species.…”

Reconstructions of individual fish trophic geographies using isotopic analysis of eye-lens amino acids

“…Considering that the present study is the only the second peer-reviewed study to reconstruct individual CSIA-AA histories and associated lifetime trophic trajectories from fish eye lenses [13], further study of migratory patterns via CSIA-AA is necessary to reveal more detailed trophic and geographic information. It should also be kept in mind that, although isoscapes on the West Florida Shelf present robust gradients, these gradients are somewhat dynamic (i.e., spatial stationarity cannot be assumed) [14], and thus one must be careful not to over-interpret data.…”

“…The present paper addresses spatio-temporal variability in baseline nitrogen using a larger mesopredator, Red Snapper (Lutjanus campechanus). Previous work by Harada et al [13] used stable-isotope analysis of eye-lens amino acids to calculate lifetime trophic positions of chub mackerel, a dedicated zooplanktivore, to suggest an increase in trophic position occurred during early life (i.e., its trophic trajectory was positive). We investigate whether CSIA-AA of eye lens laminae can be used to track trophic position changes and geographic movement over an individual fish life-history, which is known to change trophic position during life [3][4][5].…”