By Alexandra Chu and Danhong Ally Li
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For those familiar with fish archival tissues, fish otoliths are likely one of the first things that come to mind. Otoliths are indeed remarkable tools, offering insights into the water chemistry and trace elements the fish encountered while they were alive. However, we want to highlight another fascinating tissue on the rise – the fish eye lens.
To understand the rise of this novel tissue, we first need to understand the needs behind its growing use. As fish biologists, we are often fueled by these burning questions: Where did this particular fish come from? What did it eat? While their otoliths may help us answer the first question, they fall short when it comes to piecing together a diet record. Yet, understanding diet is crucial for identifying the key habitats and resources that support these populations.
Ecological systems are structured by who eats whom—and where. Understanding diet and feeding patterns helps reveal the habitats and resources that matter most. Studying a species’ diet allows scientists to reconstruct food webs, study predator-prey dynamics, and identify regions that are especially important for biodiversity (Bell‐Tilcock et al., 2021; Tzadik et al., 2017; Young et al., 2022). Tracking dietary shifts across different life stages is especially important for species with complex life cycles – like our California Chinook salmon. Over the span of 2-4 years, Chinook migrate from freshwater rivers to the open ocean and back again, relying on different habitats and food sources along the way. Their diets can reveal which habitats are critical for growth and survival and help guide efforts to protect the resources Chinook depend on. However, capturing a complete dietary history is often challenging, especially for migratory species, as traditional methods, like prey sampling and gut content analysis, offer only a snapshot of an individual’s diet at a specific time.
Traditional methods of dietary analysis, such as stomach content or prey sampling, only provide brief snapshots of an individual’s diet, often missing important dietary changes that occur over a lifetime (Curtis et al., 2020). In contrast, looking at the isotope values in fish tissues tells us about what they’ve eaten over a longer timescale. Different tissues have different isotopic turnover rates, which limits how far back in a fish’s life diet can be examined. For example, liver reflects short-term feeding (typically a few weeks), whereas muscle integrates diet over longer periods, typically one or two months (Bell-Tilcock et al., 2021; Simpson et al., 2019). As a result, these tissues primarily capture recent diet and provide limited insight into longer-term feeding history.
Eye lenses offer a way around this limitation. Although they have not commonly been used in stable isotope studies, eye lenses are increasingly recognized as an ideal archival tissue because they can preserve a lifetime record of a fish’s diet. Like tree rings, fish eye lenses contain distinct layers, which we refer to as lamina (or laminae for plural). As fish grow, each new lamina becomes inert; its molecular composition stops changing, leaving behind chemical traces of the fish’s diet at that time (see Figure 2). The fish’s eye lens acts as a comprehensive dietary journal, documenting what the fish consumed from juvenile stages through adulthood. This allows researchers to “peel back” layers to investigate how the fish’s diet changed throughout its lifetime. As research in this area grows, so does the need for a standardized process for peeling and measuring the lens layers to ensure accuracy and reproducibility. (To read more about eye lens research at UC Davis, click here)
The Results / Study
Recent eye lens isotope studies typically use lens size (diameter or radius) (Bell‐Tilcock et al., 2021; Wallace et al., 2014) as a proxy for sequential growth over a fish’s lifetime. This approach builds on earlier methods that relied more heavily on lamina counts, which involves counting discrete layers that form in the eye lens over time. Yet, no research has been done to compare the two parameters and to justify such a transition. To address this issue, our team conducted a study to compare the accuracy of two different parameters used in eye lens research: lamina number and lens diameter. Each researcher independently peeled one eye from the same fish. We then used the isotope values obtained from these lenses as a validation metric to compare these two parameters. Prior research confirmed that isotopic values between both eyes of a single fish are identical, making this approach effective for comparison (Wallace et al., 2014).
Our study revealed differences between researchers in how they peeled the layers, with one consistently removing smaller, thinner layers, and the other peeling thicker layers (see Figure 4). This variability in peeling led to variances in the total number of laminae, which could potentially cause confusion when interpreting when dietary shifts or changes in habitat occurred. Differences in lamina counts between readers could misalign isotopic results, leading to divergent interpretations of the data. For example, one researcher might identify laminae 2–7 as representing the fish’s juvenile period, while another researcher’s thicker laminae numbers 2–7 correspond to a different and potentially broader timeframe. Such discrepancies could mistakenly suggest that dietary or habitat shifts occur at incorrect developmental stages, complicating our ecological understanding of the species.
To resolve this issue and increase accuracy, we recommend using lens diameter rather than lamina number. When plotting isotope data against lens diameter (our validation metric), results from both researchers aligned (see Figure 5). Using lens diameter instead of lamina number as the standardized parameter produced consistent results across researchers, removing subjectivity and significantly improving reproducibility. Our findings clearly indicate that using lens diameter as the standard metric for delamination ensures consistent results across different researchers, providing a more reliable method for interpreting dietary histories from fish eye lenses. Importantly, adopting lens diameter as the standard measurement allows comparison of results across different studies, enhancing the broader applicability and relevance of eye lens research. By adopting diameter-based measurements, we can confidently reconstruct dietary life histories and identify key habitats/resources for Chinook Salmon. This improved resolution can better inform management decisions and support more effective conservation of Chinook and other species.
About the Authors
Danhong Ally Li is a researcher in the Salmon Ecology Lab, where she is an “eye-xpert” in decoding fish eye lenses – natural archives that record the dietary history of fish. Her eye lens work in stable isotope analysis focuses on the juvenile stage of Chinook Salmon in California’s Central Valley, where she quantifies off-channel habitat use (e.g. floodplains) to inform conservation strategies and policies aimed at preserving salmon populations. [Google Scholar Profile]
Alexandra Chu is a graduate student in the Salmon Ecology Lab. She considers herself a “forensic fish biologist,” working backward from evidence left in fish tissues to reconstruct their dietary and habitat histories. Alexandra uses stable isotopes to understand how shifts in marine diets contribute to thiamine deficiency in Chinook Salmon. [Google Scholar Profile]
Further Reading
Chu, A., Li, D. A., Bell-Tilcock, M., Lowe-Webb, M., Jeffres, C., & Johnson, R. C. (2025). Enhancing reproducibility in stable isotope analysis (SIA) of fish eye lenses: A comparison between lamina number and diameter. PLOS ONE, 20(6), e0326345. https://doi.org/10.1371/journal.pone.0326345
References
Bell‐Tilcock, M., Jeffres, C. A., Rypel, A. L., Sommer, T. R., Katz, J. V. E., Whitman, G., & Johnson, R. C. (2021). Advancing diet reconstruction in fish eye lenses. Methods in Ecology and Evolution, 12(3), 449–457. https://doi.org/10.1111/2041-210X.13543
Curtis, J., Albins, M., Peebles, E., & Stallings, C. (2020). Stable isotope analysis of eye lenses from invasive lionfish yields record of resource use. Marine Ecology Progress Series, 637, 181–194. https://doi.org/10.3354/meps13247
Simpson, S. J., Sims, D. W., & Trueman, C. N. (2019). Ontogenetic trends in resource partitioning and trophic geography of sympatric skates (Rajidae) inferred from stable isotope composition across eye lenses. Marine Ecology Progress Series, 624, 103–116. https://doi.org/10.3354/meps13030
Tzadik, O. E., Curtis, J. S., Granneman, J. E., Kurth, B. N., Pusack, T. J., Wallace, A. A., Hollander, D. J., Peebles, E. B., & Stallings, C. D. (2017). Chemical archives in fishes beyond otoliths: A review on the use of other body parts as chronological recorders of microchemical constituents for expanding interpretations of environmental, ecological, and life-history changes. Limnology and Oceanography: Methods, 15(3), 238–263. https://doi.org/10.1002/lom3.10153
Wallace, A. A., Hollander, D. J., & Peebles, E. B. (2014). Stable isotopes in fish eye lenses as potential recorders of trophic and geographic history. PLoS ONE, 9(10). https://doi.org/10.1371/journal.pone.0108935 Young, M. J., Larwood, V., Clause, J. K., Bell-Tilcock, M., Whitman, G., Johnson, R., & Feyrer, F. (2022). Eye lenses reveal ontogenetic trophic and habitat shifts in an imperiled fish, Clear Lake hitch (Lavinia exilicauda chi). Canadian Journal of Fisheries and Aquatic Sciences, 79(1), 21–30. https://doi.org/10.1139/cjfas-2020-0318
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