by Miranda Bell-Tilcock

It is strange to think of an eye as a diet journal, but a fish’s eye can tell much about what it has been eating at each point in its life. If we know what a fish has been eating and when, then we can figure out where a fish has been. You just need to know where to look and how to understand what the eye is telling you. This approach formed the basis for our publication Advancing diet reconstruction in fish eye lenses in Methods in Ecology and Evolution. In this study, we used stable isotopes of carbon, nitrogen, and sulfur in fish eyes to better understand diet and habitat history of juvenile and adult Chinook Salmon (Oncorhynchus tshawytscha).

Stable isotopes such as carbon, nitrogen, and sulfur are natural markers found in the environment and can be integrated into tissues of fishes through diet (see Halloween blog). As a fish grows, many tissues eventually are replaced with new cells that isotopically resemble the habitat where the fish is currently feeding. This makes it difficult to track diet over a fish’s lifetime, with the exception of the eye lens. Fish eye lenses are onion-like spheres, rich in protein. Similar to onions, eye lenses are composed of individual layers that accumulate throughout a fish’s lifetime. Each layer represents a different time in a fish’s life. Applying stable isotope techniques with these individual layers helps researchers understand what and where a fish was eating.

When we applied this technique to the single adult Chinook Salmon, we could see that this fish had early life history values indicative of hatchery rearing. We then could identify when the fish reached the estuary before moving into the ocean. Once you isolate what habitat fish are using, then you can begin to quantify it for long term success.

With this study as a proof of concept, the tool can be applied to reconstruct juvenile life histories, and we can now begin to use it to quantify restoration efforts. Using Chinook Salmon as an example, we want to understand the long-term benefits of restoring and managing floodplains for juvenile salmon. We are now using this isotope tool in addition to the otoliths to reconstruct the life history of fish returned to spawn and understand the amount of time spent on floodplains before out-migrating to the ocean. This work will in turn help measure the success of restoration and management actions.

While this technique was particularly insightful for Chinook Salmon, its applications are not limited to salmon in California’s Central Valley. All over the world migratory species need freshwater habitat, and many of these environments are declining in quantity and quality. Understanding which habitats are important for fishes at different life stages can aid in conservation efforts and better tailor regulations and land and water management for recovery.

Miranda Bell-Tilcock is an assistant research specialist at the Center for Watershed Sciences, UC Davis. 

Further Reading:

Video peeling a juvenile Chinook Salmon lens

Listen to Science Friday: Seeing The World Through Salmon Eyes

Eye-Popping Research Helps Inform Salmon and Floodplain Management

https://www.ucdavis.edu/news/eyes-reveal-life-history-fish

https://www.ucdavis.edu/news/podcasts-and-shows/unfold/fish-eyes-and-ears

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