A fish’s life history is stored in its ears – and we can read it with the right tools

It is fall, and a female salmon has spent the last few weeks strenuously pushing forward against the current to the spawning grounds of her natal coastal stream. She has now laid hundreds or thousands of tiny eggs in the redds. While the female will now die, another life has begun. The embryo within the egg will feed off the yolk sac for many weeks, develop and grow, and finally hatch. As a small fry, she will spend her first months to years in that freshwater stream. She will then make her way down to the mouth of the river, and for the first time, swim into the ocean. Here, she may spend one to five years, where she will obtain up to 99% of her adult body weight.

Imagine if we could follow that salmon during its entire, multi-year life cycle. How long did she stay in the natal stream before she migrated to the ocean? How many years did she stay in the ocean? Was her natal stream contaminated with heavy metals? What about the ocean? Did her mother pass on heavy metals to her? Could her metal burden cause health implications to her? Or to those who consume her?

At TrichAnalytics Inc. we can answer those questions with a small, calcified tissue called an “otolith”. Otoliths (oto = ear, lith = stone) are the small calcium-based structures in the inner ears of teleost fish (ray-finned fish) used for sensing gravity, movement, direction changes and balance. Otoliths can preserve a permanent record of the fish’s environment and diet at each age and stage of its life. How is this possible? As the fish grows, a thin new layer of calcium crystal is added to its otolith every day starting from when it was just an embryo. Other elements will also get incorporated into the structure. The changes and ratios of those elements are what we monitor to reveal the life story of that individual fish.

The figure shows the otolith of a Chinook salmon after laser ablation (you can see the ablation line made by the laser) with the measured element ratios superimposed. We look at the Strontium:Calcium ratios (blue line) and the Barium:Calcium ratios (red line) to follow the freshwater to ocean migratory timeline of the salmon. Elevated Strontium ratios are linked with life in the ocean while elevated Barium ratios indicate life spent in freshwater. The core of the otolith shows Strontium and Barium ratios that are midway between freshwater and marine due to the developing fish using the nutrition from its mother from its egg yolk while living in the freshwater stream. The concentrations of other elements such as Magnesium, Manganese, Zinc, Cadmium and Lead can also be measured and graphed in this way to differentiate between the fish's use of different bodies of freshwater and to reveal potential contamination from industry. Wow! A single analysis on a single fish tissue can provide this much information!

Enhance your aquatic monitoring program by analyzing fish otoliths to obtain accurate water quality and fish health data. Contact us today at  info@trichanalytics.com.