Reading a Salmon’s Passport: How Otoliths Reveal the Secret Life of Sockeye and Kokanee

Have you ever wondered whether a fish spent its life cruising the open ocean or staying close to home in a freshwater lake? For fisheries managers, Indigenous communities, conservation groups, and resource developers, the answer can be incredibly important. Understanding whether a fish was anadromous (migrating to the ocean and back) or resident (remaining in freshwater) helps guide conservation decisions, evaluate habitat connectivity, monitor population health, and better understand changing environmental conditions.

Fortunately, fish carry a surprisingly detailed travel diary inside their heads.

Enter the otolith. Often called an “ear bone,” an otolith is a calcium carbonate structure that grows throughout a fish’s life, forming annual growth rings, or annuli, much like a tree. As the otolith grows, it incorporates elements from the surrounding water, permanently recording where the fish has been.

One of the most useful clues locked inside an otolith is the ratio of strontium (Sr) to calcium (Ca).

Why Strontium?

Strontium naturally occurs in both freshwater and marine environments, but seawater contains considerably higher concentrations of strontium than most lakes and rivers. As fish absorb water through their gills and consume food, trace amounts of dissolved strontium enter their bloodstream. Some of that strontium becomes incorporated into the growing otolith, replacing calcium within the crystal structure.

Think of it as a chemical fingerprint. More strontium in the environment generally means more strontium recorded in the otolith.

As a result, fish that spend time in the ocean typically show elevated Sr:Ca ratios, while fish living exclusively in freshwater display lower values.

The Mother’s Signature

Now for a fascinating twist.

The very centre of the otolith, known as the core, forms during early development and contains chemical information transferred from the mother. This means the core can tell us something about the fish’s maternal origin before the fish even started swimming on its own.

If the otolith core shows an elevated Sr:Ca ratio, the mother was likely anadromous and had spent time in the marine environment before spawning. In sockeye salmon studies, this is often interpreted as evidence that the mother migrated to the ocean.

If the core exhibits low Sr:Ca values, the mother was most likely a resident freshwater kokanee.

In other words, the otolith core provides a glimpse into mom’s life history.

The Fish’s Own Story

While the core tells us about the mother, the outer edge of the otolith tells us about the fish that was actually captured.

The otolith grows from the centre outward. As we move from the core toward the edge, we are essentially travelling forward through time.

An elevated Sr:Ca ratio near the edge indicates the fish itself spent time in marine waters and is therefore considered anadromous (sockeye).

A low Sr:Ca ratio near the edge indicates the fish remained in freshwater throughout its life and is classified as resident (kokanee).

The beauty of otolith chemistry is that it allows us to identify all combinations of life histories:

• Marine-Marine: Anadromous fish, anadromous mother (Sockeye, Sockeye).
• Marine-Resident: Anadromous fish, resident mother (Sockeye, Kokanee).
• Resident-Marine: Resident fish, anadromous mother (Kokanee, Sockeye).
• Resident-Resident: Resident fish, resident mother (Kokanee, Kokanee).

As shown in the accompanying figure, elevated Sr:Ca values at the core identify maternal anadromy, while elevated values near the edge reveal the fish’s own ocean-going behaviour.

A Tiny Structure with a Big Story

At first glance, an otolith looks like an unremarkable little bone. Yet hidden within its microscopic layers is a lifelong record of migration, maternal history, and environmental exposure. By measuring Sr:Ca ratios from the core to the edge, scientists can determine not only whether a fish ventured to sea, but whether its mother did as well.

It’s a bit like reading a passport, a family tree, and a travel journal all at the same time, except the pages are made of calcium carbonate and the author smells vaguely of salmon.