How Old Is That Fish? Stones in Its Head That Grow Like Trees

Otoliths are dense structures of calcium carbonate sitting in fluid-filled chambers of a fish's inner ear, and their biological function is sensory. Because they are denser than the fish's tissue, they lag behind when the fish accelerates or changes orientation, and hair cells beneath them detect that relative movement, which supplies information about balance, orientation and acceleration. They also contribute to hearing, since sound passing through a fish's body moves the tissue and the denser stone differently. Most fish carry three pairs of different sizes, and the largest pair is the one used for research because it is easiest to handle and shows the clearest structure.

The stone accumulates material continuously, laying down structure at several timescales:

• •Daily increments, visible under a microscope, which allow very young fish to be aged in days
• •Annual bands, formed by seasonal changes in growth rate, which are counted like tree rings
• •Width of each band, which records how fast the fish grew that year
• •Chemical composition, which reflects the water the fish was living in
• •Marks left by stressful events including spawning
• •Structure continuing after growth in body length has effectively stopped

Knowing the ages of fish in a catch is fundamental to assessing a stock and cannot be obtained any other way for most species. Age composition reveals whether a population contains older individuals or has been reduced to young fish, which is the clearest single indicator of overfishing. It establishes how fast a species grows and at what age it breeds, which determines what fishing pressure a population can sustain and what minimum size limits should be. It identifies strong and weak year classes, since a good spawning year produces a cohort that can be tracked through subsequent samples. Getting the ages wrong propagates into every assessment built on them, which is why reading is cross-checked between laboratories.

Extracting an age from a stone is a skilled and slow process and the difficulties are worth knowing. The stone is removed, cleaned and usually embedded in resin and sectioned through its centre, since bands are visible only on a cut plane at the right orientation, and the section is ground and polished until thin enough to read. Bands are counted under a microscope, sometimes after burning or staining to increase contrast. Readers disagree, particularly on older fish where bands crowd together at the edge, so laboratories exchange samples and compare results to measure their consistency. Validating that the bands really are annual requires independent evidence, obtained by marking fish chemically and recapturing them later or by ageing fish of known age, and it has not been done for every species that is routinely aged.

The composition of each layer carries further information, which has opened uses nobody anticipated. Trace elements incorporated as the stone grows reflect the chemistry of the water at the time, so a fish moving between a river, an estuary and the sea records those transitions in bands of different composition, which allows an individual's migration history to be reconstructed from a single stone. Oxygen isotopes record temperature. Matching the chemistry of a fish's early layers to the water chemistry of possible nursery areas identifies where it came from, which is used to establish which spawning grounds supply a fishery. Otoliths from archaeological sites and from sediment cores extend the same reading into the past, reconstructing conditions centuries before any instrument.

The stones sit in the inner ear and lag behind the fish when it moves, which is how they supply balance and hearing. They accumulate daily and annual layers, so counting bands gives age and their width gives growth rate. Trace elements record the water the fish was living in, which lets an individual's whole migration history be read from one stone.