A fish ear bone (otolith) with red dye marks separated by 2 months of growth. The lines show where the composition of the bone was analysed.
A fish ear bone (otolith) with red dye marks separated by 2 months of growth. The lines show where the composition of the bone was analysed.
Throughout our lives, the enamel on our teeth carries the tell-tale fingerprint of where we lived as children. Containing the characteristic proportion of strontium isotopes (87Sr:86Sr) in the water we drank, teeth carry the hallmark forever. And fish are much the same, acquiring chemical isotopes in their bones as they grow. But one set of bones – the otoliths in the ear – grow continually throughout a fish's lifetime, building up tissue each day like tree growth rings, carrying a permanent isotope record, which indicates the fish's location throughout its entire life. However, less was known about the rates at which various elements, such as strontium and barium, found in water accumulate in ear bones. How quickly do changes between locations show up in the bones and might some fish acquire elements from water at faster rates than others, possibly affecting how researchers interpret ear bone histories?
To find out, Matthias Vignon and colleagues from Université de Pau et des Pays de L'Adour, France, transferred young brown trout (Salmo trutta) from water with low levels of strontium and high levels of barium, to river water with naturally high levels of strontium and very little barium, marking the transition in the fish's otoliths by bathing them in a red dye to create a visible band in the bone. After 2 months in the new water, the team bathed the fish again in red dye, creating a second band, before boosting the barium in the water for a further 2.5 months. Finally, the researchers checked how energetic the individuals were by recording how much oxygen they consumed when resting and when swimming before collecting their ear bones, to analyse how the quantities of strontium and barium in the otoliths varied over time.
Meticulously recording how the proportion of both elements varied from the first red band to the surface of the ear bone, the team saw the quantities of strontium increase as the barium fell – in line with the levels in the fish's water – before the barium levels climbed again after the researchers had supplemented the barium in the water.
But when the team compared how fast individuals absorbed the isotopes, the most energetic fish fully incorporated the barium in their ear bones within ∼10 days, while the slowest took almost a month, and some fish could almost saturate fresh bone with strontium within a month while the slowest took almost 2.4 months. However, the strontium and barium levels in the ear bones always started changing soon after the fish were transferred between the water supplies. ‘Habitat transitions should therefore be discernible after a relatively moderate/brief residence time, but the otolith may not reflect ambient water levels for weeks’, says Vignon.
But what made some fish fast absorbers while others took longer to incorporate the hallmark elements in their ear bones? Comparing the fish's resting metabolic rates with their absorbance of both elements, the fish with the fastest metabolisms recorded water composition changes in their otoliths faster than fish with slower metabolic rates, which could skew how researchers interpret the location signals from the ear bones of fish communities if the population is particularly energetic or sluggish.
Vignon says, ‘The time taken for environmental changes to be reflected in the growing otolith thus can no longer be assumed to remain a constant within populations’, advising researchers that although otoliths are a key tool for tracking fish migrations, it might be better to not read too much about an individual's precise location at a specific time from their ear bone element profiles, which could depend as much on their vigour as their location.
