Tracking ocean change with fish ear bones: do’s and don’ts

Changes in the environment are imprinted on the hard parts of many animals. Researchers use ‘hard parts’, like ear bones (otoliths), teeth, feathers and toenails, to answer many different questions: “Has river flow drastically changed through time?’, “Is there more pollution now in this estuary than there used to be?” or “Where does this seabird find food for its chicks?”. To use an animal’s hard part as an environmental recorder, we have to work out the amount of control the animal’s body (physiology) has on what it takes up from the environment, and if this changes as the animal ages (yes, it usually does). In our study, we used the ear bones of an ocean perch, a strikingly red, deep-water fish from southern Australia, to learn more about how chemical elements are taken up from the environment and how we can best use the ear bones as environmental recorders. The ocean perch live in a dynamic ocean region called an upwelling area, where deep water comes to the ocean’s surface in the summer. This causes a change in the water’s chemistry that can also be seen in the fishs’ ear bones. We blasted the ear bones with a laser, from the centre to the edge, to measure the different elements in the growth rings (think tree rings on a cross-section of a log). We used advanced statistical modeling to tie together measurements of the fishs’ growth and the chemical elements, tease apart the influence of physiology, and extract our growth and chemical chronologies (17 years worth!). Some chemical elements were highly influenced by physiology (sodium and strontium) and some barely at all (barium and lithium). We could see the summer upwelling signal in the ocean perches’ ear bones and the changes over the last 17 years—definitely year to year changes but not that much overall. If you are someone interested in creating your own biochronologies from the hard parts of animals, especially fish ear bones, we provide detailed instructions on how to do it in our newly published paper.

Full details of the paper: Grammer GL, JR Morrongiello, C Izzo, PJ. Hawthorne, JF Middleton, BM Gillanders. 2017. Coupling biogeochemical tracers with fish growth reveals physiological and environmental controls on otolith chemistry. Ecological Monographs.