What's in a fish ear bone

Did you know that fish have ear bones that provide scientists with a wealth of information. They are also used for jewellery (see left image).These tiny structures are located in the head of the fish and play a role in hearing and balance, but that’s not why we are interested in them. Much like the growth rings of a tree, the ear bones of fish also have information about their growth. This means that we can estimate their age, but if we know when they were collected we can also relate the widths of their growth increments to environmental conditions. The idea is that when the conditions are good the fish will grow faster and this will be reflected in wider growth increments. Similarly when conditions are poor, fish grow more slowly and we see this reflected in narrower growth increments. Thus we can develop long term biochronologies of fish growth which can extend back before instrumental records of environmental conditions started if the fish are old enough. Similarly, we can estimate how fish might respond to changing environmental conditions into the future. Our lab has been developing chronologies of growth for a number of species in southern Australia and New Zealand including parore (luderick), King George whiting, snapper, mulloway, black bream, yellow eye mullet, ocean perch, golden perch and Murray cod.

Besides growth information, ear bones also incorporate information on the water chemistry where the fish has been. This information is like an elemental fingerprint of the entire fish’s life from birth to death, similar to the flight recorder of a plane. The difficulty is in extracting and interpreting the information. We know these bones are composed of calcium carbonate so are more similar to limestone rock than to bone, but other elements are also incorporated into this calcium carbonate matrix often substituting for the calcium. It’s these other elements that reflect the fish’s environment. If we analyse profiles across the ear bone or specific areas of the ear bone we can make predictions about the type of environment that the fish has spent time in and movements throughout its life. We know that the edge of the otolith will reflect where the fish was caught and that the centre of the otolith will reflect where the fish spent its early life. By relating the growth information to the chemistry we have a life time record of the fish’s movement patterns.

The information obtained from fish ear bones is used for management purposes – it can feed into stock assessment models where growth or age information is required. Understanding where fish reproduce, where they spend their juvenile life, whether they return to the same area to reproduce each year can help with management actions to rebuild stocks. Similarly the sorts of information obtained using chemicals in ear bones can help determine the boundaries that are important to fish populations – if fish share the same population then they should be managed as one stock. Our lab has used chemicals in ear bones to address a range of ecological and environmental questions related to management including fisheries and conservation.

Our recent paper is a review that looked at published literature where ear bone chemistry and water data were available. It focused on two of the key elements used in ear bone chemistry research, strontium and barium. This review demonstrated that both strontium and barium were influenced by water chemistry, but also by temperature and salinity of the water, but that there may be other factors such as ecological niche (whether the fish lived in freshwater, estuarine or marine waters) that also play a role. This sort of information is important for deciphering effects of multiple factors and ensuring ear bone chemistry results are correctly interpreted.

Izzo, Reis-Santos, Gillanders (2018) Otolith chemistry does not just reflect environmental conditions. A meta-analytic evaluation. Fish and Fisheries - DOI: 10.1111/faf.12264