Investigating how enfironmental variables affect fish otolith chemistry

A project undertaken at The School of Earth and Environmental Science, The University of Adelaide, and supervised by Bronwyn Gillanders and Travis Elsdon

A major problem in fisheries ecology is being able to track the movements of individual stocks of fish. This problem exists largely due to a lack of understanding regarding fish migration. Although the best evidence of migration is the recognisable movement of fish (conventionally by either tagging or directly observing movements of individuals), obtaining reliable data in great enough quantities to aid fisheries management has proven difficult; the consequences of which are the collapses of fisheries. In response to these problems, the elements in calcified structures such as fish otoliths (earbones) are being investigated as tools to answer ecological questions about fish movements.

Otoliths are calcium carbonate structures (CaCO3) used by fish for hearing and balance. The formation of new otolith material occurs daily, hence, otoliths are accurate chronological recorders. Incorporated into the lattice structure of otoliths, in conjunction with these daily increments, are elements such as strontium (Sr) and barium (Ba) that are derived from the surrounding environment. In addition, elemental concentrations in otoliths are affected by temperature and salinity. Thus, elemental concentrations in otoliths can be linked to the environment fish have occupied making it possible to identify specific estuaries or water bodies of importance for different species. However, many of the underlying assumptions of how elements are incorporated into otoliths are yet to be addressed. Therefore, the extent to which we can deduce migratory patterns of fish is unknown.

The aims of our research are to understand the importance of processes that influence elemental incorporation into otoliths. There are two principal objectives of this research, each relating to an individual experiment that will be done using black bream, Acanthopagrus butcheri, a species endemic to Southern Australian waters. These are to:

  • Determine how interactions between water temperature, salinity, and ambient elemental concentrations influence otolith chemistry.
  • Determine if otolith chemistry can be used to track fish that have been reared through fluctuating water temperatures and salinities (i.e., "migratory simulations").

This research will increase our understanding of how environmental factors influence otolith chemistry, by examining the effect that temperature, salinity, and ambient elemental concentrations have on otolith chemistry. This information will form the foundation upon which methods of determining migratory patterns of fish can be modelled. Importantly, determining how environmental variables influence otolith chemistry is the logical step that we must take if otolith chemistry is to become applicable to answering ecological questions. Such knowledge will provide a much clearer understanding of how accurately we can deduce migratory patterns using otolith chemistry, with any derived migrations benefiting the conservation of fish species and the habitat they occupy.

Figure 1. Scanning electron micrograph of a fish otolith.