Fish Otoliths

Platform Recruited Reef Fish:
Phase II: TM and SEM study of fish otoliths from the Mississippi River

Figure 1. Reference image from sefcpanamalab.noaa.gov/OtolithGuide/

Figure 2. Sample Red Snapper otolith from this research project.

Monitoring and evaluating the environmental effects of human activities requires gathering accurate, precise, and reliable information on the fate and transport of chemical entities of interest. At best these data should establish a historic record, so chemical trends can be evaluated. Reef corals, fish otoliths, tree rings, and layered sediments are some examples of naturally occurring data repositories that include a time domain. During this project we will evaluate otoliths of Red Snapper Lutjanus campechanus as indicators of the transition, heavy metal, and actinide trace element and isotopic compositions of natural waters.

Otoliths are polycrystalline structures composed of aragonite and a protein matrix. Unlike bones, otoliths grow by a cycle of daily deposition of material at a typical rate of 1-5 µm per day that makes chemical life history analysis practical at the monthly level. Like all aragonite, otoliths have an affinity for bone-seeking uranium series metal ions and other bone-building elements including Sr and Pb. Fish, particularly deep ocean fish, are wide ranging migrants and can potentially provide a very effective sampling/concentrating mechanism for radio nuclide dispersion in the ocean from a variety of sources (e.g., sunken submarines and other material disposed of in the ocean or in river systems).

In addition to actinides, otoliths incorporate a variety of trace elements (Pb, Zn, Li, Mg, Mn, Sr, Ba). While lighter elements (transition metals) may be chemically or biologically fractionated, heavier elements (Pb, actinides) most likely reflect source fingerprints. Therefore, chemically dependent fractionation of lighter elements may identify biological and chemical pathways taken by nutrients from source to otolith. Heavier element (Pb, Rare Earth Elements, U, for example) systematics may reveal source characteristics. Therefore, both processes and sources releasing chemical tracers to water bodies can potentially be studied and understood.

Figure 3. Rings of growth used for establishment of temporal relations can be seen on these otoliths.

The purpose of this study is to evaluate the viability of a comprehensive suite of elements and isotopes (Pb, REE, actinides, transition and heavy metals, alkaline-earths) as chemical indicators. We will measure chemical systematics in Red Snapper otoliths from the Gulf of Mexico, where oil platforms may be significantly influencing fish spawning and harvesting rates, and a second location, to be determined, where significant quantities of actinides and potentially, fission products, have been released to the environment.

We will use Thermal Ionization Mass Spectroscopy and Inductively-Coupled Plasma Mass Spectroscopy for our analyses, thereby reducing the required sample size and increasing our ability to measure chemical concentrations and isotope ratios at low levels and for more elements.