Determining temporal trends of metal exposure in Lake Trout (Salvelinus namaycush) otoliths using microchemical analysis
Carroll, Leslie C
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Otoliths are calcified structures located in the inner ear of teleost fish. They are formed by the crystallisation of calcium carbonate in the form of aragonite onto a protein matrix. Otoliths grow continuously during the lifespan of the fish by the deposition of concentric layers of aragonite and protein. During the formation of otoliths, trace elements are potentially incorporated into the otolith either by substitution for Ca or through co-precipitation with other carbonates. Since it has been suggested that otolith composition is reflective of the external environment (i.e. food and/or water) otoliths have been used as a temporal record to address many fisheries questions. Generally, toxicological studies focus on soft tissues (e.g. liver, kidneys, muscle) to determine metal exposure in fish. However, interpretation of the metal concentrations can be challenging due to the labile nature of metals in these tissues. A more recent approach to investigate temporal trends of metal exposure involves the use of otoliths. Otoliths are metabolically inert so that, once metals are incorporated they are not subject to remobilization. Red Lake, located in the Canadian Shield in Northwestern Ontario is recognized as a popular fishing area. However, over the last decade lake trout have experienced a drastic decline, with concurrent increase in the age distribution. One hypothesis for the recruitment failure has focused on the metals present in the water as a result of past mining activities. A retrospective analysis of the elemental signatures in the lake trout otoliths was performed to determine whether trace element concentrations have changed in Red Lake over the past three decades. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) was used to determine trace elements in otoliths recovered from lake trout from 1960 to 2008. Mn was incorporated into the otoliths and there was a suggestion that concentrations peaked between 1980 and 1989 in Red Lake. Concentrations of Mn in lake trout otoliths ranged from 0 mg/g to 10.1 mg/g. Potential contributors to the elevated concentrations could have been associated to the local geology. In addition to geology further uncertainties concerning the water and diet to otolith relationships could also contributed to the Mn concentrations. Augmented diets failed to increase metal concentrations in otoliths. Further research is required to further investigate the relationship between Mn in the environment and the incorporation into the otolith.