PROBING FOR STRONTIUM PATTERNS IN OTOLITHS OF COMMON BULLIES FROM THE CLUTHA RIVER, NEW ZEALAND

Common bullies, Gobiomorphus cotidianus, are a widespread small New Zealand freshwater fish, with a life-cycle that may include an optional juvenile marine phase. We used the strontium variation in fish otoliths in an attempt to track the life history of the individual fish since the interlayer variation reflect the exposure of fish to background environmental levels of Sr at the time of deposition. One possible mechanism by which Sr can be enriched in otoliths is by seawater, which usually has a higher Sr/Ca ratio than fresh water. Locally resolved elemental measurements with a proton microprobe enable therefore the detection of variation in Sr that may reflect single or multiple migrations of freshwater fish into seawater. The most striking feature of this study is the observation of high Sr/Ca ratios in the cores of all otoliths, including those from fish caught 50 km inland. This suggests different environments in the life cycle of common bullies in the lower reaches of the Clutha river. A marine juvenile phase may be a common feature. Preliminary area scans were used to select suitable transects for detailed line scans which gave greatly improved statistics.