Abstract
The giant kokopu, Galaxias argenteus, is a native New Zealand freshwater fish which optionally undergoes a marine larval phase after hatching near a river mouth. The marine phase is indicated by a high Sr : Ca ratio in the core of the otolith of the adult fish. Elemental scanning for Sr and Ca with a proton microprobe is one of the most useful techniques for identifying and quantifying the presence of the marine core. However, many individuals, whether or not they show the marine core, exhibit one or more concentric rings of slightly elevated Sr levels outside the core. Examples of this behaviour are given and quantitative estimates of the degree of elevation of the Sr levels are made from both area and line scans using high efficiency detectors for Sr detection. It is suggested that these Sr variations reflect adult fish migration within a freshwater habitat, perhaps seeking a brackish zone in which to spawn.
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