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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.

Otoliths are small structures in fish ears made of calcium carbonate which carry a record of the environment in which the fish live. Traditionally, in order to study their microchemistry by a scanning technique such as PIXE the otoliths have been either ground down by hand or thin sectioned to expose the otolith core. However this technique is subject to human error in judging the core position. In this study we have scanned successive layers of otoliths 50 and 100 µm apart by removing the otolith material in a lapping machine which can be set to a few µm precision. In one study by comparing data from otoliths from the two ears of a freshwater species we found that polishing by hand could miss the core and thus give misleading results as to the life cycle of the fish. In another example we showed detail in a marine species which could be used to build a three dimensional picture of the Sr distribution.