Narrow Results

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.

Micro-PIXE has been used to measure the trace element distribution in otoliths from several species of ocean fish, in order to investigate its possible use in stock discrimination. Trace elements detected include Sr, Fe, Mn, Ni, Zn, Cu, Se, Cd, Br, Hg and Pb. Trace elements Na, K, Cl, S and Cl were detected with the electron microprobe. The high sensitivity of PIXE demands a meticulous sample preparation procedure to avoid contamination problems. Practical problems associated with the application of the technique were investigated in detail. Preliminary results indicate that most trace elements except Sr, are present at close to the limits of detection at few ppm, but biologically significant data can be obtained for stock discrimination applications.

PIXE analysis was applied to estimate mineral concentration in red sea bream otoliths without cutting or polishing. Detected elements include Sr, Fe, Mn and Zn, which are commonly found in the otoliths in marine fishes. Strontium-Calcium concentration ratio is calculated by means of combined X-ray yields. The ratio doesn’t indicate dear correlaion with mean reared seawater temperature. It is caused by the diffraction error induced by rough topographies of the otoliths surface.

In-air PIXE was used to analyze trace elements in otoliths from several species of teleost fish in order to examine the correlation between the trace elements concentrations and environmental conditions. Mn, Fe, Zn and Sr were detected accurately in the order of ppm by using the in-air PIXE. It seemed that concentrations of Sr and Zn in red sea bream otoliths increased in proportion to higher seawater temperature. In addition, there were significant differences in trace element composition between that of reared red sea bream and rockfish and that of wild ones. Preliminary results indicate that the PIXE is a powerful technique to investigate fish otoliths.

Fish otoliths are continuously dsposited from fish birth to its death along with encoding environmental information. In order to decode the information, PIXE was adopted as trace elemental analysis of the otoliths. Strontium to calcium concentration ratios of red sea bream otoliths varied among rearing stations. The Sr/Ca ratios of Lake Biwa catfishes also varied between male and female and among fishing grounds. The PIXE analysis was applied to the fish stock discrimination.

PIXE was adopted to analyze trace elements in otoliths of Japanese flounder to discriminate among several local fish stocks. The otoliths were removed from samples caught at five different sea areas along with the coast of the Sea of Japan: Akita, Ishikawa, Kyoto (2 stations), and Fukuoka. Besides calcium as main component, strontium, manganese, and zinc were detected. Especially Sr concentrations were different among 4 areas except between 2 stations in Kyoto. It suggested that the fish in the 2 stations in Kyoto were the same stock differed to the others.

PIXE was adopted for analysis of trace elements in otoliths of anadromous fish, masu salmon Oncorhynchus masou masou and amago salmon O. masou ishikawae. Mn, Zn and Sr were detected in order of ppm. Two different types of spectra were obtained from masu salmon otoliths. The concentrations of Sr and Zn in the two types were quite different. The difference seems to be a result of the difference in periods when the individuals stayed in freshwater environment until they were caught. Amago salmon otoliths contain significantly less Sr than masu salmon otoliths. It is considered to be due to different migration pattern between the two subspecies. Anadromous fish generally experiences freshwater and sea water environment in its migration. Trace elemental concentrations in sea water is quite different from those in freshwater. The environment, which varies along with the migration, would affect the incorporation of trace elements into otoliths of anadromous fish.

Japanese sea bass (Lateolabrax japonicus) is a typical euryhaline marine fish and frequently migrates from salt to freshwater environments during early life stages. We hypothesized that strontium concentrations in the otolith could be a useful index to examine freshwater entry because of its lower concentration in freshwater. Otoliths of Japanese sea bass juveniles collected in the Chikugo river and estuary were analyzed by Particle Induced X-ray Emission (PIXE) to see relationship between strontium concentration and ambient salinity. Strontium concentrations in otoliths of sea bass juveniles are significantly lower in the river samples than in brackish water samples.

PIXE was adopted for analysis of trace elements in otoliths of Masu salmon Oncorhynchus masou masou to examine relationship between trace elements and environmental salinity. The otoliths were removed from salmon juveniles reared in four values of salinity and wild ones. The otolith Sr concentrations of reared individuals are positively related to salinity and there is significant difference between freshwater and seawater. The otoliths of smolts contain more Sr than those of parrs. It seems that the Sr concentrations in otoliths of Masu salmon reflect salinity where they had stayed and show the migration pattern.

For the purpose of obtaining basic data to understand the population dynamics of the Japanese flounder, Paralichthys olivaceus, inhabiting the Sanriku coastal waters, the concentration ratios of Ca and Sr in otoliths of juvenile fishes being cultivated for releasing to the regions, and those of adult fishes captured in both the Sanriku area (Aomori, Iwate and Miyagi) and Shizuoka prefecture coastal regions (as a comparison) were analysed using a particle induced X-ray emission (PIXE) technique.

The Sr/Ca ratios of otoliths taken from juvenile Japanese flounders had significant differences between Sanriku and Shizuoka sea-farming groups. The differences in otolith Sr/Ca ratios between Sanriku and Shizuoka sea-farming stations would thus differentiate flounder populations. No significant difference in otolith Sr/Ca ratios was observed among the Sanriku group. However, the values for Aomori group formed by small fishes in the Sanriku group seemed to be lower in proportion to their body size. Therefore, genetic characteristics of the juvenile Japanese flounder being reared at the sea-farming stations in Iwate and Miyagi prefectures are possibly different from those at an Aomori station.

On the other hand, statistically significant differences in the otolith Sr/Ca ratios among Aomori, Iwate, Miyagi and Shizuoka groups were found in the adult Japanese flounder. That is, higher values for the otolith Sr/Ca ratios were found in the groups inhabiting in the northern regions. The differences in otolith Sr/Ca ratios among these groups probably indicate that there are differences in the fish populations among these sample sites.