Electron Microprobe Study of Otolith: Migratory Behavior and Habitat of Three Major Temperate Species of Eels

An analytical technique for measurement of strontium (Sr) and calcium (Ca) concentrations in otolith was developed using an electron probe micro-analyzer (EPMA). Precisions of measurement for Sr and Ca were examined with various electron beam conditions, which were a combination of different beam currents, beam diameter, X-ray counting time, and diffracting crystals on carbonate standards and eel otolith. The precision decreased when beam power densities exceeded 1 μW/μm2 and/or beam diameter was less than 2 μm,
resulting from beam damage of carbonates. The optimum analytical conditions for measuring the Sr/Ca ratios in the otoliths are suggested to be with a beam current at 3 nA, a 5 × 4 μm2 beam, counting times of Sr and Ca for 80 sec and 20 sec at the peak, and of Sr and Ca for 20 sec and 10 sec at back-ground intensities at an acceleration voltage of 15 kV with strontianite and aragonite standards, respectively. In these conditions, it is possible to obtain precise measurements of Sr/Ca ratios at 10 μm intervals along a transect from the primordium (core) to the edge of individual otolith with an error less than 0.05 wt% in Sr, as these have been found to discriminate between brackish- and fresh-waters. We have studied more than 1,000 temperate eels from different water bodies where the East Asia (Japanese eels Anguilla japonica), the Northeast Ameria (American eels Anguilla rostrata), and the Northwest Europe (European Eel Anguilla anguilla). Patterns of otolith Sr/Ca ratios from core to edge indicated similar but multiple life histories, at least four to six type of migration behaviors. Additionally, the Sr/Ca patterns of otoliths clearly illustrate eels’ life between naturallyrecruited and artificially-restocked. Since Anguillid eels are the endangered species, the revealed habitat information from otolith microchemistry are variable and useful for conservation and cultivate studies.