| Summary: | 博士 === 國立臺灣大學 === 漁業科學研究所 === 100 === To understand the distribution, migratory behaviors and habitats use characteristics of Japanese eels Anguilla japonica in the continental waters, 3263 wild and cultured-origin yellow eels and 4800 elvers were marked and released in the Kao-ping River estuary and Da-pong Bay in southwestern Taiwan over four years since 2003-2006. Five different tagging methods were used to mark the eel, including: (1) liquid nitrogen marking on the fish body surface, (2) fin clipped, (3) oxytetracycline and alizarin complexone marking on otoliths, (4) micro-chips, and (5) coded wire tags. The recapture of marked eels were monitored after release by local fisherman using the traditional shrimp net in the 3 stations in the Da-pong Bay and the 15 stations in the Kao-ping River. The chemical composition in water and otolith of the eel including strontium: calcium ratios (Sr/Ca), Barium: Ca ratios (Ba/Ca) and Sr isotope ratios (87Sr/86Sr) were measured by Inductively Coupled Plasma Mass Spectrometry (ICPMS), Electron Probe Micro Analyzer (EPMA) and Thermal Ionization Mass Spectrometry (TIMS), respectively. The recapture rate of marked eel sharply decreased with increasing distance from the released site and the maximum dispersal distance of the eel was less than 2 km, indicating that the eel may have a territorial behavior after recruitment to the river. The recapture rate also sharply decreased with time, indicating that heavy harvesting of the eel occurred over a short time after release. According to the level of otolith Sr/Ca ratios of the wild eel, it revealed that estuarine contingents (river: 75.5%, bay: 60%) were higher than freshwater contingents (river: 22%, bay: 25%) and seawater contingents (river: 2.5%, bay: 15%). The otolith Sr/Ca ratios of recaptured eels indicated that the marked eels also preferred a brackish environment similar to wild population.
The otolith Sr isotope ratios are more sensitive than Sr/Ca ratios which could separate freshwater contingents into lower reach and middle reach groups. The combination of mark-recapture methods and otolith chemical composition can look insight into migratory behavior and micro habitats use of the eels in continental waters.
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