Use of otolith Sr:Ca ratios to study the riverine migratory behaviors of Japanese eel Anguilla japonica

To understand the migratory behavior and habitat use of the Japanese eel Anguilla japonica in the Kaoping River, SW Taiwan, the temporal changes of strontium (Sr) and calcium (Ca) contents in otoliths of the eels in combination with age data were examined by wavelength dispersive X-ray spectrometry with an electron probe microanalyzer. Ages of the eel were determined by the annulus mark in their otolith. The pattern of the Sr:Ca ratios in the otoliths, before the elver stage, was similar among all specimens. Post-elver stage Sr:Ca ratios indicated that the eels experienced different salinity histories in their growth phase yellow stage. The mean (±SD) Sr:Ca ratios in otoliths beyond elver check of the 6 yellow eels from the freshwater middle reach were 1.8 ± 0.2 x 10-3 with a maximum value of 3.73 x 10-3. Sr:Ca ratios of less than 4 x 10-3 were used to discriminate the freshwater from seawater resident eels. Eels from the lower reach of the river were classified into 3 types: (1) freshwater contingents, Sr:Ca ratio <4 x 10-3, constituted 14% of the eels examined; (2) seawater contingent, Sr:Ca ratio 5.1 ± 1.1 x 10-3 (5%); and (3) estuarine contingent, Sr:Ca ratios ranged from 0 to 10 x 10-3, with migration between freshwater and seawater (81%). The frequency distribution of the 3 contingents differed between yellow and silver eel stages (0.01 < p < 0.05 for each case) and changed with age of the eel, indicating that most of the eels stayed in the estuary for the first year then migrated to the freshwater until 6 yr old. The eel population in the river system was dominated by the estuarine contingent, probably because the estuarine environment was more stable and had a larger carrying capacity than the freshwater middle reach did, and also due to a preference for brackish water by the growth-phase, yellow eel.