Habitat use and habitat transitions in the tropical eel, Anguilla bicolor bicolor

Recently, the migratory history of temperate species of anguillid eels has been intensively studied using analytical microchemical techniques to determine the ratios of strontium to calcium (Sr:Ca) in the otoliths. However, there have been few studies of this nature on tropical anguillid eel species. To elucidate the migration ecology of the freshwater eel genus Anguilla, as well as its life history and habitat use, we examined the otolith Sr and Ca concentrations of a tropical anguillid eel Anguilla bicolor bicolor collected in Indonesian waters. In A. bicolor bicolor collected in a lagoon area, the change in Sr:Ca ratios outside the high Sr:Ca core showed two patterns of habitat use, (1) constantly living in either brackish or sea waters with no freshwater life (residents, 37%), and (2) habitat shift from freshwater to brackish or sea waters (migrants, 63%). For migrant fishes, most fish shifted their residence between 520 and 950 mu m (mean: 740 mu m) from the otolith core. These transitions suggest that these specimens moved to a higher salinity environment. The wide range of otolith Sr:Ca ratios (0.02 x 10(-3) to 12.79 x 10(-3) in all analytical points outside of 150 mu m from the core of all otoliths) in A. bicolor bicolor indicated that the habitat use of the tropical eel was facultative among fresh, brackish and marine waters during their growth phases after recruitment to the coastal areas. This flexible habitat use was the same as that of temperate eels. Thus, the migrations of anguillid eels into freshwater is clearly not an obligatory behavior.