Evidence of different habitat use by New Zealand freshwater eels Anguilla australis and A-dieffenbachii, as revealed by otolith microchemistry

The apparent use of marine and freshwater habitats by Anguilla australis and A. dieffenbachii was examined by analyzing the strontium (Sr) and calcium (Ca) concentrations in otoliths of silver eels collected from Lake Ellesmere, which is a shallow brackish-water coastal lagoon in New Zealand. The age and growth of these eels was also examined using their otolith annuli. Size and ages of females were greater than those of males for both species. Growth rates were similar among sex and species, but the highest growth rates were observed in eels that experienced saline environments. Line analyses of Sr:Ca ratios along a life-history transect in each otolith showed peaks (ca. 15 to 21 x 10(-3) in A. australis, 14 to 20 x 10(-3) in A. dieffenbachii) between the core and elver mark, which corresponded to the period of their leptocephalus and early glass eel stage in the ocean. Outside the elver mark, the Sr:Ca ratios indicated that eels had remained in different habitats that included freshwater (average Sr:Ca ratios, 1.8 to 2.4 x 10(-3)), areas with relatively high salinities (average Sr:Ca ratios, 3.0 to 7.4 x 10(-3)), and in some cases individuals showed clear evidence of shifts in the salinity of their environments. These shifts either indicated movements between different locations, or changes in the salinity of the lake. There were more individuals of A. australis that used areas with intermediate or high salinities, at least for a short time (85% of individuals), than A. dieffenbachii (30%). These findings suggest that these 2 southern temperate species may have the same behavioral plasticity regarding whether or not to enter freshwater or remain in marine environments, as has been recently documented in several northern temperate anguillid species.