Habitat use by the European eel Anguilla anguilla in Irish waters

The apparent use of marine and freshwater habitats by European eel Anguilla anguilla was examined by analyzing the strontium (Sr) and calcium (Ca) concentrations in otoliths of the eels collected from Irish coastal and fresh waters. The age and growth of eels were also examined using their otolith annuli. The sizes and ages of the female eels were greater than those of the males. The somatic growth rates ranged from 15 to 62 mm/year, which is typical for Ireland and other European countries. Analyses of Sr:Ca ratios along a life history transect in each otolith showed peaks (maximum more than 25 × 10⁻³) between the core and elver mark corresponding to the period of their leptocephalus and early glass eel stages 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, 0.98–1.78 × 10⁻³) and areas with relatively high salinities (average Sr:Ca ratios, 6.73–8.89 × 10⁻³). Some individuals showed clear evidence of shifts from sea to fresh waters. These findings suggest that Irish eels have the same behavioral plasticity regarding whether or not to enter freshwater or remain in marine environments as has been recently documented in this species and several other temperate anguillid species. However, patterns of habitat use in Irish waters were somewhat different than those previously reported for other habitats.