Otolith Sr:Ca ratios were examined to evaluate the contribution of the stocked eel Anguilla anguilla elvers, which have been stocked in Lithuanian waters and mixed with naturally recruited eels for several decades, to the native eel population. Stocked eels were identified by the freshwater signature (Sr:Ca ratios <2Á24 Â 10 À3 ) on the otolith after the glass eel stage. Naturally recruited eels, that had migrated through the North and Baltic Seas, were characterized by an extended seawater and brackish-water signature (Sr:Ca ratios >3Á23 Â 10 À3 ) after the glass eel stage. Of 108 eels analysed, 21 eels had otolith Sr:Ca ratio profiles consistent with stocking while 87 showed patterns of natural recruitment. The ages of naturally recruited eels arriving in Lithuanian fresh waters varied from 1 to 10 years, with a mean AE S.D. age of 5Á2 AE 2Á1 years. Eels from the inland Lake Baluosˇai were all freshwater residents of stocked origin. Stocked eels, however, accounted for only 20% of the eels from the Curonian Lagoon and 2% of eels sampled in Baltic coastal waters. This finding does not support the hypothesis that the eel fishery in the Curonian Lagoon depends mostly on stocking.

Most fish otoliths are composed of aragonite, but occasionally they form a mosaic with its polymorph, vaterite. We found an unusual mosaic structure of extremely low strontium to calcium (Sr/Ca) ratio in the aragonite otoliths of European eels Anguilla anguilla. The mosaic crystal structure in the eel otolith appeared opaque under reflected light after EDTA etching and was confirmed to be a vaterite crystal by Raman spectroscopy. Analysis by laser ablation inductively coupled plasma mass spectrometry indicated that the elements sodium (Na), strontium (Sr) and barium (Ba) were lower in abundance and magnesium (Mg) and manganese (Mn) were higher in the vaterite than in the aragonite. Otoliths with vaterite inclusions were found in 48.1% of the 108 eels examined. If the mosaic vaterite in otoliths is not identified and avoided, the migratory environmental history of fish could be misidentified when otolith elemental signatures are used as biological tracers.