Journal of Fish Biology (2009) 75, 2709–2722 doi:10.1111/j.1095-8649.2009.02471.x, available online at www.interscience.wiley.com Do otolith annular structures correspond to the first freshwater entry for yellow European eels Anguilla anguilla in the Baltic countries? Y.-J. Lin*, J.-C. Shiao†, L. Loˇ zys‡, M. Plikˇ ss§, A. Minde§, Y. Iizuka‖, I. Raˇ sals¶ and W.-N. Tzeng* , ** , †† *Institute of Fisheries Science, National Taiwan University, Taipei, 10617, Taiwan, R.O.C., †Institute of Oceanography, National Taiwan University, Taipei, 10617, Taiwan, R.O.C., ‡Institute of Ecology of Vilnius University, Vilnius, LT-08412, Lithuania, §Latvian Fish Resources Agency, State Agency, Ministry of Agriculture, Riga, LV-1048, Republic of Latvia, ‖Institute of Earth Sciences, Academia Sinica, Nankang, Taipei, 10617, Taiwan, ¶Institute of Biology, University of Latvia, Salaspils, LV-2169, Republic of Latvia and **Department of Life Science, National Taiwan University, Taipei, 10617, Taiwan, R.O.C. (Received 17 April 2009, Accepted 30 September 2009) To examine the relationship between freshwater entry and otolith annular structures, a total of 113 naturally recruited European eels Anguilla anguilla from Lithuania and Latvia that entered fresh water at least once were collected. In some individuals (8·3–11·3%), the first freshwater entry coin- cided with a dark check that was distinctly different from neighbouring annuli. In most individuals (81·7–84·9%), the first freshwater entry occurred on rings and increments indistinguishable from other annuli. For the remaining individuals (3·8–10%), the first freshwater entry did not correspond to any otolith ring, band or annulus. According to recent evidence, the observed high correspon- dence between the first freshwater entry and otolith annuli was more likely due to the movement into fresh water during winter when the annulus was deposited, rather than stress resulting from habitat change. Consequently, the age estimation based on otoliths might be less influenced by this habitat change during the yellow eel stage. © 2009 The Authors Journal compilation © 2009 The Fisheries Society of the British Isles Key words: Anguilla anguilla ; annulus; habitat change; otolith; Sr:Ca ratios. INTRODUCTION The age of fishes is usually estimated by counting annuli in otoliths (Campana & Thorrold, 2001) and the deposition of annuli in otoliths is considered to be the consequence of the unfavourable environment of low temperature and starvation in winter (Liew, 1974; Jearld, 1983). Other factors, however, such as sudden changes in temperature (Volk et al., 1999), developmental transitions (Wright et al., 2002) and starvation in other seasons (Graynoth, 1999) might also have the potential to influence ††Author to whom correspondence should be addressed: Tel.: +886 2 33662887; fax: +886 2 23629570; email: wnt@ntu.edu.tw 2709 © 2009 The Authors Journal compilation © 2009 The Fisheries Society of the British Isles