Abstract: The selection of an otolith optimal zone where to obtain interpretable chronological data (e.g. chemicaltransect, increment width) and valid age estimation is fundamental if otoliths are to be used as an effectivetool for investigating life history traits or environments experienced by fish. In the remote sub-AntarcticKerguelen Islands (49◦S, 70◦E), introduced brown trout populations are invading the archipelago sincethe 50’s and life histories information is of great interest to understand the invasion dynamics. Unfortu-nately, few studies have used otolith as a tool to estimate brown trout age at these extreme latitudes andlittle is known about otolith growth and annulus formation in these sub-polar conditions. Furthermore,no formal procedure exists at the population level to select an otolith optimal zone where to concomi-tantly extract chronological data aiming at investigating life history traits and growth chronologies. Inthis context, the aims of this study were (1) to validate the otolith age estimation method by the deter-mination of the position of the first annulus and the annual formation of subsequent annuli; (2) to definean optimal zone where chronological data could be coupled to age estimates. A brown trout populationlocated near the Port-au-Franc¸ ais station was studied and sampled repeatedly using mark-recapturemethod and Alizarin Red S marking. After to 2 years of recaptures 53 sagittae were analysed. A compositeindex was created to select an optimal zone where chronological data transects have the maximum prob-abilities to cross readable annuli. Results showed that brown trout forms annuli at the end of September,starting with the first winter after the December hatching. Annulus formation up to 2 years after mark-ing was validated. The area between 80◦and 120◦perpendicular to the rostrum/core axis was optimalfor the positioning of data transect. The proposed formal approach allows distinguishing the zone thatidentifies the best compromise between spatial resolution and precise annulus positioning along otolithchronological series.AcknowledgementsWe thank the French Polar Institute (IPEV, Institut PolairePaul-Emile Victor) for the financial support of the Program 1041SALMEVOL; we thank Jacques Labonne for his management of theprogram from 2008 to 2013.We thank Margaret Lang for providingwriting assistance that lead to the clarity of the English language.We thank Jacques Rives and Franc¸ ois Guéraud for otolith and scaleanalysis. We thank two anonymous reviewers for comments thatlead to substantial improvements in the manuscript. We gratefullyacknowledge the logistical support provided by the TAAF adminis-tration and the Southern French Natural Reserve at Kerguelen. Thefieldwork would not have been possible without the help and sup-port of the IPEV logistics team (Romuald Bellec, Yann Lemeur, NinaMarchand) and all the people involved in the SALMEVOL summercampaign (Stephane Betoulle, Eduardo Vicente Dopico-Rodriguez,Emily Farcy, Franc¸ ois Guéraud, José-Luis Horreo-Escandon, RenaudKaeuffer, Jacques Rives, Thibault Thellier, Julien Tremblay).