Age validation of the Kerguelen Islands brown trout, Salmo trutta L., and selection of the otolith optimal zone for investigating chronological data series

The selection of an otolith optimal zone where to obtain interpretable chronological data (e.g. chemical transect, increment width) and valid age estimation is fundamental if otoliths are to be used as an effective tool for investigating life history traits or environments experienced by fish. In the remote sub-Antarctic Kerguelen Islands (49 degrees S, 70 degrees E), introduced brown trout populations are invading the archipelago since the 50's and life histories information is of great interest to understand the invasion dynamics. Unfortunately, few studies have used otolith as a tool to estimate brown trout age at these extreme latitudes and little 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 concomitantly extract chronological data aiming at investigating life history traits and growth chronologies. In this context, the aims of this study were (1) to validate the otolith age estimation method by the determination of the position of the first annulus and the annual formation of subsequent annuli; (2) to define an optimal zone where chronological data could be coupled to age estimates. A brown trout population located near the Port-au-Francais station was studied and sampled repeatedly using mark-recapture method and Alizarin Red S marking. After to 2 years of recaptures 53 sagittae were analysed. A composite index was created to select an optimal zone where chronological data transects have the maximum probabilities 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 marking was validated. The area between 80 degrees and 120 degrees perpendicular to the rostrum/core axis was optimal for the positioning of data transect. The proposed formal approach allows distinguishing the zone that identifies the best compromise between spatial resolution and precise annulus positioning along otolith chronological series. (C) 2015 Elsevier B.V. All rights reserved.