Assessment of winter size-selective mortality of young-of-the-year Atlantic salmon (Salmo salar) using otolith microstructure analysis* Un resumen en espan ˜ ol se incluye detra ´s del texto principal de este artı ´culo. Introduction The winter season is generally recognized as being a long period of harsh environmental conditions often causing a bottleneck in stream dwelling salmonid populations (Bjornn 1971; Mason 1976). Many stud- ies have characterized this high winter mortality as having either shown size-selection against small individuals (Lindroth 1965; Hunt 1969; Smith & Griffith 1994; Meyer & Griffith 1997) or random mortality (Lund et al. 2003), whereas other studies have shown spatiotemporal and intercohort variations in the direction of the selection (Aubin-Horth 2002; Hendry et al. 2003). In these studies, a number of methods were used to characterize size-selective mortality, including controlled experiments, tracking of marked or known individuals, follow-up of length– frequency distributions through time and monitoring of the characteristics of survivors in natural popula- tions (reviewed in Sogard 1997). The latter, however, is considered as the most concrete and relevant method of assessing size-selective mortality (Sogard 1997). This approach was employed for juvenile Atlantic salmon (Salmo salar L.), by Good et al. (2001) to assess summer size-selective mortality and by Aubin- Horth & Dodson (2004) to study the link between size-selective mortality and precocious maturation. Both studies used otolith-based back calculations to determine fish lengths. This method allowed the detection of size-selective mortality, which showed in both studies an interannual variability that was suggested to be related to various habitat characteris- tics (i.e. hydroclimatic conditions, habitat availability). Based on these insights into the interaction of populations with environmental conditions, the use of otolith microstructure analysis to assess size- selective mortality of juvenile salmon during the winter season appears promising. The first objective of this study was to assess the usefulness of otolith microstructure analysis to Ecology of Freshwater Fish 2005 Printed in Denmark Æ All rights reserved Copyright Ó Blackwell Munksgaard 2005 ECOLOGY OF FRESHWATER FISH doi: 10.1111/j.1600-0633.2005.00089.x 1 Johnston P, Bergeron NE, Dodson JJ. Assessment of winter size-selective mortality of young-of-the-year Atlantic salmon (Salmo salar) using otolith microstructure analysis. Ecology of Freshwater Fish 2005. Ó Blackwell Munksgaard, 2005 Abstract – The objectives of this study were, first, to assess the usefulness of otolith microstructure analysis to examine winter size-selective mortality of young-of-the-year (YOY) Atlantic salmon and, secondly, to validate various hypotheses relating to the dynamics of two populations with different winter survival. By examining otolith microstructure, we back-calculated body size at hatching and at emergence of YOY salmon sampled in fall 2000 and in early summer 2001 on the Petite Cascape ´dia River and the Bonaventure River (Que ´bec, Canada). The results of the study did not reveal any size-selective mortality of YOY salmon in the Petite Cascape ´dia River, while in the Bonaventure River, size-selective mortality of the smaller individuals of the cohort was detected. This case study allowed not only a better comprehension of the population dynamics of those rivers but demonstrated the usefulness of otolith analysis to detect winter size-selective mortality under a natural environment. P. Johnston 1 , N. E. Bergeron 1 , J. J. Dodson 2 1 INRS-Eau, Terre et Environnement, Que ´bec, 2 De ´partement de biologie, Universite ´ Laval, Cite ´ universitaire, Que ´bec, Canada Key words: size-selective mortality; winter; Salmo salar P. Johnston, INRS-Eau, Terre et Environnement, 490 rue de la Couronne, Que ´bec, QC, G1K 9A9 Canada; e-mail: Patricia_Johnston@inrs- ete.uquebec.ca *This is a contribution to the scientific program of CIRSA (Centre Interuniversitaire de Recherche sur le Saumon Atlantique). Accepted for publication December 23, 2004