Salvelinus species are one of the most thoroughly studied groups of fishes. Many reasons explain this intense interest in charr biology. Charrs have a Holarctic distribution encompassing many Asian, North American, and European countries and occupy diverse marine and freshwater environments. Furthermore, the current distribution of charr includes areas that were directly influenced by climate and topographic change associated with the many Pleistocene glaciations. Undoubtedly, these conditions have promoted much of the tremendous morphological, ecological, and genetic variability and plasticity within Salvelinus species and they make charr very good models to study evolutionary processes 'in action'. Many charr species also exhibit demographic characteristics such as slow growth, late maturity, and life in extreme environments, that may increase their susceptibility to extinction from habitat changes and overexploitation, especially in depauperate aquatic habitats. This vulnerability makes understanding their biology of great relevance to biodiversity and conservation. Finally, charr are of great cultural, commercial, and recreational significance to many communities, and their intimate linkage with human societies has stimulated much interest in this enigmatic genus. This volume comprises a selection of papers presented at the fourth International Charr Symposium held in Trois-RiviA]res (QuA(c)bec, Canada), from 26 June to 1 July 2000. It includes 31 papers on ecological interactions and behaviour, trophic polymorphism, movement and migration, ecophysiology and evolutionary genetics, ecological parasitology, environmental stress and conservation. These studies cannot cover all recentdevelopments in the ecology, behaviour and conservation of Salvelinus species, but collecting them into a special volume should bring attention to current research on this important genus and stimulate further work on Salvelinus species.

This book was conceived from the realization that there was a need to update recent work on invariants in a single volume providing a useful set of references and pointers to related work. Since the publication in 1992 of J L Mundy and A Zisserman's Geometric Invariance in Computer Vision, the subject has been evolving rapidly. New approaches to invariants have been proposed and novel ways of defining and applying invariants to practical problem solving are testimony to the fundamental importance of the study of invariants to machine vision. This book represents a snapshot of current research around the world.

A version of this collection of papers has appeared in the International Journal of Pattern Recognition and Artificial Intelligence (December 1999). The papers in this book are extended versions of the original material published in the journal. They are organized into two categories: foundations and applications. Foundation papers present new ways of defining or analyzing invariants, and application papers present novel ways in which known invariant theory is extended and effectively applied to real-world problems in interesting and difficult contexts. Each category contains roughly half of the papers, but there is considerable overlap. All papers carry an element of novelty and generalization that will be useful to theoreticians and practitioners alike. It is hoped that this volume will be not only useful but also inspirational to researchers in image processing, pattern recognition and computer vision at large.