Arching effect on fish body shape in geometric morphometric studies

Upward and downward arching of the body was observed during a study on redfishes Sebastes sp. population structure in the north-west Atlantic Ocean. The present study investigated the potential causes of this arching artefact. The results suggested that it is not related to biological factors (size or species) or to the preservation technique (freezing), but is rather due to slight posture differences between fishes during landmark capture. The consequences of the arching artefact on data analysis are discussed. An approach coupling a PCA-based model of the arching with Burnaby's orthogonal projection is proposed for removing the artefact from the data. (C) 2008 The Authors Journal compilation (C) 2008 The Fisheries Society of the British Isles.