Automatic fish fillet inspection, i.e. detection of flaws and irregularities, is often done on fast moving conveyer belts using line scan imaging. Recent work shows that imaging spectroscopy is a promising method for automatic fish fillet inspection. This requires fast algorithms that can handle large amount of data and make reliable decisions in fractions of a second. One important step in these algorithms is segmentation, where different regions in the image are labelled. For cod fillets, this can be to identify which part of the fillet belongs to the loin, belly flap, centre cut and tail. How severe a detected flaw is depends on which part of the fillet it is located in. Segmentation requires a robust spatial reference system which is invariant to rotation and warping of the fillet. The centreline, consisting of veins and arteries cut off during filleting, is always visible on cod fillets and hence a good reference for segmentation. We show how to enhance the centreline by using the absorption characteristics of haemoglobin, and how a novel ridge detection method can detect the centreline in cod fillets. The results show that the centreline can be detected with an average accuracy of 1 mm from the tail and 77% into the fillet relative to its total length. The average error increases rapidly in the neck region and typical errors of 4 mm is reported. (C) 2008 Elsevier Ltd. All rights reserved.