Comparison of specimen and image-based morphometrics for Cisco

Morphometric data from fish are typically generated using one of two methods: from direct measurements made on a specimen, or by extracting distances from a digital picture. We compared data on twelve morphometrics collected with these two different methods on the same collection of Cisco Coregonus artedi to assess the degree of bias in measurements made directly on a specimen versus an image-based method. We also assessed the degree of reproducibility within the image-based method by evaluating the amount of variation between different analysts for each morphometric. Our results indicate specific morphometrics may be more prone to bias across the two methods and between analysts. Four out of twelve morphometrics evaluated showed significant deviation from a 1:1 relationship that would be expected if specimen-based measurements were accurately reproduced from the image-based method. Pelvic fin length and pelvic-anal fin distance had the highest between-analyst variation for image-based landmarks, indicating low reproducibility for these metrics, compared to pectoral fin or total length which had lower between-analyst variation. While some morphometric measurements can be accurately obtained with either method, and therefore potentially used interchangeably in studies on Cisco morphology, our findings highlight the importance of considering method bias in morphometric studies that use data collected by different methods.