different tasks:
calibration
- intrinsic camera calibration (projection matrix, lens distortion)
- extrinsic calibration (camera–surface pose): put a checkerboard or ARUCO/CHARUCO in frame
assuming the fish is flat and on the surface, now you can get physical lengths from the picture.
segmentation
use a different color for the background. the fish itself appears white in parts. use a color that will certainly not appear on the fish.
threshold to get a mask.