Ruler from a photo
Place an object of known size next to the fish (a coin, a card, a banknote), photograph them from above in the same plane, then tap the ends of the object and the ends of the fish in the photo.
Place an object of known size next to the fish (a coin, a card, a banknote), photograph them from above in the same plane, then tap the ends of the object and the ends of the fish in the photo.