Visually Separating the Longnose Butterflyfishes: Forcepsfish versus Big Longnose
The two fish Forcepsfish, Forcipiger flavissimus, and Big Longnose, F. longirostris, are very similar. Observers often confuse them when seen while diving or snorkeling. There are several ways to tell them apart.
In only a few cases can color be used to tell them apart. The Big Longnose can occur in an all brown morph, and rarely intermediate patterns that are muddy yellow or yellow with brown patches. But the yellow form that looks like Forcepsfish is the most common.
Differences in the angle of the fish to the camera and how the fins are being held in the photos below are what makes the shape of the yellow area look a little different between the two. This is not a field mark.
Look for black speckling on the "chin", the white area below the eye and pectoral fin. If you see many tiny black dots there, then it is Big Longnose. However, that species does not always show this, so lack of speckling does not tell you anything.
Compare the snout length to the head width. Compare the distance from the front of the eye to the yellow part of the body (the head width) with the distance from the front of the eye to the end of the snout (the snout length). In the Forcepsfish, the snout length is less than 1.5 times the head width, while in the Big Longnose it is more than 1.5 times the head width, often more than twice. Beware if you are looking at a photo where the head is angled towards or away from the camera, as perspective can make the snout appear larger or smaller than it is.
Given a sharp photo, it is sometimes possible to count the spines in the dorsal fin to separate these. Count only the stiff spines in the front of the dorsal fin, not the flexible rays in the rear part of the fin. Forcepsfish has 12-13 spines, whereas the Big Longnose has 10-11.
As a general rule, Forcepsfish are smaller than Big Longoses, but this is not a good way to separate them, as fish vary in size depending on age, and the largest recorded specimens of each are about 22cm (8.7 inches).