You might find a few answers with Marley et al. 's Using Tooth Rakes to Monitor Population and Sex Differences in Aggressive Behaviour in Bottlenose Dolphins (Tursiops truncatus) (2013)

They use a, I quote, Dorsal Fin Nick Percentage (DFNP) : " The dorsal fin often has nicks and tears from aggressive interactions. The length of each nick and total height of each dorsal fin was mea-sured for each photograph of each individual. The total length of all nicks in each photograph was divided by the height of the fin to calculate the percentage of the fin missing, and an overall average was taken for each individual. "

thank you Gregoire! Unfortunately, I am given an average DF and I do not obtain that measurement from my photos by from the literature. That's why i am confused about how to derive the distance in CM between rakes. Take care!

Hi Chiara, if I understand correctly, in ImageJ you will specify that the dorsal fin in the picture is XX cm tall (the average that your received) and then the program will be using this as the reference measurement when you measure anything else. It will only be an estimate that assumes that every dolphin you measure is of average size, but this is only way to do this (that I know of) when you do not have any other information which would allow you to calculate the actual size of the dolphin (ie. distance between the dolphin and the photographer etc.).

thank you Charla - I have to re-download ImageJ. I figure I can convert pixels into cm and vice versa so that way I can navigate around using DH and distance between rakes.

If you dont have the true size of the dorsal fin your measures of tooth rake distance will be relative and vice versa. It would bias your measures if you use an average for the species, and not absolute measures of each animal, the resulting measures will be pretty variable i think

Definitely. When you tell imageJ that that dorsal fin is XX cm high (in the set scale option), it will then calculate for itself that X pixels in that specific photograph = 1 cm.