Cheap and accessible method to aid individual photo-identification of grey seals, Halichoerus grypus

Abstract

A method was developed to assist in the photo-identification of individual grey seals (Halichoerus grypus). Fixed reference points of the eye and ear were used to define a rectangular area of pelage on the seal's head, termed the identifier patch (IP). Extracted IPs were converted to binary masks, and the percentage of black pixelation within each mask was evaluated. Multiple images of the same head profile gave masks with similar patterns and percentage of black pixelation (mean standard deviation 3.3% (±1.6%)). The low level of ‘within seal’ variability enabled ‘between seal’ images to be separated on the basis of percentage of black pixelation. Subsequent matching was achieved by eye using the mask pattern to complete the identification process. Successful matching of seal images, collected two years and nine months apart, at a North Wales field site was accomplished using this method. Pattern-matching trials using 88 untrained volunteers showed the extracted black and white mask to be a superior visual cue for separating individual seals compared to straightforward inspection of photographs. All volunteers identified a greater number of correct matches using the mask-matching method. This method shows promise in providing a cheap and accessible system to order and search image databases.