Evaluation of photo-ID technique to estimate harbour seal numbers in Gullmarsfjorden, Sweden

Abstract: To be able to preserve areas in nature in a careful way a good understanding of the biodiversity living there is needed. The harbour seals are a top predator that affect and is affected by the whole food chain and plays a vital role in the ecosystem. Some seals migrate long distances and some are relatively stationary, it is therefore very important that we can get to know the population on an individual level. Earlier, the way to identify an individual has been through physically tag them, this thesis explored both the possibility to use the natural patterns in the fur to identify individual harbour seals by Photo-ID. The images were processed using the software Wild-ID, the population sizes were estimated in two different locations in Gullmarsfjorden, Sweden. The software worked well, 58 potentially unique individuals were managed to be identified during the project, 22 of these with both their left and right side of their face. The similarity scores the software gave the matched images were to 99% scored beneath 0.3 (0 = no similarity, 1 = exact copies) which is a relatively low score. Still, the software ranked about 75% of the matches as the best suggestion and 95% as one of the top 10 suggestions. This shows that even though the scores were very low, the software could still usually pick out the correct match. With the information gathered and capture-recapture statistics, population sizes of two different locations in the fjord was estimated, 26 (12 – 60) individuals on one site and to 73 (47 – 129) on the other. This is the first time identification of harbour seal individuals in Sweden has been done with the help of photo-ID and it’s a important step for better understand migration and spatial structures by the harbour seal population. This information can lead to better management and conservation of our marine ecosystems.