29 January 2010

Narwhals can be photo-identified

Photo-identification is a simple tool: you have a camera and an animal, and you take a photo of the natural markings and peculiar features that may help identify an individual.

It is a widespread technique used by cetacean researchers worldwide. Dorsal fins, flukes and body pigmentation are often used to tell one dolphin or whale from another. For some species, however, it gets more difficult and the narwhal (Monodon monoceros) is an example.

Until recently, photo-identifying these gentle creatures was considered too difficult, mainly because they do not have a dorsal fin, and body pigmentation changes over time.

But some researchers did not give up and found out that narwhals can be identified. Researchers noticed that narwhals have nicks and notches on the dorsal ridge, located at the posterior half of the back, and this part of the body is always visible during breathing. Although the dorsal ridge isn't an ‘obvious’ dorsal fin, natural marks are found on more than 90% of the individuals, and they differ in location, numbers, shape and size.

Now researchers hope to better understand the narwhal’s ecology, and investigate their population abundance as well as social organisation and migrations through the analysis of photo-identification data.

Silvia Bonizzoni

Photo: dorsal ridge’s narwhal, by Marie Auger-Méthé.

Auger-Méthé M., Marcoux M., Whitehead H. 2009. Nicks and notches of the dorsal ridge: promising mark types for the photo-identification of narwhals. Marine Mammal Science. DOI: 10.1111/j.1748-7692.2009.00369.x
Abstract - The narwhal is a hunted species for which we have many knowledge gaps. Photoidentification, which uses photographs of naturalmarkings to identify individuals, is widely used in cetacean studies and can address a broad range of biological questions. However, it has not been developed for narwhals. The marks used for other cetaceans are inappropriate for this species either because narwhals lack the body part on which these marks are found or because the marks are known to change with time. We investigated the marks apparent in photographs of narwhals. Nicks and notches on the dorsal ridge are the mark types most promising for photo-identification. They are found on 91%–98% of the individuals, thus allowing the identification of a large part of the population. They can be used to differentiate between individuals, in part because they are variable in their location, numbers, shape, and size. Although our results suggest that nicks and notches are relatively stable over time, rates of change should be formally measured to assess the probability of photographic matches over multiple years. However, we are confident that these marks can be used in studies spanning at least a field season.

No comments: