Sunday, August 25, 2013

Common dolphins

The only thing worse than being stuck in the Bay of Algeciras waiting for the fog to lift is discovering that in our rush to leave we mistook salt for sugar in our iced coffee.  The saving grace is that the bay is completely filled with common dolphins. 

The weather has been quite unpredictable over the last few days.  We've gone out to sea, but fog or high wind has kept us from being able to work with the pilot whales.  Instead, we head to the Bay of Algeciras, next to Gibraltar, where CIRCE is collecting photo-identification data on the dolphins who live there.

Aixa Morata (front) and Mar Hernandez (rear) taking
photo-ID pictures of a short-beaked common dolphin
A fundamental issue in marine mammal research is being able to recognize individual animals.  This allows you to address basic issues such as where and when animals have been sighted or what animals they regularly associate with.  This information also enables higher level studies of population dynamics such as population size or birth and mortality rates.

One way to identify individual animals is through photos of distinctive features.  We often recognize humans by photos of their faces, but different species require different body parts.  With humpbacks and sperm-whales, we use tail flukes, and with pilot whales and dolphins, nicks and notches on the dorsal fins and scars on the back allow us to discriminate between animals.   

A group of short-beaked common dolphins
In the field, this means taking high-quality time-stamped photos of dolphin and pilot whale dorsal fins and comparing them to a catalogue of known individuals to identify the animals.  Unlike in many parts of the world, most of the pilot whales here are surprisingly well marked, but others have very few markings and are extremely difficult to distinguish. Over the last 13 years, CIRCE has built up a long-term catalogue that enables identification of nearly all the resident and immigrant pilot whales in the area.

A close up of a dorsal fin from a long-finned pilot whale
allowing for photo identification
This is why we are here. The information CIRCE has collected is crucial for our studies of social dynamics. We rely on identifying animals in the field so that we may select highly associated individuals - individuals that are seen consistently together - for tagging. Toothed whales are very capable vocal learners, and it is likely that they change their calls when associating with specific individuals for long time periods. The way animals interact within a social group is also likely to differ depending on whether animals grew up together or not. Thus, the long-term data on association patterns available here may help us interpret datasets.

Nicholas Macfarlane and Frants Jensen

No comments:

Post a Comment