Counting seals in Cape Cod with satellites

Grey seals were nearly driven to extinction in North America by hunting in the early 1970’s. Populations have since then rebounded. In the past few decades, the number of animals has grown dramatically and, until a few months ago, there was no accurate count of current populations.

David Johnston, Associate Professor of the Practice of Marine Conservation Ecology at Duke University, was working on a project to monitor seal populations with cellphone tags  — to gather information on location, dive depth, and animal habits — when he noticed he was able to spot colonies from Google Earth imagery of beaches.

In order to monitor the animals, the class would load up a KML file extracted from the location trackers to assess what the seals were doing. When they realized they could see the seals well enough with satellite data to get an estimate, Johnston decided to work with students on getting a population census for the seals in the area.

Visible seals can be spotted on a beach in Cape Cod via satellite imagery available on Google Earth.

Visible seals can be spotted on a beach in Cape Cod via satellite imagery available on Google Earth.

Previous beach counts had estimated the population to be around 20,000-30,000 animals but none of them were comprehensive and, most importantly, they could not connect these isolated numbers with the existing telemetry data.

The telemetry devices provided the team information about the proportion of time the animals spend on the beach. Combined with the number of seals they could identify in the satellite image and the data they obtained when the images were taken, these datasets allowed the team to understand what the whole population could be in the area. With the two datasets, they estimated the population to be between 30,000 and 50,000 animals, probably closer to 50,000 which could be more than double previous estimates. This study was recently published in the journal Bioscience, providing the first population estimate of gray seals in the Northeastern United State.


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