Counting Seals in Cape Cod With Satellites

April 28, 2022

In the early 1970s, grey seal populations were nearly extinct in North America. Since then, populations have rebounded. Until a few months ago, there was no accurate count of current populations because the number of animals has grown dramatically in the last few decades.

During a project to monitor seal populations with cellphone tags — to gather information about their location, dive depth, and behaviour — David Johnston, Associate Professor of Marine Conservation Ecology at Duke University, noticed he could spot colonies on Google Earth imagery.

The class would load up a KML file extracted from their location trackers to assess what the seals were doing. The students and Johnston decided to work together to get a population census of seals in the area when they realized they could see them well enough with satellite data to get an estimate.

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

According to previous beach counts, the population was estimated to be between 20,000 and 30,000 animals, but they were not comprehensive and could not be linked to existing telemetry data.

The telemetry devices gave the team details about how long the animals spent on the beach. Coupled with the number of seals that could be seen in the satellite image and corresponding data, this enabled them to estimate the whole population in the area. They predicted a count of 30,000 and 50,000 animals, possibly even closer to 50,000; this was almost double previous estimates. The research has since been published in Bioscience – the first time a grey seal population count has been made for the Northeastern United States.

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