Why is it important to know the gray seal population size?

One of the major questions concerning the return of grey seals to the US east coast is simply how many are present. In order to understand the impact of gray seals in the ecology of the New England coast, it is vital to have an accurate estimate of the population size at both local and regional scales. In addition, knowledge of the population size and age structure are key for the creation and assessment of management practices.

Challenges of estimating the grey seal population size!

Photo Credit: Mark Benson, Flickr

Photo Credit: Mark Benson, Flickr

One reason that we do not yet have an accurate population estimate for grey seals along the US east   coast is because of the difficulties associated with counting animals that spend much of their time in the water. Grey seals can be counted with relative ease when they are hauled out, however in order to get an accurate count of grey seals, beach counts must be corrected for by taking into account the amount of time spent in the water as compared to the amount of time spent hauled out on the beach. To get around this problem, a popular method of estimating population in seals is to count the number of pups born in a season. Combining this information with pup mortality and adult reproductive rates, the total population can be estimated. However, determining reproductive and mortality rates have challenges of their own, and are not yet known for the gray seal population in New England.

How is population size estimated?

Aerial survey of harbor seals in Glacier Bay National ParkMost commonly, aerial surveys are conducted to count the pups and can involve photographing and counting based on the images taken and/or real-time sighting by trained professionals aboard the plane. Another method that could potentially become useful is to utilize satellite imagery to count pups, but this is not yet commonly done.