Success story: Comeback of the Gray Seals

Published 2017-06-20

Photo: Northeast Fisheries Science Center, NOAA

The gray seal (Halichoerus grypus) is found on both the western and eastern shores of the Atlantic Ocean. Gray seals eat a diverse diet of fish, octopus, lobster, skates, and eels and can reach weights of up to 880lb (males) and 550lb (females) in the western Atlantic. Pups are born in the winter in the western Atlantic and in the autumn in the eastern Atlantic. The gray seals were hunted to near extinction in the western Atlantic through the mid 1900’s for oil, meat, and skin. In the 1980’s, after the passing of the 1972 Marine Mammal Protection Act, sightings of the seals started to increase. Recent findings suggest that efforts to conserve seals are working and highlight a conservation success story.

In 1973, a survey documented only 30 gray seals along the coast of Maine. By 2009, thousands of gray seals were sighted on the east coast and in 2011, that number was increased to over 15,000. These past surveys relied on traditional methods of counting individual seals on beaches, islands, and ice, including observations by manned aircrafts. However, seals spend a large amount of time in the water and the pups' coat provides camouflage to their surroundings, limiting the ability of researchers to reliably assess the population by visual means.

A new study published in Bioscience by Johnston and colleagues details a new technological approach to conducting population assessments of the grey seal. The researchers used a combination of Google Earth imagery, aerial surveys, and telemetry data to generate a new estimate of the number of gray seals – between 30,000 and 50,000 individuals. The use of technological advances in these surveys allowed the researchers to count individuals that were out at sea and camouflaged pups by using thermal imagery. The images and data collected in these surveys were analyzed in two ways – first by human observation and manual counting of individuals and second through an automated computer algorithm. The two methods showed less than a 5% difference, indicating that automated methods may be just as reliable as manual methods while saving time and resources.

This new assessment of the gray seal population in the western Atlantic highlights a conservation success story for marine mammals.

Journal Reference:
1. Jerry H. Moxley, Andrea Bogomolni, Mike O. Hammill, Kathleen M. T. Moore, Michael J. Polito, Lisa Sette, W. Brian Sharp, Gordon T. Waring, James R. Gilbert, Patrick N. Halpin, David W. Johnston. Google Haul Out: Earth Observation Imagery and Digital Aerial Surveys in Coastal Wildlife Management and Abundance Estimation. BioScience, 2017; DOI: 10.1093/biosci/bix059