New Statistical Model for Estimating North Atlantic Right Whale Abundance Revealed

Figuring out how many North Atlantic right whales exist has always been a difficult task. After all, these migratory animals travel from their winter grounds in the Caribbean and off the southeastern U.S. along the Eastern seaboard into Massachusetts Bay and the Gulf of Maine each year. Researchers have long used photographs taken of the whales at the surface to estimate the number living and their reproductive rates. Thousands of those photographs reside in the New England Aquarium’s Right Whale Catalog (http://rwcatalog.neaq.org/ Terms.aspx).

The catalogue forms the basis of a breadth of population estimates, including population viability, migration patterns, distribution and demographics, reproduction, mortality rates, genetic substructuring, patterns of chemical exposure, association patterns, mating strategies, patterns of health, and incidence of past human interactions.

The problem with using photographs and annual at-sea surveys to assess the right whale population is that changes in the whales’ behavior may make them harder to find and vagaries in funding for annual surveys may change the surveys’ scope and frequency. For example, a warmer Gulf of Maine has affected the location and density of the copepod species that forms the basis of the right whales’ diet. Once the right whales turned up in great numbers off Grand Manan Island in the late summer and early fall; in recent years that pattern has changed. If, during the annual surveys, a specific whale is not seen for six consecutive years, it is assumed to have died.

In September, Richard Pace and Peter Corkeron, large whale researchers at NOAA’s Northeast Fisheries Science Center, and Scott Kraus of the New England Aquarium published an article in the journal Ecology and Evolution detailing a new statistical modelling approach developed to better estimate right whale population trends. The new estimates are less affected by changes in whale distribution, less reliant on sighting frequency, and better account for animals that are still alive but are not frequently seen.

For the study, data from more than 61,000 sightings were reviewed by the authors. Analysis included sighting histories from 658 whales, including 280 females, 328 males and 50 animals of unknown sex. Of the 658 whales seen during the study period of 1990 to 2015, 247 were first seen before 1990.

Using the new model, the researchers concluded that the population has shrunk slightly in the past few years. Between 1990 and 2010, the abundance of North Atlantic right whales increased by slightly less than 3% per year, from about 270 animals in 1990 to 482 in 2010. After relatively steady increases over that time, abundance has declined each year since 2010, to 458 animals in 2015. In addition, the number of female right whales has declined, leading to a growing gap between the sexes.

The new model shows a slight declining trend but only covers the period through 2015. Given the high level of right whale mortalities in 2016 and 2017, the model will reflect a much steeper decline when those years are included. However, the results of this new model produce a higher population estimate than the previous methods used in the right whale stock assessment because scientists no longer have to observe every individual whale for it to be included in the population. Given the distribution shifts of whales, this has become more difficult in recent years. The older methods of estimating the population would have indicated a much steeper decline in the right whale population.

The new model comes at a time when federal authorities in the U.S. and Canada are discussing new measures to protect right whales after 16 of the animals were found dead in the two countries this summer.

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