Historical distribution and abundance of the North Atlantic right whale

The North Atlantic right whale (Eubalaena glacialis) is one of the world’s most threatened whale species, having come close to extinction after nearly a millennium of exploitation, and currently persisting as a population of only ca. 500 individuals. Setting appropriate conservation targets for this species requires an understanding of its historical population size, as a baseline for measuring levels of depletion and progress towards recovery. This is made difficult by the scarcity of records over this species’ long whaling history. We reconstructed the potential historical summer distribution and abundance of the North Atlantic right whale through statistical models combining historical records and environmental data. 

[Photo: Florida Fish and Wildlife Conservation Commission/NOAA]

We took advantage of a spatially explicit dataset on historical catches of North Pacific right whales (Eubalaena japonica) to model the relationship between right whale relative density and the environment during the summer feeding season. Assuming that the two right whale species select similar environmental conditions, we projected this model into the North Atlantic to predict how the relative abundance of NARWs varied across their range. Our results suggest that this species’ feeding range extended from the eastern coast of North America to northern Norway, well beyond its current feeding areas off eastern North America.

Top: Predicted historical distribution of the North Atlantic right whale in the summer months based on a species distribution model fitted to whaling records for the North Pacific right whale, and extrapolated into the Atlantic. Shades of red indicate progressively higher environmental suitability as predicted by the model; blue cells correspond to areas where the species is predicted as absent; white cells are areas for which no reliable predictions can be made. Bottom: Historical records in relation to the envelope of predicted presence (pink). Symbols correspond to historical (pre-1950) records (in red) and recent (post-1950) records outside the main summer grounds (black crosses). [Figure reproduced from Monsarrat et al. (2015); see this paper for more details].

In a separate model, we then predicted the relative abundance of North Pacific right whales within their predicted range, by modeling the relationships between whale strikes for the period 1840-1849 and the associated environmental conditions, to obtain predictions of abundance conditional on the species’ presence. We calibrated these relative abundances with estimates of the North Pacific right whale total pre-whaling population size. By projecting this model into the North Atlantic, we predicted how the relative abundance of North Atlantic right whales varied across their range, as well as the total abundance across the entire range. We estimate that there were 9,075 to 21,328 right whales in the North Atlantic. If so, the current NARW population is less than 6% of the historical North Atlantic carrying capacity and has enormous scope for recovery. We further predict that in the summer North Atlantic right whales were concentrated in two main feeding areas: east of the Grand Banks of Newfoundland and in the Norwegian Sea. These two areas may not only become important in the future as additional feeding grounds, they may already be playing a role in the conservation of this endangered species.

Predicted abundance from a Generalized Additive Model fitted to historical catches of North Pacific right whales and extrapolated to the North Atlantic within the envelope of predicted distribution; white cells are outside this envelope. “Rel.” are values of relative abundance (from the sample of whaling data analyzed); “Low” and “High” are values of absolute abundance per cell, after calibration with total mortality data across the North Pacific. [Figure reproduced from Monsarrat et al. in press; see this paper for more details].