Updated analysis of abundance and population structure of seasonal gray whales in the Pacific Northwest, 1996-2017
John Calambokidis, Alie Perez, and Jeffrey Laake - Uploaded by Calambokidis
International Whaling Commission
We update the results of a 22-year (1996-2017) collaborative study examining the abundance and the population structure of these animals conducted over a number of regions from Northern California to British Columbia using photographic identification. Some 22,847 identifications representing 1,944 unique gray whales were obtained during 1996-2017 from Southern California to Alaska. Gray whales seen from 1 June - 30 Nov (after the northward and before southward migrations) were more likely to be seen repeatedly and in multiple regions and years; therefore only whales seen during those data were included in the abundance estimates. Gray whales using the Pacific Northwest in summer and fall include two groups: 1) whales that return frequently and account for the majority of the sightings and 2) transients seen in only one year, generally for shorter periods and in more limited areas. A time series of abundance estimates of the non-transient whales for 1996-2017 was constructed for the region from N. California (NCA) to N. Vancouver Island (NBC). The most recent estimate for 2017 was 232 whales (se=25.2). The estimated abundance increased in the late 1990s and early 2000s during the period when the eastern North Pacific gray whale population was experiencing a high mortality event and this created an apparent influx of whales into the area. The earlier estimates for 1996-1997 are biased low because the survey coverage area was much smaller but those data were included to improve estimates later in the time series. This report updates our estimates in previous similar reports using data from two new years (2016 and 2017). The abundance estimates since the early 2000s were relatively stable but increased from 2010 to 2015 before decreasing slightly in 2016.