Preliminary report of abundance estimation for the North Pacific humpback whales using IWC-POWER data
Kanako Inai, Koji Matsuoka and Toshihide Kitakado
International Whaling Commission
The abundance of humpback whales in a summer feeding ground in the North Pacific was estimated using sighting data from 2010-2012 and 2017 surveys of the â€œInternational Whaling Commission Pacific Ocean Whale and Ecosystem Research (IWC-POWER)â€. A design-based line transect method was primarily used for the estimation of density and abundance. In the estimation of detection function, both of half-normal and hazard-rate functions were used with covariates/factors such as â€œyear of surveyâ€, â€œschool sizeâ€, â€œcueâ€ and â€œvisibilityâ€ as well as some likely interaction terms among them. The best model was then selected by an information criterion, AIC. The density and abundance were estimated by Horvitz-Thompson-like estimators to account for possible heterogeneity in detection process within the leg. In addition, a spatial modelling approach was tested as a model-based method using the GAM with potential covariates of longitude, latitude, SST and distance from the land to see any differences in estimates among two estimation methods. As a result, in the design-based model, the abundance of humpback whales migrating to the southern Aleutian archipelago (2010-2012 survey areas; north of 40Â°N, south of the Alaskan coast including both the US and Canadian EEZs between 170Â°E-135Â°W) in summer (July - August) was estimated as 9,868 (CV=0.53) under the hazard-rate model with the survey year, school size and visibility. The northern Aleutian archipelago (2017 survey areas; eastern Bering Sea) in summer (July - August) was estimated as 4,539 (CV=0.64) under the half-normal model with the school size and visibility. In total, 14,407 (CV=0.56). An initial trial for the model-based method provided an estimate in 2010-2012 survey areas as 16,961, which is somewhat larger than the estimate from the design-based estimates, and therefore it warrants further researches to improve the spatial modelling and compare the results. The authors conclude that the model-based estimates in the south area (2010-2012 survey areas) can be used for future in-depth assessment of this species in the North Pacific Ocean.