Spatial ecology of eastern North Pacific blue whales

Reliable trends of population abundance are fundamental to conservation assessments and measuring the success of conservation actions. Abundance of eastern North Pacific (ENP) blue whales has previously been estimated using conventional closed capture–recapture and distance sampling methods, but the estimates show differing trends which are compounded by statistical uncertainty. Exploratory analyses of photo-identification, telemetry and line transect survey data indicate spatial variation in distribution over time, which could explain differences in trends and motivated a spatial approach to estimate abundance. I developed a spatial capture-recapture (SCR) approach to model blue whale photo-identification data collected from small boat surveys with associated effort along the United States west coast from 1991 to 2023. Density was modelled as a flexible term of latitude (space) and year (time), and identified cyclical spatiotemporal variation in abundance and indicated a possible northward shift in density over the study period. To identify the drivers of this variation, I developed the SCR approach further, modelling spatiotemporal patterns of density as functions of ecologically informed predictors of distribution. Abundance estimates showed marked interannual variation highlighting the importance of considering ecological drivers of distribution to enhance understanding of biological processes. Identification of these ecological relationships may enable prediction of future distribution in response to projected changes in environmental covariates. To improve the approach further, I investigated the impacts of data integration on statistical precision and biological inferences, using a five-year subset of the SCR model which was tested with opportunistic photo-identification and telemetry data integrated, separately and then together. This model found greater precision around abundance estimates and identified a different pattern of space use. These results further our understanding of ENP blue abundance and distribution, and deliver methodological developments relevant to many wide-ranging marine species, which are important steps towards informed population monitoring in an era of ecological change.