Population estimate and demographic trends of humpback whale breeding stock G in Ecuador

Insights from 15 years of land-based monitoring

Abstract

Population abundance and trends of the humpback whale Breeding Stock G (BSG) were estimated from land-based monitoring at the Santa Elena Peninsula tip, Ecuador (2°11.35′S, 81°0.63′W). The study was conducted between May and December from 2010 to 2024. The effort included 3,736.5 hours of observation, during which 7,528 whale groups were recorded. Boat-based data were used to complement land-based observations. The first whales arrived in early May, and the last departed in late December. The northbound migration peak occurred in late June. The observation rate increased from 0.699 whales per hour (whales/h, SD = 0.63) in 2010 to 4.19 whales/h (SD = 3.45) in 2024, concordant with a population increase. Based on an estimated intrinsic population growth rate of 0.107, we estimated the population to be 27,362 whales in 2024. The calf production rate was 0.165 (SD = 0.04). The average annual survival rate of the population was 0.956 (SD = 0.212). These findings suggest that the population has recovered and is likely approaching carrying capacity. The observed interannual variability was associated with ENSO’s impact on whale migration behavior. During strong El Niño years (2015 and 2023), the birth rate increased, likely because a portion of the population did not migrate or reach the breeding area, artificially inflating the rate. In the years following a strong El Niño (2016 and 2024), the birth rate decreased, while the population increased considerably as whales that did not migrate the previous year joined with pregnant females in the former year. This relationship between migration behavior and environmental conditions warrants further analysis, particularly in light of increasing ocean climate variability and its implications for population assessments.