Four decades of annual monitoring reveal declining reproductive success of a migratory baleen whale

Abstract

Understanding and mitigating threats to species survival relies on the ability to track key demographic processes over time. Among these processes, reproduction stands out as a primary biological driver of population growth and resilience. Multiple factors affect reproductive success, including environmental variables and resource availability. Long-term trends in reproductive parameters of long-lived species can thus serve as indicators of ecosystem health over meaningful timescales. This study analyses the reproductive performance of southern right whales (Eubalaena australis) in South Africa using forty years of individual sighting histories. Results reveal that the median and average calving intervals increased post-2010, reaching an average maximum of 7.2 years in 2022. The average calving rate decreased significantly across the four decades, from 0.48 calves/year in the 1980s to 0.19 calves/year in the 2010s. Although based on observational data, these findings align with the hypothesis of a trade-off linked to the reduced maternal body condition in this population, likely indicative of diminished foraging success associated with recorded regional Antarctic sea ice declines and reduced prey availability. The study adds to the growing evidence that baleen whale reproduction is influenced by environmental change, reinforcing the need for long-term monitoring to understand population dynamics and their potential role as sentinels of ocean health.