The goal of the present study was to complement existing data of testosterone and progesterone in blue whale (Balaenoptera musculus) blubber from the eastern North Pacific Ocean to evaluate effects of seasonality and location on these hormones and to better assess reproductive status of individuals. Physiological parameters regarding reproduction are fundamental for describing population dynamics, and hormones can be a valid tool to estimate those for wildlife populations. In this study, blubber tissue was validated for testosterone and progesterone assays. Hormone concentrations were measured in 69 (35 males and 34 females) blubber samples from live (n = 66) and stranded (n = 3) animals collected between 2002 and 2016 from a known winter reproductive ground in the Gulf of California (GoC) and summer feeding areas along the United States West Coast (USWC), specifically off the states of California and Oregon. Results were combined with sighting histories as a tool to determine reproductive status of individual whales. Testosterone concentrations in adult male blue whales were significantly higher (p < 0.05) in blubber biopsies sampled off the USWC between the months of June and November compared to those sampled in the GoC between February and April. Elevated testosterone concentrations likely indicate physiological preparation for reproductive activity while the animals were present off the USWC. Progesterone concentrations were significantly elevated in pregnant females, confirming progesterone as an indicator of pregnancy in blue whales. Probabilities of being pregnant were estimated for adult females with unknown sighting histories based on progesterone concentrations. Testosterone in females was detected and measured only in pregnant whales suggesting its biosynthesis or metabolism is altered during gestation. These results provide updated and new information on the reproductive cycle of blue whales in the eastern North Pacific, posing new milestones to better estimate the timing of the mating season for this endangered population.

本研究的目的是补充蓝鲸（Balaenoptera musculus）中睾酮和孕酮的现有数据来自北太平洋东部的鲸脂，以评估季节性和位置对这些激素的影响，并更好地评估个体的生殖状况。有关繁殖的生理参数是描述种群动态的基础，而激素可以成为估计野生动物种群的有效工具。在这项研究中，鲸脂组织经过验证可用于睾酮和孕酮测定。在 2002 年至 2016 年期间，从加利福尼亚湾 (GoC) 已知的冬季繁殖地和美国西海岸 (USWC) 的夏季觅食区，特别是加利福尼亚州和俄勒冈州附近。结果与目击历史相结合，作为确定个体鲸鱼繁殖状况的工具。与 2 月至 4 月期间在 GoC 采样的相比，在 6 月至 11 月期间从 USWC 取样的鲸脂活检中，成年雄性蓝鲸的睾酮浓度显着更高 (p < 0.05)。当动物出现在 USWC 之外时，睾酮浓度升高可能表明生殖活动的生理准备。怀孕雌性的黄体酮浓度显着升高，证实黄体酮是蓝鲸怀孕的指标。根据黄体酮浓度估计有未知目击史的成年女性怀孕的概率。仅在怀孕的鲸鱼中检测和测量了雌性的睾酮，这表明其生物合成或新陈代谢在怀孕期间发生了改变。这些结果提供了有关北太平洋东部蓝鲸繁殖周期的最新信息，为更好地估计这一濒临灭绝种群的交配季节时间提供了新的里程碑。