The World’s Northernmost Harbour Seal Population–How Many Are There?

This study presents the first abundance estimate for the world’s northernmost harbour seal (Phoca vitulina) population, which resides in Svalbard, Norway, based on three digital stereoscopic photographic surveys conducted in 2009 and 2010. The counts from these high resolution 3D images were combined with a novel method for estimating correction factors for animals that were in the water at the time of the surveys, in which extensive behavioural data from radio-tagged harbour seals were used together with age distribution data to estimate the proportion of seals of various age and sex classes hauled out at the times of the surveys. To detect possible seasonal shifts in age distribution between surveys, lengths of hauled out seals were measured from the stereoscopic images. No body-length differences were detected between the surveys; but, this may be due to a high degree of sexual dimorphism exhibited in this population. Applying the modelled correction factors, a total of 1888 (95% CI: 1660–3023), 1742 (1381–3549) and 1812 (1656–4418) harbour seals were estimated for the surveys flown on 01 August 2009, 01 August 2010 and 19 August 2010, respectively. The similarity between the three survey estimates (despite significant differences in the number of animals actually counted on the photos from each survey effort) suggests that the variation in numbers of hauled out seals is reasonably accurately adjusted for by the haul-out probability model. The low population size, the limited spatial distribution of the population and its reduced genetic diversity make this population vulnerable to chance events, such as disease epidemics.

本研究基于2009年与2010年开展的三次立体数码摄影调查，针对栖息于挪威斯瓦尔巴群岛的全球最北港海豹（Phoca vitulina）种群，完成了首次种群丰度估算。本次调查获取的高分辨率三维图像计数结果，结合一种全新的校正因子估算方法，用于修正调查时段内处于水中的个体造成的统计偏差：该方法依托无线电标记港海豹的大量行为数据与年龄分布数据，估算不同年龄与性别类群的海豹在调查时刻的上岸比例。为检测不同调查间年龄分布可能存在的季节性偏移，研究人员从立体图像中测量了上岸海豹的体长。结果未发现调查间存在体长差异，但这可能源于该种群显著的性别二态性。应用建模得到的校正因子后，2009年8月1日、2010年8月1日及2010年8月19日三次调查对应的港海豹种群丰度估算值分别为1888只（95%置信区间：1660–3023）、1742只（1381–3549）及1812只（1656–4418）。尽管三次调查实际从照片中统计的个体数量存在显著差异，但三者的估算结果较为接近，这表明上岸概率模型已较为精准地校正了上岸海豹数量的波动。该种群规模偏小、空间分布有限且遗传多样性降低，使其极易受到疫病流行等偶然事件的冲击。