Sea ice habitat characteristics and number of narwhal (Monodon monoceros) sightings in Disko Bay, West Greenland

There is a paucity of information on abundance, densities, and habitat selection of narwhals Monodon monoceros in the offshore pack ice of Baffin Bay, West Greenland, despite the critical importance of winter foraging regions and considerable sea ice declines in the past decades. We conducted a double-platform visual aerial survey over a narwhal wintering ground to obtain pack ice densities and develop the first fully corrected abundance estimate using point conditional mark-recapture distance sampling. Continuous video recording and digital images taken along the trackline allowed for in situ quantification of winter narwhal habitat and for the estimation of fine-scale narwhal habitat selection and habitat-specific sighting probabilities. Abundance at the surface was estimated at 3484 (coefficient of variation [CV] = 0.46) including whales missed by observers. The fully corrected abundance of narwhals was 18 044 (CV = 0.46), or approximately one-quarter of the entire Baffin Bay population. The narwhal wintering ground surveyed (~9500 km**2) had 2.4 to 3.2% open water based on estimates from satellite imagery (NASA Moderate Resolution Imaging Spectroradiometer) and 1565 digital photographic images collected on the trackline. Thus, the ~18 000 narwhals had access to 233 km**2 of open water, resulting in an average density of ~77 narwhals/km**2 open water. Narwhal sighting probability near habitats with <10% or 10 to 50% open water was significantly higher than sighting probability in habitats with >50% open water, suggesting narwhals select optimal foraging areas in dense pack ice regardless of open water availability. This study provides the first quantitative ecological data on densities and habitat selection of narwhals in pack ice foraging regions that are rapidly being altered with climate change.尽管冬季觅食区的重要性不言而喻，且过去数十年间海冰面积已出现显著减少，但目前关于西格陵兰巴芬湾近海浮冰区内独角鲸（Monodon monoceros）的种群丰度、密度及栖息地选择的相关信息仍较为匮乏。本研究在独角鲸越冬地开展双平台目视航空调查，以获取浮冰密度数据，并首次采用点条件标记-重捕距离抽样法完成了完全校正的种群丰度估算。沿测线采集的连续录像与数码影像，可用于原位量化冬季独角鲸的栖息地特征，同时实现精细尺度的独角鲸栖息地选择与栖息地专属目击概率估算。表层可见个体的种群丰度估算值为3484头（变异系数（coefficient of variation，CV）= 0.46），该数值包含观测者漏检的个体；经完全校正后的独角鲸总丰度为18044头（CV = 0.46），约占巴芬湾整个种群的四分之一。本次调查的独角鲸越冬地面积约9500平方千米，基于美国国家航空航天局（National Aeronautics and Space Administration，NASA）中分辨率成像光谱仪（Moderate Resolution Imaging Spectroradiometer）的卫星影像以及测线沿途采集的1565张数码影像估算，该区域内开阔水域占比为2.4%~3.2%。据此推算，约18000头独角鲸可利用的开阔水域总面积为233平方千米，对应开阔水域内的平均种群密度约为77头/平方千米。在开阔水域占比低于10%或处于10%~50%区间的栖息地附近，独角鲸的目击概率显著高于开阔水域占比超过50%的栖息地，这表明独角鲸会优先选择密集浮冰中的最优觅食区域，而非依赖开阔水域的可获得性。本研究首次提供了受气候变化快速影响的浮冰觅食区内独角鲸种群密度与栖息地选择的定量生态学数据。