Observations on Abundance of Bluntnose Sixgill Sharks, Hexanchus griseus, in an Urban Waterway in Puget Sound, 2003-2005

The bluntnose sixgill shark, Hexanchus griseus, is a widely distributed but poorly understood large, apex predator. Anecdotal reports of diver-shark encounters in the late 1990’s and early 2000’s in the Pacific Northwest stimulated interest in the normally deep-dwelling shark and its presence in the shallow waters of Puget Sound. Analysis of underwater video documenting sharks at the Seattle Aquarium’s sixgill research site in Elliott Bay and mark-resight techniques were used to answer research questions about abundance and seasonality. Seasonal changes in relative abundance in Puget Sound from 2003–2005 are reported here. At the Seattle Aquarium study site, 45 sixgills were tagged with modified Floy visual marker tags, along with an estimated 197 observations of untagged sharks plus 31 returning tagged sharks, for a total of 273 sixgill observations recorded. A mark-resight statistical model based on analysis of underwater video estimated a range of abundance from a high of 98 sharks seen in July of 2004 to a low of 32 sharks seen in March of 2004. Both analyses found sixgills significantly more abundant in the summer months at the Seattle Aquarium’s research station.

扁头六鳃鲨（bluntnose sixgill shark，Hexanchus griseus）是一种分布广泛但认知匮乏的大型顶级捕食者。1990年代末至21世纪初，太平洋西北地区流传的潜水员与该鲨遭遇的轶事报告，引发了学界对这种通常栖息于深水的鲨鱼及其在普吉特湾浅水区出现情况的研究兴趣。本研究采用西雅图水族馆埃利奥特湾六鳃鲨研究站点的水下视频分析数据与标记重捕技术，解答关于该鲨种群丰度与季节特征的研究问题，并在此报告2003至2005年普吉特湾内该鲨相对丰度的季节变化规律。在该研究站点中，研究人员对45头六鳃鲨使用改良型Floy视觉标记标签进行标记，同时记录到约197次未标记个体的观测记录，以及31次标记个体的重观测记录，累计获得273次六鳃鲨的有效观测事件。基于水下视频分析构建的标记重捕统计模型，估算出该站点的种群丰度范围为：2004年7月的峰值98头，与2004年3月的低谷32头。两项分析均显示，西雅图水族馆研究站点内的六鳃鲨种群丰度在夏季月份显著更高。