White shark tagging off New Zealand

Regional Population Connectivity, Oceanic Habitat, and Return Migration Revealed by Satellite Tagging of White Sharks, Carcharodon carcharias, at New Zealand Aggregation Sites. A joint NIWA/Department of Conservation (DOC) tagging programme was launched in 2005. Hi-tech electronic tags are being used to gather information on where the sharks are and when, and to record their depth and the temperature of the surrounding water. Three tag types have been used: popup archival tags, acoustic tags, and dorsal fin tags. Popup tags are implanted in the muscle under the dorsal fin with a tagging pole, and record depth, temperature and location, storing the data for up to a year. They then release themselves from the shark, float to the surface, and transmit summaries of the data to a satellite. If the tags are physically recovered, the high resolution data collected at one minute intervals can be downloaded. Popup tags provide only approximate location data, so they are most useful for tracking long-distance migrations. Acoustic tags send out coded, individually identifiable sound ‘pings’ that can be detected up to a kilometre away by acoustic data loggers. The tag batteries last long enough to monitor the presence of white sharks in the region in the vicinity of a data logger for two years. Acoustic tags provide accurate fine-scale information on sharks at specific locations. Dorsal fin tags bridge the gap between popup and acoustic tags. They provide accurate location information by transmitting to orbiting satellites every time the shark is at the surface and the dorsal fin and the tag’s aerial are exposed to air. Their batteries can last for more than one year, so both fine scale and large scale movement patterns can be recorded. However, dorsal fin tags are more difficult to deploy: the shark has to be caught and restrained while the tag is attached to the dorsal fin. So far, only a few of these tags have been deployed in New Zealand. Since 2005, 44 white sharks have been tagged with popup tags, mainly at the Chatham Islands and Stewart Island. These islands support large colonies of fur seals, which are a major food source for white sharks. Our research has focussed on Stewart Island since 2009. The population there is dominated by males (about 2.2 males for every female). Nearly all of the females are immature, being shorter than the female length at maturity of 4.5 to 5 m. Only about one-third of the males are longer than the male length at maturity of about 3.6 m. This indicates that the white shark aggregations at Stewart Island are not related to mating, and are most likely driven by the abundance of seals for food. Large, mature females are rarely seen anywhere in New Zealand and their distribution, habitat and behaviour are almost completely unknown. Tagging results show that most New Zealand white sharks make annual migrations to tropical waters in winter, travelling as far as 3,300 km away. Sharks have migrated to the Great Barrier Reef in Australia, the Coral Sea, New Caledonia, Vanuatu, Norfolk Island, Fiji and Tonga. They don't cross the equator. Most of the sharks from Stewart Island headed northwest of New Zealand, whereas most Chatham Islands sharks headed north. However, some sharks from the two tagging locations overlap in the tropics. Surprisingly, we have not detected any direct movement between Stewart and Chatham islands. Some popup tags have remained on the sharks for long enough (up to one year) to reveal that the sharks returned to their tagging locations after their tropical holiday. This has been confirmed by photo-identification work at Stewart Island. Each shark has a unique colour pattern, particularly around the gills and on the tail. Some individuals have been seen at Stewart Island each year for multiple years. This indicates that white sharks have a sophisticated navigation mechanism that enables them to make major direct oceanic migrations, and then return to precisely the spot they left from. We do not know how they achieve this.新西兰聚集海域大白鲨（Carcharodon carcharias）卫星标记揭示的区域种群连通性、海洋栖息地与回迁行为。新西兰国家水与大气研究所（NIWA）与环境保护部（DOC）于2005年联合启动了大白鲨标记计划。研究采用高端电子标签采集大白鲨的时空分布数据，并记录其潜水深度与周边海水温度。本次研究共使用三类标记标签：弹出式档案标签（popup archival tags）、声学标签（acoustic tags）与背鳍标签（dorsal fin tags）。弹出式档案标签通过标记杆植入背鳍下方肌肉组织，可记录深度、温度与位置信息，数据存储时长可达一年。随后标签会自动脱离鲨鱼，上浮至海面并将数据摘要传输至卫星。若物理回收该标签，则可下载每分钟采集的高分辨率原始数据。此类标签仅能提供近似位置信息，因此最适用于长距离迁徙追踪。声学标签可发射经编码的唯一识别声脉冲信号，最远可在1公里范围内被声学数据记录仪捕获。标签电池续航足以在数据记录仪周边区域监测大白鲨的出现情况长达两年。声学标签可提供特定区域内大白鲨的高精度精细尺度活动信息。背鳍标签填补了弹出式档案标签与声学标签之间的技术空白。每当鲨鱼上浮至水面、背鳍及标签天线暴露于空气中时，该标签会向在轨卫星传输精准位置信息。其电池续航可超过一年，因此可同时记录精细尺度与大尺度的活动模式。但背鳍标签部署难度更高：需将鲨鱼捕获并固定后，才能将标签附着于背鳍。截至目前，新西兰仅部署过少量此类标签。自2005年起，研究人员已在查塔姆群岛与斯图尔特岛周边为44头大白鲨植入了弹出式档案标签。这两座岛屿栖息着大量毛皮海豹种群，而毛皮海豹是大白鲨的主要食物来源。自2009年起，研究重点转向斯图尔特岛海域：该海域的大白鲨种群以雄性为主，雌雄比例约为2.2:1。几乎所有雌性个体均未性成熟，体长均低于雌性性成熟体长（4.5至5米）；仅有约三分之一的雄性个体体长超过雄性性成熟体长（约3.6米）。这表明斯图尔特岛的大白鲨聚集行为与交配无关，最有可能是为了利用充足的海豹食物资源。大型成熟雌性大白鲨在新西兰海域极为罕见，其分布、栖息地与行为几乎完全未被探明。标记结果显示，多数新西兰大白鲨会在冬季进行年度性迁徙，前往热带海域，最远迁徙距离可达3300公里。已记录的迁徙目的地包括澳大利亚大堡礁、珊瑚海、新喀里多尼亚、瓦努阿图、诺福克岛、斐济与汤加，但它们不会穿越赤道。来自斯图尔特岛的大白鲨大多向新西兰西北方向迁徙，而来自查塔姆群岛的个体则大多向北迁徙。不过部分个体的热带迁徙区域存在重叠。令人意外的是，研究未检测到斯图尔特岛与查塔姆群岛之间存在直接的个体移动。部分弹出式档案标签的续航时长（可达一年）足以记录到鲨鱼在完成热带海域活动后返回原标记地点的行为，这一结论已通过斯图尔特岛的照片识别研究得到证实：每头大白鲨都拥有独特的体色图案，尤其是在鳃部与尾部区域。部分个体连续多年每年都在斯图尔特岛被观测到。这表明大白鲨拥有复杂的导航机制，使其能够完成长距离远洋迁徙后精准返回出发地点，但其具体实现方式至今仍未明确。