ABSTRACT: Effective management of wild populations of animals relies on having accurate methods for estimating their densities from field surveys. Surveying sharks and other highly mobile marine predators is particularly difficult, requiring specialised methods that can entail a range of potential biases. Recent simulation studies have suggested that non-instantaneous visual and video survey methods may overestimate densities of highly mobile fishes. A non-instantaneous method is one where each point in space is surveyed for a non-negligible period of time, rather than in an instantaneous ‘snapshot’, allowing animals to move into the survey space during the survey period. We conducted instantaneous and non-instantaneous surveys of sharks (Sphyrna lewini and Carcharhinus galapagensis) around Darwin and Wolf Islands (Galápagos Islands, Ecuador) using diver-operated stereo-video (stereo-DOV) systems with towed GPS. We provide the first empirical evidence of non-instantaneous bias and show that the degree of bias is negatively related to diver swimming speed. At a speed of 40 m min^-1, the non-instantaneous method recorded twice as many sharks as the instantaneous method. The bias may be as high as 10 fold at slow speeds (23 m min^-1) and negligible at very fast speeds (65 m min^-1). Moreover, we used GPS data to produce fine-scale distribution maps, revealing high densities in areas of strong currents along the south-eastern sides of both islands. We recommend the use of stereo-DOV systems with GPS and fast swimming speeds for surveying highly mobile sharks, to produce more realistic estimates of densities on which to base management of these species.

中文翻译：

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加拉帕戈斯群岛海洋保护区高机动性鲨鱼的瞬时与非瞬时潜水操作立体视频（DOV）调查

摘要：有效管理野生动物种群的方法有赖于通过实地调查估算其密度的准确方法。测量鲨鱼和其他机动性强的海洋捕食者特别困难，需要采用可能导致一系列潜在偏差的专门方法。最近的模拟研究表明，非即时的视觉和视频调查方法可能会高估高流动性鱼类的密度。非瞬时方法是在一个不容忽视的时间段内对空间中的每个点进行调查的方法，而不是在瞬时的“快照”中进行调查的方法，该方法可以使动物在调查期间进入调查空间。我们对鲨鱼（Sphyrna lewini和Carcharhinus galapagensis）进行了瞬时和非瞬时调查）在达尔文和沃尔夫群岛（厄瓜多尔加拉帕戈斯群岛）周围使用带拖曳GPS的潜水员操作的立体声视频（stereo-DOV）系统。我们提供了非瞬时偏见的第一个经验证据，并表明偏倚程度与潜水员的游泳速度负相关。以40 m min ^-1的速度，非瞬时方法记录的鲨鱼数量是瞬时方法的两倍。在慢速（23 m min ^-1）下，偏差可能高达10倍，而在非常快的速度（65 m min ^-1）下可以忽略不计）。此外，我们使用GPS数据生成了精细的分布图，揭示了两个岛屿东南侧沿强流区域的高密度。我们建议使用具有GPS和快速游泳速度的立体声DOV系统来调查高度活动的鲨鱼，以对这些物种进行管理时得出更现实的密度估计。