Limitations of acoustic monitoring at wind turbines to evaluate fatality risk of bats

Wind turbines (WTs) frequently kill bats worldwide. During environmental impact assessments, consultant ecologists often use automated ultrasonic detectors (AUDs) to estimate the activity and identity of bats in the zone of highest mortality risk at WTs in order to formulate mitigation schemes, such as increased curtailment speeds to prevent casualties. While acknowledging the potential of acoustic monitoring, we evaluate the limitations of AUDs for monitoring bats at WTs and highlight directions for future research. We show that geometric and atmospheric attenuation of ultrasonic echolocation calls, in conjunction with limited sensitivity of ultrasonic microphones, severely constrain detection distances of bats at WTs. Taking into account the acoustic shadow produced by the nacelle, AUDs cover only approximately 23% of the risk zone for a bat calling at 20 kHz and 4% for a bat calling at 40 kHz, assuming a 60 m blade length. This percentage will further decrease with increasing blade lengths in modern WTs. Additionally, the directionality of echolocation calls and the dynamic flight behaviour of bats constrain the detectability of bats. If a call can be detected, the low interspecific and high intraspecific variation of echolocation call characteristics may impair species identification, limiting the power to predict population-level effects of fatalities. We conclude that technical, physical, and biological factors severely constrain acoustic monitoring in its current form. We suggest the use of several AUDs, installed at complementary sites at WTs, and the testing of other techniques, such as radar, cameras, and thermal imaging, to inform stakeholders on the mortality risk of bats at WTs.全球范围内，风力涡轮机（Wind Turbines, WTs）常会造成蝙蝠死亡。在环境影响评价工作中，咨询生态学家常借助全自动超声波探测器（Automated Ultrasonic Detectors, AUDs），对风力涡轮机周边最高致死风险区域内的蝙蝠活动情况与物种种类进行评估，以此制定蝙蝠伤亡减缓方案——例如提高切出转速以避免蝙蝠撞击致死。尽管认可声学监测的应用潜力，本研究仍评估了全自动超声波探测器用于风力涡轮机周边蝙蝠监测的局限性，并指明了未来研究方向。研究表明，超声波回声定位叫声的几何衰减与大气衰减，结合超声波麦克风有限的灵敏度，严重限制了风力涡轮机周边蝙蝠的有效探测距离。若假设叶片长度为60米，同时考虑机舱产生的声学遮蔽效应，全自动超声波探测器仅能覆盖约23%的20 kHz叫声蝙蝠的风险区域，以及4%的40 kHz叫声蝙蝠的风险区域。随着现代风力涡轮机叶片长度的增加，这一覆盖比例还会进一步降低。此外，回声定位叫声的指向性以及蝙蝠的动态飞行行为，也会限制蝙蝠的可探测性。即便能够探测到蝙蝠的回声定位叫声，回声定位叫声特征的种间差异较小、种内差异较大的特点，可能会干扰物种识别，进而限制了对蝙蝠死亡事件的种群层面影响进行预测的能力。综上，技术、物理与生物因素均严重制约了当前形式下的声学监测应用。本研究建议，可在风力涡轮机的互补点位部署多台全自动超声波探测器，并测试雷达、摄像机与热成像等其他技术手段，以便为利益相关方提供风力涡轮机周边蝙蝠致死风险的相关信息。