Table_1_Changes of Oceanic Conditions Drive Chagos Whale Migration Patterns in the Central Indian Ocean.docx

Marine ecosystems are experiencing rapid shifts under climate change scenarios and baleen whales are vulnerable to environmental change, although not all impacts are yet clear. We identify how the migration behaviour of the Chagos whale, likely a pygmy blue whale (Balaenoptera musculus brevicauda), has changed in association with shifts in environmental factors. We used up to 18 years of continuous underwater acoustic recordings to analyse the relationships between whale acoustic presence and sea surface temperature (SST), chlorophyll-a concentration, El-Niño Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD). We compared these relationships between two independent sites Diego Garcia southeast (DGS) and Diego Garcia northwest (DGN) where Chagos whales are detected and are suspected to move interannually across the Chagos-Laccadive ridge. We showed that the number of whale songs detected increased on average by 7.7% and 12.6% annually at DGS and DGN respectively. At the DGS site, Chagos whales shifted their arrival time earlier by 4.2 ± 2.0 days/year ± SE and were detected for a longer period by 7.3 ± 1.2 days/year ± SE across 18 years. A larger number of songs were detected during periods of higher chlorophyll-a concentration, and with positive IOD phases. At the DGN site, we did not see an earlier shift in arrival and songs were not detected for a longer period across the 13 years. Whale presence at DGN had a weaker but opposite relationship with chlorophyll-a and IOD. The oceanic conditions in the Indian Ocean are predicted to change under future climate scenarios and this will likely influence Chagos whale migratory behaviour. Understanding how environmental factors influence whale movement patterns can help predict how whales may respond to future environmental change. We demonstrate the value of long-term acoustic monitoring of marine fauna to determine how they may be affected by changing environmental conditions.

气候变化背景下，海洋生态系统正经历快速变迁，须鲸（baleen whales）对环境变化较为敏感，尽管目前其受影响的具体效应尚未完全明晰。本研究针对查戈斯海域疑似为小蓝鲸（pygmy blue whale, *Balaenoptera musculus brevicauda*）的种群，探讨其洄游行为随环境因子变化的响应模式。
我们采用长达18年的连续水下声学记录（underwater acoustic recordings），分析鲸类声学检出频次与海表温度（sea surface temperature, SST）、叶绿素a浓度（chlorophyll-a concentration）、厄尔尼诺-南方涛动（El-Niño Southern Oscillation, ENSO）以及印度洋偶极子（Indian Ocean Dipole, IOD）之间的关联。
我们在两处独立监测站点——迭戈加西亚东南海域（Diego Garcia southeast, DGS）与迭戈加西亚西北海域（Diego Garcia northwest, DGN）——开展上述关联分析，两处均有查戈斯鲸类的声学检出记录，且推测种群会年际间跨查戈斯-拉克代夫海岭（Chagos-Laccadive ridge）移动。
研究显示，DGS与DGN站点的鲸鸣检出数量年均分别增长7.7%与12.6%。在DGS站点，18年间查戈斯鲸类的抵达时间年均提前4.2±2.0天（标准误，±SE），且每年的监测持续时长年均延长7.3±1.2天（标准误，±SE）。在叶绿素a浓度较高及印度洋偶极子正相位阶段，检测到的鲸鸣数量更多。
而在DGN站点，未观测到抵达时间提前的趋势，且13年间的监测持续时长亦未出现显著延长。DGN站点的鲸类检出频次与叶绿素a浓度、IOD的关联强度较弱，且方向与DGS站点相反。
未来气候变化情景下，印度洋海洋环境预计将发生改变，这大概率会影响查戈斯鲸类的洄游行为。明确环境因子如何调控鲸类移动模式，有助于预测鲸类对未来环境变化的响应。本研究证实了长期水下声学监测在解析海洋动物响应环境变化机制中的重要应用价值。