2020 Aerial survey data of southern right whales (Eubalaena australis) off southern Australia

Annual aerial surveys of southern right whales have been conducted off the southern Australian coast, between Cape Leeuwin (W.A.) and Ceduna (S.A.) over a 28 year period between 1993 and 2020, to monitor the recovery of this species following commercial whaling. We conducted an aerial survey of southern right whales between the 20th and 24th August 2020, to continue these annual series of surveys and inform the long-term population trend. The comparable count for the 2020 survey utilised the maximum count for each leg and incorporated a correction for the unsurveyed area between Head of the Bight to Ceduna due to the inability to cover whole survey as a result of COVID-19 restrictions between State borders. This resulted in 384 individuals, consisting of 156 cows accompanied by calves of the year and 72 unaccompanied adults. Of these, 126 images of individual whales have been selected for photo-identification matching. This is a significant decrease in overall sightings that has not been observed for over 13 years when compared to long term trend data for the population; last seen in 2007 (N = 286 individuals). The subsequent population estimate for the Australian ‘western’ subpopulation is 2,585 whales, which is also a significant decrease in estimated population size from 3,164 in 2019 to 2,585 in 2020. The extremely low number of unaccompanied adults (N = 68) had the greatest impact on the overall number of sightings in 2020, and is the lowest number sighted since 1993 (N = 47). Previous surveys in 2007 and 2015 have been noted as years of low whale counts that had been deemed anomalous years, although the low numbers from this survey questions this and may suggest the 3-year female breeding cycle is becoming more unpredictable. Considerable inter-annual variation in whale numbers, and cycles in population growth, makes it difficult to detect consistent and reliable changes in abundance from one year to the next, or even over longer periods of time. This severely inhibits our ability to identify immediate threats to the population and strongly supports continued annual population surveys.

1993至2020年的28年间，研究人员在澳大利亚南部海岸李绿园角（Cape Leeuwin，西澳）至塞杜纳（Ceduna，南澳）海域开展南露脊鲸（southern right whales）年度航空调查，以监测该物种在商业捕鲸活动后的种群恢复情况。2020年8月20日至24日，我们再次开展南露脊鲸航空调查，以延续该年度调查序列，并为长期种群趋势研究提供数据支撑。本次2020年调查的可比计数采用各航段的最大观测数，并针对因跨州边境新冠疫情限制无法完成全程调查的大望角海岬（Head of the Bight）至塞杜纳之间的未覆盖海域进行了校正。最终统计得到384头个体，其中包括156头带幼崽的雌鲸以及72头单独成年鲸。其中已选取126头个体的鲸类图像用于照片识别（photo-identification）匹配。与该种群的长期趋势数据相比，2020年的总目击数量出现了13年来未出现过的显著下降——上一次出现此类情况是在2007年，当时统计个体数为286头。澳大利亚‘西部’亚种群的后续种群估计值为2585头，这同样较2019年的3164头出现了显著的种群规模下降。单独成年鲸的数量极低（仅68头），对2020年的总目击数量造成了最大影响，这也是自1993年以来目击数量最低的一次（1993年为47头）。以往2007年和2015年的调查被认为是鲸类目击数量偏低的异常年份，但本次调查的低数量数据对此提出了质疑，或表明雌性鲸类3年的繁殖周期正变得愈发不可预测。鲸类种群数量存在显著的年际波动，且种群增长存在周期性规律，这使得我们难以在单年度甚至更长时间段内检测到种群丰度的一致且可靠的变化。这极大地限制了我们识别该种群当前面临威胁的能力，也有力支持了继续开展年度种群调查的必要性。