Seals in the Elbe estuary from Wedel to Cuxhaven 2018/2019

Aerial surveys were conducted in the Elbe Estuary, Germany, to count harbour seals on their haul-out sites. Haul-out sites were located within the intertidal zone, usually close to the waterway or Tidal creeks. The north-west border of the study area was located at river-km 728 (city of Cuxhaven) and the south-east border was located at river-km 632 (Mühlenberger Loch). Flights were performed monthly from August 2018, until July 2019 (12 lights in total) at low tide. Within each month, a flight date in the middle of the month what chosen, depending on the tide and weather conditions. Flights were performed with a twin-engined aircraft at a height of approximate 600 feet and at an average speed of 100 knots. The flight routes were tracked via GPS. Sighted harbour seals were photographed on their haul-out sites and the responding geographical position was recorded. In the office, the photos were used to count the animals. In the beginning of the Pupping season in June, adult and juvenile individuals were counted separately. The sizes of individual seals were used to distinguish between both age classes. Later in the year, juvenile seals could no longer visually be distmented from adult individuals due to the PUPS’s rapid growth. Consequently, count data of juvenile harbour seals are only available for June 2019.本研究在德国易北河口（Elbe Estuary）开展航空调查，以统计港海豹（harbour seal）在其休憩位点（haul-out site）的种群数量。休憩位点均位于潮间带（intertidal zone），通常临近航道或潮汐水道。研究区域的西北边界位于易北河728公里处（库克斯港（Cuxhaven）市域范围内），东南边界位于632公里处（米伦贝格 Loch（Mühlenberger Loch））。调查飞行于2018年8月至2019年7月每月开展1次，总计12架次（原文疑似将flights笔误写为lights），所有飞行均在低潮时段进行。每月选取月中某一日期开展飞行任务，具体日期需结合潮汐与天气条件确定。本次飞行采用双引擎飞机执行，飞行高度约600英尺，平均航速为100节（knot）。飞行航线通过全球定位系统（GPS）进行追踪。在休憩位点观测到的港海豹会被拍摄记录，同时同步记录对应地理位置。后续在室内通过航拍照片完成个体数量统计。每年6月的产仔季初期，会分别统计成年与幼年港海豹的数量，本次调查通过个体体型差异区分这两个年龄组。当年后续时段，由于幼崽快速生长，无法再通过视觉区分幼年与成年港海豹，因此幼年港海豹的统计数据仅在2019年6月可获取。