The North Sea is one of the most heavily used shelf regions worldwide with a diversity of human impacts, including shipping, pollution, fisheries, and offshore constructions. These stressors on the environment can have consequences for marine organisms, such as our study species, the harbor porpoise (Phocoena phocoena), which is regarded as a sentinel species and hence has a high conservation priority in the European Union (EU). As EU member states are obliged to monitor the population status, the present study aims to estimate trends in absolute harbor porpoise abundance in the German North Sea based on almost two decades of aerial surveys (2002–2019) using line-transect methodology. Furthermore, we were interested in trends in three Natura2000 Special Areas of Conservation (SACs), which include the harbor porpoise as designated feature. Trends were estimated for each SAC and two seasons (spring and summer) as well as the complete area of the German North Sea. For the trend analysis we applied a Bayesian framework to a series of replicated visual surveys, allowing to propagate the error structure of the original abundance estimates to the final trend estimate and designed to deal with spatio-temporal heterogeneity and other sources of uncertainty. In general, harbor porpoise abundance decreased in northern areas and increased in the south, such as in the SAC Borkum Reef Ground. A particularly strong decline with a high probability (94.9%) was detected in the core area and main reproduction site in summer, the SAC Sylt Outer Reef (−3.79% per year). The overall trend for the German North Sea revealed a decrease in harbor porpoise abundance over the whole study period (−1.79% per year) with high probability (95.1%). The assessment of these trends in abundance based on systematic monitoring should now form the basis for adaptive management, especially in the SAC Sylt Outer Reef, where the underlying causes and drivers for the large decline remain unknown and deserve further investigation, also in a regional North Sea wide context.

中文翻译：

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人类高利用区的小型鲸类动物：过去二十年北海港湾鼠海豚数量的趋势

北海是世界上使用最频繁的陆架区域之一，人类影响多种多样，包括航运、污染、渔业和近海建设。这些环境压力源会对海洋生物产生影响，例如我们的研究物种港湾鼠海豚 (Phocoena phocoena)，它被视为哨兵物种，因此在欧盟 (EU) 中具有高度的保护优先权。由于欧盟成员国有义务监测种群状况，本研究旨在基于近 20 年的航空调查（2002-2019 年）使用线断面方法估计德国北海港湾鼠海豚绝对数量的趋势。此外，我们对三个 Natura2000 特别保护区 (SAC) 的趋势很感兴趣，其中包括作为指定特征的港湾鼠海豚。对每个 SAC 和两个季节（春季和夏季）以及德国北海的整个区域的趋势进行了估计。对于趋势分析，我们将贝叶斯框架应用于一系列重复的视觉调查，允许将原始丰度估计的误差结构传播到最终趋势估计，并旨在处理时空异质性和其他不确定性来源。总的来说，北部地区的港湾鼠海豚数量减少，南部地区增加，例如 SAC Borkum Reef Ground。在夏季的核心区域和主要繁殖地点，SAC Sylt 外礁（每年 -3.79%）检测到特别强烈的下降（94.9%）。德国北海的总体趋势表明，在整个研究期间 (-1. 79% 每年），概率很高（95.1%）。基于系统监测对这些丰度趋势的评估现在应该成为适应性管理的基础，特别是在 SAC Sylt 外礁，大量下降的根本原因和驱动因素仍然未知，值得进一步调查，同样在北部区域海阔环境。