Abstract
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.
Highlights
The harbor porpoise (Phocoena phocoena) is the smallest and by far most common cetacean species in the North Sea, including German waters (Benke et al, 1998; Siebert et al, 2006a; Hammond et al, 2013)
Our evaluation presented here can be regarded reliable in the sense of (i) being based on adequate data collected over a sufficiently long-time frame and (ii) resulting from an analysis designed to deal with spatio-temporal heterogeneity and other sources of uncertainty (Johnson and Fritz, 2014)
Harbor porpoise abundance in summer has decreased in the German Exclusive Economic Zone (EEZ) between 2002 and 2019
Summary
The harbor porpoise (Phocoena phocoena) is the smallest and by far most common cetacean species in the North Sea, including German waters (Benke et al, 1998; Siebert et al, 2006a; Hammond et al, 2013). Being described as a continental shelf species, it is distributed throughout the shallow North Sea, from coastal waters up to the Dogger Bank in the central North Sea (Gilles et al, 2009, 2011, 2016). In the North Sea, they are usually encountered solitary or in small groups (Benke et al, 1998; Gilles et al, 2009; Hammond et al, 2013). The North-East Atlantic population which includes the North Sea, seems to have lower pregnancy rates (Pierce et al, 2008; Learmonth et al, 2014; Murphy et al, 2015)
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