Small Cetacean in a Human High-Use Area: Trends in Harbor Porpoise Abundance in the North Sea Over Two Decades

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.

Predicting Harbor Porpoise Strandings Based on Near-Shore Sightings Indicates Elevated Temporal Mortality Rates

“…The rapid growth in the porpoise sighting and stranding rate in the southern North Sea has been reported extensively (Camphuysen, 2004;Hammond et al, 2013;Gilles et al, 2016;IJsseldijk et al, 2020;Nachtsheim et al, 2021). It has been suggested that this increase is linked to North Sea wide changes in prey distribution and abundance, although currently solid evidence is lacking (Camphuysen, 2004;Hammond et al, 2013).…”

“…Apparently, porpoises move away from the nearshore regions out of sight from the Seabird watchers, who detect porpoises predominantly within 2 km from shore. These porpoises possibly move to nearby offshore waters (e.g., Geelhoed and Scheidat, 2018) or elsewhere (e.g., toward German and Danish waters) to give birth and nurse (Gilles et al, 2009;IJsseldijk et al, 2020;Nachtsheim et al, 2021). Porpoise coastal sighting rates slowly increases from August and into autumn.…”

“…This comes with the need to assess their abundance and distribution. For cetaceans, population assessment is mainly achieved through ship-based or aerial line-transect surveys (Hammond et al, 2002;Scheidat et al, 2012;Geelhoed and Scheidat, 2018;Nachtsheim et al, 2021). Disadvantages of linetransect surveys are that they are costly, logistically challenging and often only sample a very small proportion of the population, despite a considerable effort.…”

“…Elevated mortality could be a result of increased threats and pressures that negatively affect the population. The North Sea is a hotspot of anthropogenic activities, where many anthropogenic stressors occur and overlap (Halpern et al, 2008(Halpern et al, , 2015Nachtsheim et al, 2021). Harbor porpoises in the North Sea are impacted by fishery activities (bycatch and competition) (Kirkwood et al, 1997;Leeney et al, 2008;IJsseldijk et al, 2018c), chemical pollution (Pierce et al, 2008;Weijs et al, 2009;Jepson et al, 2016), noise pollution from shipping (Wisniewska et al, 2018), seismic surveys and underwater explosions (von Benda-Beckmann et al, 2015;Aarts et al, 2016), and more recently, habitat loss due to the rapid growth of offshore activities related to the construction of wind farms (Madsen et al, 2006;Gilles et al, 2009).…”

Return of the Salish Sea Harbor Porpoise, Phocoena phocoena: Knowledge Gaps, Current Research, and What We Need to Do to Protect Their Future

“…Urbanization of the Salish Sea is likely one of the greatest contemporary threats to harbor porpoise in this region, as is the case in other areas. For example, declines in harbor porpoise abundance in parts of the well-studied North Sea over the last 20 years have been correlated with human highuse areas including activities like shipping, pollution, fisheries and offshore construction (Nachtsheim et al, 2021). Similarly, associated with urbanization of the Salish Sea is a myriad of vessel types ranging from recreational traffic, to essential services like ferries and the United States and Canadian Coast Guards, to the commercial economic livelihood sector including fishing, tourism (e.g., wildlife viewing and cruise ships), transportation of goods by ocean-going vessels, tug boats and barges, United States and Canadian national border services, research vessels, specialty services vessels (e.g., cable laying etc.)…”