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No. 398: Harbour porpoises and the construction of Nord Stream 2 gas pipeline

Tougaard, Jakob and Griffiths, Emily T. 2020. Harbour porpoises and the construction of Nord Stream 2 gas pipeline. Assessment of the impact on porpoises in the Natura2000 Hoburg’s Bank and Midsjöbanks, Swedish Baltic. Aarhus University, DCE – Danish Centre for Environment and Energy, 29 pp. Scientific Report No. 398 http://dce2.au.dk/pub/SR398.pdf



Nord Stream 2 recently constructed a gas pipeline through the Swedish part of the Baltic Sea. During the environmental impact assessment (prior to construction) particular focus was paid to the possible negative impacts on harbour porpoises. Harbour porpoises in the Baltic Proper are distinct from porpoises in the Western Baltic; the population size is very low and the status is assessed as critically endangered. In the summer months, the Baltic Proper population of porpoises aggregate in the Natura2000 area Hoburg’s Bank and Midsjöbanks likely for breeding and nursing, which designates it as critical habitat. The pipeline route discussed in this report was constructed through this area.

In the impact assessment, it was clear that underwater noise was the only real concern with respect to impact on porpoises. The most significant sources of underwater noise were considered to be the pipe laying vessel and support ships, and particularly noisy single activities including rock placement before the pipe is laid down and trenching of the pipe into the sediment.

Based on measurements performed during previous construction works, including the Nord Stream pipeline in 2012, it was predicted that noise emissions from the pipe laying vessel, as well as from rock placement and trenching, would be comparable to or lower than the noise from commercial vessels also using the habitat area. The main difference to the commercial ships is the slower speed of the pipe laying vessel and trenching plough.

Based on these assumptions, together with precautionary assumptions on reactions of porpoises to ships, the temporary habitat loss caused by construction of the pipeline was modelled. The results showed that the construction would constitute a significant increase of the habitat loss caused by the commercial ships in the area. However, the absolute magnitude of the habitat loss was predicted to be very low, therefore leading to the conclusion of the impact assessment that the Nord Stream 2 pipeline could be constructed without affecting the Baltic Proper porpoise population or the integrity of the Natura2000 area. The measurements underlying this conclusion came from surveys performed during the construction of the Nord Stream pipeline and these recordings were limited in frequency range to only the very low frequencies, where porpoises have poor hearing. To close this gap in knowledge, an ambitious monitoring program was designed in dialog with the Swedish authorities, which obtained full bandwidth recordings of both ambient noise and construction related noise during construction of Nord Stream 2 through the Natura2000 area. In addition to measuring underwater noise the presence of harbour porpoises was also monitored by deployment of passive acoustic detectors (C-PODs) in different distances from the pipeline route.

The detailed results of this monitoring program is presented in the monitoring report (Stöber et al., 2020), but several conclusions can be drawn from the results, with implications for evaluating the actual impact on porpoises due to construction activities. These are summarized as: 

  • Underwater noise radiated from the pipe laying vessel (Solitaire) and support ships was as expected comparable in level and frequency content to noise radiated from commercial cargo ships in the area.
  • Underwater noise from rock placement was likewise lower than or comparable to ship noise and with comparable frequency spectrum.
  • These observations support the precautionary assumption used in the impact assessment that porpoises would not react to the pipe laying vessel, support ships, rock placement and trenching beyond 1 km from the vessel.
  • Pipe laying operation during Nord Stream 2 construction was faster than during construction of Nord Stream, which was used as input to assessment of impact. This means that the time spent inside the Natura2000 area was lower for construction of Nord Stream 2 than what was assumed in the impact assessment.
  • These results lead to the conclusion that the actual temporary habitat loss due to pipe line construction was not larger than what was anticipated in the impact assessment and most likely smaller.
  • This conclusion was supported by rerunning the habitat loss model used for the impact assessment with the traffic data from the actual construction of Nord Stream 2.
  • Harbour porpoises were detected at low rates throughout the monitoring program, which extended from before the pipelaying vessel arrived to the Natura2000 area until after it left. The low detection rate was anticipated based on the very low density of porpoises in the Baltic Proper and indicate that porpoises did use the Natura2000 area also when construction took place.
  • Although detection rates are too low to allow for any kind of robust statistical analysis, these detections are consistent with the low level of disturbance anticipated in the impact assessment.
  • Therefore, nothing in the results of the monitoring program indicate that porpoises were adversely affected beyond the low and negligible impact anticipated in the impact assessment. 

Although this may seem as a vague non-conclusion, given the very low abundance of porpoises in the central Baltic Sea and challenging conditions for field work, this is a significant achievement, both practically and analytically. The measurements obtained will also remain a valuable addition not only to the knowledge about impact of pipeline construction on harbour porpoises, but also on the general acoustic soundscape of the central Baltic Sea.