Relatively little is known about the distribution and diversity of marine mammals around offshore anthropogenic structures. We present results obtained from incidental sightings of marine mammals around oil and gas installations located 200 km off the Danish coast. A total of 131 sightings corresponding to about 288 animals were reported between May 2013 and May 2016. A total of seven marine mammal species were identified, five cetaceans: harbour porpoise (Phocoena phocoena), minke whale (Balaenoptera acutorostrata), white-beaked dolphin (Lagenorhynchus albirostris), killer whale (Orcinus orca), pilot whales (Globicephala spp.) and two species of pinnipeds: harbour (Phoca vitulina) and grey seals (Halichoerus grypus). The most sighted species were harbour porpoise (41%) and minke whale (31%). Relative counts and biodiversity of marine mammals observed around installations corresponded well with the expected distribution in the central North Sea. Several taxon-specific correlations were identified between number of sightings and environmental parameters (depth and latitude) or installation characteristics (installation aerial footprint). Furthermore, 85% of sightings were made during spring and summer and it is unclear whether the pattern observed reflected a natural seasonal occurrence of marine mammals in the area or an effect of reduced effort during autumn and winter. Despite the potential caveats, results obtained during this programme provide an insight into the relationship between marine mammals and oil and gas offshore installations in the North Sea.
The Nord Stream Pipeline (a two-pipeline system) from Russia to Germany through the Baltic Sea was constructed during 2010–2012. In Swedish waters, subsea post-lay trenching by ploughing was carried out at minimum 4 km distance to two Natura 2000 areas1, which was considered by the environmental authorities to be relatively close. The Swedish construction permit required that the excess Suspended Sediment Concentration (SSC; often the indirect measure turbidity is used) caused by sediment spill as a result of the construction works (particularly post-lay trenching by ploughing) never exceeded 15 mg/l above natural background at the borders of the Natura 2000 areas. Because natural turbidity levels were not well-described in this area, a monitoring programme combining passive, real-time turbidity monitoring and active, vessel-based spill monitoring combined with a feedback mechanism was established. The method proved successful in documenting compliance with permit requirements with regard to threshold limits for excess SSC caused by construction activities during construction of Line 1. This ensured that trenching activities could be completed as planned, and it also enabled a significant reduction in the scope of the monitoring programme for Line 2. The monitoring was also successful in verifying that numerical modelling of sediment spreading carried out as part of the pre-construction impact assessment was correct. Finally, the new knowledge on spill rates acquired from the vessel-based monitoring will enable a more accurate assessment of the environmental impact of future subsea ploughing operations.
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