Marine mammals and man-made structures

Recent research has demonstrated that harbour (Phoca vitulina) and grey (Halichoerus grypus) seals target wind turbines and pipelines as foraging locations (Russell et al. (2014); This research is in agreement with Ocean Science Consulting’s (OSC) on going rigs-to-reefs research around offshore installations in the North Sea. Results so far have indicated that harbour porpoises are likely foraging around these structures (Todd et al. (2009);, and new data being collected currently, appears to support this theory.

Given her expertise in this area, OSC’s Dr Victoria Todd was asked by New Scientist to comment on the recent publication (, but we have expanded on those thoughts here.

  • It is unsurprising that seals target offshore windfarms or pipelines as foraging locations, because these structures concentrate food as part of the ‘reef effect’. During a decade of research, studying harbour porpoise (and other cetacean) acoustics and behaviour around offshore installations (Todd (2013); we have observed seals feeding regularly, mostly on flatfish, and using any haul-out opportunities that offshore structures may present (e.g. rig and platform legs), around remote oil and gas rigs and platforms, mostly in the central North Sea (i.e. as far from land as is possible in the North Sea), but also on drilling rigs located near to Alpha Ventus. The animals are likely not just targeting windfarms, pipelines and met masts, but any offshore structures that aggregate fish. As part of our latest research (forming part of an undergraduate thesis), ROV data also show seals swimming along subsea gas pipelines, apparently using these features like a ‘marine mammal highway’, and picking off nearby fish periodically;
  • High-resolution data on fine scale movement and activity patterns of individual animals around offshore man-made structures is required urgently, so this paper is a step in the right direction, although collaboration with scientists is required to put all theories to bed. The workers have the licences to tag animals that scientists working for industry such as ourselves are not able to obtain, but scientists like us have funding, permission and access to remote offshore oil and gas structures that academics have difficulty obtaining;
  • The authors note that we are in a ‘time of unprecedented developments in marine infrastructure’, but this isn’t true. Oil and gas installations have been around for fifty years (since 1964 in the North Sea). Indeed the authors cite ‘The windfarms considered here were new, and prevalence of such behaviour may increase with time, especially if the artificial reefs are not yet fully established’. Offshore oil and gas platforms are thriving, fully-established artificial reefs, hence the importance of including them in this kind of work. Moreover, in the North Sea, many of these structures are coming to the end of their operational lifetime, and thus have decommissioning implications;
  • The authors, like many academics, are careful to avoid talking about oil and gas. Everybody needs to get over this, and work with the industry, rather than against it. Today, the most prevalent man-made structures in the ocean are oil and gas related (and they are also the first marine structures to approach the end of their operational lifetime), so they cannot be ignored;
  • With regards to implications of such behaviour, if animals are targeting man-made structures, then routine population estimates need to account for this grouped behaviour in their density and abundance estimation methods, or animals may be missed, potentially resulting in inaccurate population estimates. There are a plethora of other implications, such as long-term exposure to noise generated from these structures, or other emissions, but pollution from man-made structures in the North Sea is negligible; and
  • Whether man-made structures can compensate for the loss of habitat is complicated. For marine mammals threatened with unsustainable rates of fisheries bycatch, pollutant accumulation and starvation due to overfishing of their prey species, yes, artificial reefs more than likely offer alternative high-quality foraging habitat (especially around oil and gas rigs and platforms, with 500 m shipping exclusion zones), but it depends on the artificial reef type, and acoustic (and other) emissions must be considered (i.e. some man-made structures produce noise, that while negligible for some marine mammals, may have effects on others). For other species, more research is required. For example, coral reefs are highly complex and mostly irreplaceable structures. In the North Sea, I believe that offshore man-made structures may well provide feeding oases for marine mammals in an otherwise overfished environment (the primary threat), and may be relied upon heavily to the extent that, if these structures were removed through decommissioning, species population declines might result.


Russell, DJF, Brasseur, SMJM, Thompson, D, Hastie, GD, Janik, VM, Aarts, G, McClintock, BT, Matthiopoulos, J, Moss, SEW, and McConnell, B (2014): Marine mammals trace anthropogenic structures at sea. Current Biology 24, R638-R639.

Todd, VLG (2013): Rigs – to decommission or not to decommission? Marine Scientist 43, 22-25.

Todd, VLG, Pearse, WD, Tregenza, NC, Lepper, PA, and Todd, IB (2009): Diel echolocation activity of harbour porpoises (Phocoena phocoena) around North Sea offshore gas installations. ICES Journal of Marine Science 66, 734 – 745.


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