Ecology Human impacts Noise Survey

Oil rigs, the food trucks of the sea?

Todd, V. L. G., Warley, J. C., & Todd, I. B. (2016). Meals on wheels? A decade of megafaunal visual and acoustic observations from offshore Oil & Gas rigs and platforms in the North and Irish Seas. PLoS ONE, 11(4). doi:10.1371/journal.pone.0153320


It isn’t exactly news that humans have greatly impacted the planet. Throughout human history, we have sought greater and (now) more sustainable sources of energy. However, new sources of energy require time and different technology to properly harness, meaning we continue to rely on older sources in the interim. Oil and natural gas are two of the sources we’re reliant on, but as reserves dwindle, drilling rigs are being dismantled and removed.

The process of dismantling ocean-based rigs can be very expensive, so some countries and companies have come to agreements where rigs can instead be sunk to create artificial reefs which benefit local species. This financial and environmental benefit, though, only makes sense if proper surveys exist, documenting the presence/absence of local species. Unfortunately, many assessments of local species are only reported in confidential client reports, and usually done to ensure the process of rig installation and operation does not irrevocably harm animals. Aside from being confidential, these surveys do not account for any upheaval the removal of the rig could cause when its well is dry. We can’t change the landscape and then change it back without expecting some type of consequence. Therefore, scientists are starting to push for increased access to platforms to conduct visual and auditory surveys of local wildlife.


Methods and Challenges

Victoria Todd and her colleagues managed to compile a dataset of visual and auditory observations taken over a decade at various platforms in the North and Irish Seas, specifically focusing on marine mammals and other megafauna (large animals). Visual and auditory monitoring occurred on board stationary oil platforms as well as floating rigs that could be moved to different locations (Fig. 1).

Figure 1: Oilrig and platform locations in the North and Irish Seas. (Taken from Todd et al. (2016) Figure 1.)
Figure 1: Oilrig and platform locations in the North and Irish Seas. (Taken from Todd et al. (2016) Figure 1.)

Visual observations were naturally difficult to collect—for one, the ocean is a big place, and trying to find a porpoise on the horizon can be as hard to find as a needle in a haystack. Second, the North Sea isn’t known for its tranquil waters; high waves and surf can often obscure an observer’s view. Third, most oilrig operators are busy with their specific tasks; their noticing local fauna is rarely ever recorded with any degree of accuracy. In this study, the most reliable observations were therefore made and recorded by trained Marine Mammal Observers (MMOs).

Auditory observations also had their fair share of difficulties. Hydrophones are incredibly useful tools, and passive acoustic monitoring (PAM) has steadily gained popularity as a means to gauge the types of species living under the waves. However, there are also limitations. An oilrig in production mode generates its own spectrum of noise, potentially drowning out sound produced by mammals. In addition, mammals whose sounds are higher in pitch would only be detected at closer range to the hydrophone (high pitch sounds fizzle out at shorter distances).

Regardless of these challenges, Todd and her crew were able to make visual observations on 45 separate days from 2004-2014, totaling 238 observation hours. Auditory observations only took place in 2014 and recorded 160 hours over the span of 12 days. Although this is a relatively minimal dataset, it is important to note this is one of the first scientific surveys like this done in the region of northern Europe.



Despite the relatively few hours of observation (compared to the total hours in a decade), the MMOs recorded 47 sightings of marine mammals while aboard rigs that were being towed to a new location (five individual tows over 25 days). Fifteen sightings were recorded while MMOs were on stationary platforms (six observation periods over 20 days, Fig. 2).

Figure 2: Photographic evidence of mammals around oilrigs (Taken from Todd et al. (2016) Figure 11.)
Figure 2: Photographic evidence of mammals around oilrigs (Taken from Todd et al. (2016) Figure 11.)

The acoustic monitoring revealed that harbor porpoises were the predominant species present, at least based on vocal clicks and whistles recorded by the hydrophones. The scientists found this interesting given that harbor porpoises are one of the smallest species of porpoise and maintain very low levels of body fat. They would need to have a plentiful food supply to warrant their continued presence so far offshore. Since the acoustic dataset was only from 2014, though, long-term conclusions could not be drawn.


Big Picture

Scientists have many hypotheses about how offshore structures like oilrigs affect the local wildlife. The precautions are already in place when it comes to their construction since the auditory shockwaves produced by pile driving can be deafening for animals that are too close to the site. However, once these structures are built and fully operational, not many studies have looked at the ecological implications, good or bad, of them being there. Structures like these can provide shelter and habitats for invertebrates and fish, which in turn can draw in marine megafauna. The permanent removal of the structures, after the resources have been removed, could be detrimental to populations that come to seasonally rely on the rigs as a food source. While Todd and her colleagues make many good points using singular examples from the dataset, they recognize that the number of observations limits the strength of their conclusions. Therefore, a logical conclusion would be to increase the amount of monitoring going on around the rigs. This isn’t always easy to do, especially if funding is minimal.

But what do you think? Do you think there needs to be more observation? And if so, do you think there are cost-effective and reliable ways to do so?

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