Tuesday, 18 September 2012

Field work in Angola: Part 1

So, after three weeks away to the west coast of Africa, I'm back to tell the tale! But before I launch into everything that we did out there, I'd better give you a bit of background on why we went.

Part of my PhD work is looking at the effects of deep-water oil production on the behaviour and composition of fish communities over time. While shalllow-water effects have now been studied in depth for a number of years, there is virtually no data available on the effects that oil production in the deep sea (>1000m) may have on the benthic (seabed) fauna, particularly on mobile species such as fish.

In shallow waters, oil rigs are known to provide artificial habitat for a variety of fish species, and the abundance of fish at such oil platforms can be far higher than in the surrounding waters. This knowledge has led to programmes such as 'Rigs-to-Reefs' in the US which convert decommissioned oil platforms into man-made reefs to provide artificial habitat for fish and invertebrates as well as protection from fishing activities such as trawling. Many shallow-water fish species are attracted to vertical structures and will use such open, complex structures to hide from predators or as feeding grounds once the reefs become colonised by invertebrate life. Even while they are operational, static platforms still provide hard substrate for invertebrates to live on and for mobile animals to live in and around.

Fish attracted to a decommissioned oil rig.
Image credit: http://www.guyharveymagazine.com/

However, the role of oil platforms in the deep-sea and their potential for providing artificial reef habitats are as yet unknown. While we might expect the effects to be similar in deep water as they are in shallow water we need to consider a couple of important differences in the behaviour of the fish themselves and the type of oil production structures present.

In the deep-sea, there is relatively little hard substrate; the majority of the seabed is comprised of soft sediments like muds and clays, and is typically rather featureless, particularly in the areas targeted for deep-water oil production fields. Consequently, we could expect the fish species that live in these areas to have relatively low affinity for hard substrates since natural reef structures are rare, and therefore that the presence of any reef structure would have relatively little effect on aggregating those species without some other attraction (e.g. a good food source).
Secondly, oil production structures in deep water are physically very different to those in shallow waters, and tend to be far more spread out over the seabed. The oil wells themselves are capped by wellheads which are a few metres in diameter, and which connect via pipelines to a central 'hub' which in turn connects a flexible pipeline to a floating surface production vessel. This means that the actual amount of 'reef space' provided by the structures is relatively low compared to shallow, static rigs and it is therefore unclear how much of an effect this might have on local fish populations.

An example of a deep-sea oil field with surface production vessels.
Image credit: http://angolarising.blogspot.co.uk/2011/11/big-oil.html

This is where I come in. As a part of my PhD, I'm using data collected by two seafloor observatory platforms situated off the west coast of Angola in 1400m of seawater and installed by BP through the DELOS project. One of the platforms sits around 50m from an active oil well (the 'Near Field' platform) while the other is located approximately 15km away from any production activity (the 'Far Field' platform) and acts as a control site. The platforms each contain a number of 'modules' which hold time-lapse cameras, environmental sensors, active and passive sonar sensors and a sediment trap at the far-field and we can use these to investigate how the environment varies naturally over time at each site and whether we can see any effects of either the oil production structures or natural environmental change on the fish communities present.

The DELOS platform frame (image from www.delos-project.org)

An ROV conducting an inspection of one of the DELOS platforms

We've been collecting data from these observatories since February 2009, and although there were some early teething problems which needed to be resolved, we've now got a very nice dataset covering three years from an area of the world about which very little is known about the marine environment at all, never mind the deep sea! However, while the platforms are designed to operate autonomously for the most part, they do need to be serviced each year for routine maintenance (swapping batteries etc.) and to download the data that's been collected over the previous 12 months. This all means that once a year, technicians from Oceanlab (University of Aberdeen) have to head south to the Angolan capital city of Luanda, and then offshore via helicopter to the ROV servicing vessel, work around the clock to get everything done in as short a time as possible, reinstall the modules into the observatories and head back home again with all the data.

Sounds pretty straightforward doesn't it?

Well, this year I was finally able to join the technical team and head down there myself to see the equipment that's been feeding me data for the last two years and learn how it all gets done! It wasn't quite as smooth sailing as we'd anticipated though...

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