Q&A: How might fishing be impacting the carbon cycle?

Evidence is starting to build that fishing affects the way the ocean takes up carbon from the atmosphere, affecting climate change.

The ocean is part of the global ‘carbon cycle’, which shifts carbon between reservoirs including plants, soil, water bodies, and the atmosphere. The ocean is largely a ‘sink’ of carbon, drawing it out of the atmosphere and reducing levels of carbon dioxide, which affect global warming.

However, there are many ways the ocean’s carbon sinking powers can be disrupted, and the possibility that fishing is causing significant impacts has recently been in the research spotlight.

Dr Emma Cavan, from the Department of Life Sciences at Imperial, and Dr Simeon Hill, from the British Antarctic Survey, have just published a new paper in Global Change Biology that identified some key regions where fishing may be having the greatest impact on the ocean’s carbon sink.

We speak to them about the latest thinking and what needs to be done to find out just how large a problem fishing might be – and how the problem can be addressed.

You can listen to the full interview or read their answers below (edited for length and clarity).

What part of the oceanic carbon sink have you been studying?

EC: Most carbon is stored in soil, both on land and also in marine sediments. The carbon can get into these marine sediments through things like plankton – tiny single-celled plants and algae that photosynthesise in the surface ocean and trap atmospheric CO2, which then sinks down to the deep ocean and gets buried in the deep ocean or sediments.

This carbon is really important in making sure that we get carbon dioxide out of the atmosphere and stored away, and so anything that disrupts that cycle is not helping us as we try and maintain a normal, stable atmosphere.

How might fishing impact this process?

SH: Fishing is essentially an operation that removes large amounts of biomass – living organisms that perform valuable functions within the seas. One of those functions is to play a role in these carbon cycles.

What happens is that is that fish and other marine organisms eat food, some of which is made out of phytoplankton. Some of this food is used for growth and maintenance, and some of it is egested – passed out of the body as faeces. That faeces sinks to the bottom of the ocean and if the carbon it contains reaches the seabed it has a high chance of being kept out of the atmosphere for a long period of time.

So simply by removing biomass of fish, fishing could be disrupting this process, reducing its efficiency and reducing the amount of carbon that's being stored on the seabed.

Fishing also disturbs marine ecosystems in various ways and what we did in our study was to explore the potential routes of disturbance.

What did your new study look at?

EC: We explored where there could be a spatial overlap of fishing practices and this plankton carbon sink. We used freely satellite global data of both fishing intensity and a global picture of what the carbon sink is like.

Two important ecosystem services – fish for food and carbon to maintain a natural carbon cycle – are happening in the same place, so any negative impacts of fishing could be really damaging to this carbon sink. Dr Emma Cavan

This is from satellite data on surface chlorophyll – the green pigment in plants that you can see from satellites – and an algorithm which predicts how much of that chlorophyll or that phytoplankton sinking out of the surface waters.

What we found is that as we expected, both fishing and this plankton carbon sink are really high near the coasts and on shallow continental shelves, particularly around the UK and Europe.

We expected to find this because both fishing and the carbon sink are dependent on plankton, so this wasn't ‘wow we found this crazy amazing new thing that we never expected’, it was really just highlighting that these two important ecosystem services – fish for food and carbon to maintain a natural carbon cycle – are happening in the same place, so any negative impacts of fishing could be really damaging to this carbon sink.

What needs to happen next – how can we know exactly what impact fishing is having?

SH: Our study shows that there's a great deal of overlap between these two ecosystems services, but what we haven't been able to do at present is to quantify the extent of the impact of fishing. The next step is to identify whether this overlap will lead to significant impacts on the carbon cycle.

EC: A lot of research needs to happen in this space. It's really genuinely only in the last 12 months that these studies similar to ours have been published linking the carbon cycle and fishing practices and there's a lot of policy needs for this information.

We've already had different NGOs, charity organisations as well as policymakers both in Europe and in the UK being in contact to ask for this information. We need to work with the scientists and policymakers to get this information.

If fishing is shown to be significantly disrupting the ocean’s ability to take in carbon from the atmosphere, what can be done about it?

SH: We do need more information to quantify the extent of this problem, but there are some fairly obvious solutions, one of which is to rebuild overexploited stocks. Where fishing has driven fish populations down to very low levels, we need to allow them to recover. Another possible outcome of this type of research is to highlight hot spots of carbon sequestration in the ocean and afford them some special protected status.

EC: The kind of policies that could be in place are things like changing fishing practices, such as moving away from things like bottom trawling, where huge, weighted nets are trawled along the sea floor, which resuspends a lot the carbon. That's another way that fishing can be really damaging, not just removing biomass but also disturbing sediments that are rich in carbon.

There are also marine protected areas (MPAs), where it can be easier for politicians to designate an area of ocean as a protected area, and on paper it can be seen as X percentage of our seas and territorial waters are protected, but actually patrolling that and making sure there's no fishing is much harder.

So now NGOs are really excited about this research because they can say to politicians that if protecting marine life from overfishing and protecting dolphins from bycatch isn't enough of an incentive to make these policies actually mean anything, perhaps reaching net zero and leaving carbon locked in these sediments will be a good motive to do it.

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‘Commercial fishery disturbance of the global ocean biological carbon sink’ by Cavan, E. L & Hill, S. L is published in Global Change Biology.