June 5, 2017


This article summarizes research published on June 5, 2017 in the Proceedings of the National Academy of Sciences.  It represents the most recent science-based thinking on the value of Marine Protected Areas.



By Dr. Callum Roberts and Dr. Beth O’Leary

Environment Department, University of York, UK; callum.roberts@york.ac.uk

It is hard not to be at least a little pessimistic about the future. Regardless of what cuts in greenhouse gas emissions we make today, the global temperature will rise at least another half a degree centigrade, and sea levels will rise a couple of metres or more over the next 200 years. Ocean acidification, from dissolved carbon dioxide emissions, is changing ocean chemistry in ways that will strain the ability of creatures to grow chalky shells and skeletons for thousands of years to come.

Surface waters of the oceans have warmed by nearly one degree centigrade since pre-industrial times and there is more to come. Warming is reducing ocean productivity. It slows mixing between a warm surface layer of water and colder water below, starving the surface layer of nutrients necessary for plant growth, and deeper waters of life-sustaining oxygen. Warming is leading to a global diaspora of fish and other marine life as they colonise newly favourable regions and abandon those where conditions have soured. Taken together, these effects will likely lead to lower fish catches, especially for countries in tropical and warm-temperate regions. As if this were not enough, rising seas also imperil some of the world’s most densely populated lands, and most productive agricultural regions, around low-lying river deltas. Increasing human needs will soon collide with falling supplies.

It is easy to feel powerless and despondent in the face of such threats, particularly knowing the political roadblocks that need to be cleared for effective action to reduce greenhouse gas emissions. But there is something that we can do to steer a safer course into the future while politicians and diplomats battle. History holds the key.

Over the last two hundred years, industrial fishing has spread across the global ocean, reaching its remotest corners and to thousands of metres deep. The story of fishing has been repeated all over: large, high value and easy to catch animals are targeted first. As they are depleted, fishing switches to other species, pursued using progressively more active, intensive and destructive fishing methods. In places exploited for long periods ecosystems have been stripped of larger predators, species like sharks, billfish, cod and halibut, leaving behind smaller, more resilient creatures like prawns and crabs. Many species have declined more than 90% in abundance, rendering fisheries for them far less productive. Furthermore, in the process of switching from hook-and-line and net, to trawl and dredge, complex and highly diverse seabed habitats built by shellfish, seaweeds and corals were converted to rubble, sand and mud.

It is clear from these trends that we have already lost a great deal of fisheries productivity through mismanagement. The scale of these losses eclipses the likely downturn from future climate change stress. This means that by reversing the declines, which we can do by protecting places in marine reserves and better managing exploitation outside them, will go a long way to mitigating climate change impacts (Roberts et al. 2017).

Marine reserves are places that are protected from all fishing. Within them, species increase in abundance and size. Big animals produce many more offspring than small animals – they are the engines of reproduction and replenishment. Their offspring drift as eggs and larvae on ocean currents, spreading 10s or even 100+ kilometres to reseed surrounding fishing grounds. As protected populations grow, animals seek less crowded conditions elsewhere, spilling over into unprotected areas where they can be caught. Spared the destructive effects of industrial fishing gears and other damaging human impacts, marine reserves promote habitat recovery, which leads to higher productivity and greater population sizes.

It is through these processes of increased reproduction, habitat recovery, export of offspring and spillover – the reversal of historical declines – that marine reserves can mitigate some of the impacts of climate change. And through a quirk of physiology – fish produce highly soluble carbonate granules in their guts – more abundant fish may also help buffer rising ocean acidity in shallow seas.

There is more. Recent research is revealing the enormous value of intact coastal wetlands like mangrove forests, salt marshes and seagrass beds. Long-maligned as fly infested swamps, they have often been valued only as places to clear or develop. There has been massive loss of these coastal wetlands in the last hundred years, especially in Asia and Central and South America where thousands of square kilometres have been converted to aquaculture ponds and other uses. Globally, approximately one third of mangroves have been lost in the last 50 years, and about 30% of seagrass beds since the late 19th century. The Philippines has lost 80% of its mangroves.

Healthy wetlands, it turns out, are vigorous carbon sinks, taking up atmospheric carbon dioxide and storing it in deep, muddy sediments. They can grow upward as sea levels rise, helping protect vulnerable low-lying coasts. This function may be particularly important as a warmer world is expected to experience more extreme weather. Protection and restoration of coastal wetlands in marine reserves should help mitigate climate change and help local communities adapt to its effects.

It is apparent that a monumental environmental upheaval is underway as we enter the Anthropocene. Marine reserves will not halt climate change nor prevent harm from transboundary threats such as pollution. They must be established alongside other solutions, like restrictions on fishing effort and methods, and better management of runoff from land. While reserves have many benefits at local scales, they must also be scaled up to make a meaningful contribution to offsetting climate change impacts at a global level. The Sustainable Development Goal target of 10% protection by 2020, while ambitious in timing, is only a waypoint, not the endpoint, on the road to effective ocean management. Scientific research argues in favour of a much higher target (O’Leary et al. 2016), with figures of 30% by 2030, or even 50% by 2050, justified by the evidence. We must therefore also extend protection to areas beyond national jurisdiction, which comes with its own difficulties.

Marine reserves are not a panacea nor are they a substitute for urgent reductions in greenhouse gas emissions. They have limitations that must be addressed through further research and creativity, such as how to better resource them, improve monitoring, surveillance and enforcement, and achieve better social outcomes. Nor can they adequately protect all ecosystems, some of which appear to be much more sensitive than others. But given their broad portfolio of positive outcomes, it is hard to conceive of any circumstances under which extensive networks of well-resourced and managed, highly protected marine reserves would not increase future human wellbeing. They represent a positive, proactive step that local communities, regions, nations and the international community can pursue to build resilience and soften the blows that lie ahead.

Acknowledgements: Beth O’Leary is supported by the Pew Charitable Trusts.

Literature cited:

Roberts, C.M., B.C. O’Leary, D.J. McCauley, P.M. Cury, C.M. Duarte, J. Lubchenco, D. Pauly, A. Saenz-Arroyo, U.R. Sumaila, R.W. Wilson, B. Worm and J.C. Castilla (2017) Marine reserves can mitigate and promote adaptation to climate change. Proceedings of the National Academy of Sciences. www.pnas.org/cgi/doi/10.1073/pnas.1701262114

O’Leary, B.C., M. Winther-Janson, J.M. Bainbridge, J. Aitken, J.P. Hawkins and C.M. Roberts (2016) Effective coverage targets for ocean protection. Conservation Letters. doi: 0.1111/conl.12247