Ocean supplement: The science of decline

The world’s oceans comprise 1.3bn cubic kilometres of water and has a mean depth of around 4,000m. The Mariana Trench, in the western Pacific, drops down to 11,000m (6.8 miles). Fewer people have been to such a depth (three) than have walked on the moon (12) and we have better maps of Mars than of the deep seabed.

Yet scientists are starting to understand how important the ocean is as a part of the Earth’s complex environmental systems. The ocean not only supplies us with food independently of any human cultivation, but also plays a crucial role in the regulation of atmospheric gases, global temperatures, major bio-geochemical cycles and it also soaks up large amounts of manmade pollutants. It is also the medium for the transport of the goods of global trade and contains a large proportion of future oil and gas reserves—should we decide to use them.

There is currently great interest in the development of maritime economic activity, with a focus on new methods of aquaculture, deep-ocean mining and exploitation of the genetic resources within the great diversity of ocean life. We are on the brink of a new industrial revolution in the oceans. But unhappily, this comes at a time when the seas are showing signs of widespread degradation, and a corresponding decline in the services that marine ecosystems are able to provide for humankind.

Of all human activity, fishing has the greatest direct effect on the oceans. We fish across all latitudes, including into the Arctic and Antarctic seas, and the deepest fisheries go to a depth of 2,000m. The over-exploitation of marine fish stocks is a well-known and understood problem. It arises from overcapacity in fishing fleets and the poor management of fisheries where quotas, if set at all, are too high, and often exceed the recommendations of scientists. Bad subsidies such as cheap fuel also contribute to the problem. The World Bank has recently published a report entitled “Sunken Billions Revisted,” in which it estimates that the world loses $83bn every year through the mismanagement of fish stocks.

The illegal trade Illegal, unreported and unregulated (IUU) fishing is another significant problem encouraged by poor international cooperation. It is estimated that one in five fish caught is caught illegally. Because many of the fish species targeted play important roles in marine ecosystems, either as top predators, suppressing the abundance of their prey, or as important food for predators, depletion of stocks means changes in marine ecosystems. This can make them less productive and less resilient to shocks from environmental changes of the sort that are associated with rising CO2 emissions and other human activities.

Bycatch is the name given to fish that are caught but not wanted, and in wild capture fisheries this can reach 10kg for every 1kg of the target fish species caught. It is one of the prime drivers in the growing threat of extinction for many ocean predators such as seabirds, whales and sharks. Some forms of fishing are damaging to the ecosystem in other ways. Bottom trawling, especially of deep-sea ecosystems which show low natural levels of disturbance, can destroy coral reefs and coral gardens. Trawling also flattens the seabed and resuspends sediments, reshaping the sea floor and making it less suitable for marine life. Overall, these mechanical impacts reduce the abundance and diversity of life on the seabed. In the worst cases they leave behind deserts where previously coral allowed the existence of a host of other animals, providing an important habitat for the early life stages of some fish.

Pollution Pollution remains largely out of sight and out of mind but is pervasive. Nutrient enrichment of the coastal ocean, mainly caused by the run off of agricultural fertilisers into rivers, leads to a great increase in microbial activity in the water. This uses up the oxygen in the water and in severe cases, such as in the Baltic Sea and the northern Gulf of Mexico, dead zones can arise in which animals are suffocated in anoxic waters depleted of oxygen. These dead zones have spread, with numbers doubling nearly every decade since the 1960s and have now been reported in more than 400 areas.

There have been fewer oil spills over recent years but the Deepwater Horizon disaster in the Gulf of Mexico demonstrates that accidental release can damage the ocean and people’s livelihoods. The exploitation of oil and gas reserves in the Arctic or in the deep sea brings new risks, especially as these parts of the ocean may be more vulnerable to the impacts of spills than temperate or tropical ecosystems.

Heavy metals and organic chemicals also end up in the ocean. These chemicals can be concentrated in food chains with predators accumulating particularly high concentrations. Orca in the North Pacific have accumulated high concentrations of chemicals such as polychlorinated biphenyls (PCBs), which are used widely in industry as flame retardants. These chemicals affect the physiology of animals in subtle ways, damaging the immune system, and affecting reproductive and developmental physiology. PCBs were banned by the US in 1979 and replaced by polybrominated diphenyl ethers (PBDEs) but now these have also been found to have toxic effects and as a result are also being phased out. And now a cocktail of new chemicals including sunscreens, perfumes, pharmaceuticals and even recreational drugs are ending up in the ocean. There is little understanding of what the effects of this might be.

Then there is plastic litter. It has been estimated that somewhere between 4.8 and 12.7m tonnes of plastic enters the ocean each year (based on 2010 figures). Walking on the beach, or even sailing across the Pacific Ocean, will reveal this problem, in the form of scattered plastic bottles, plastic bags, packaging materials and pieces of fishing line or nets. This plastic is a hazard to marine life, especially animals such as turtles, which mistake it for their prey—jellyfish—and ingest it. The plastic becomes degraded and is ground up into small, microplastic particles of less than one centimetre in length.

In 2011 we were sampling remote underwater mountains in the southern Indian Ocean at depths down to 1.5km, looking at the impacts of deep-sea fishing. On returning to the UK, it was found that our sediment samples and even the corals we had sampled with a deep-sea robot contained microplastic samples. These were in such a high concentration that it was subsequently estimated that the deep sea is a major repository for this material. Microplastics can be ground up into such small particles that they are ingested by zooplankton: crustaceans, molluscs and jellyfish.

Climate change The ocean is also becoming increasingly affected by climate change. Human CO2 emissions cause global warming, and the ocean is absorbing a large proportion of this additional heat. The effects are far reaching, with marine life already showing dramatic shifts in distribution to the north or south (depending on hemisphere), changes in the timing of events such as reproduction, and also in the structure of entire biological communities.

Perhaps the most dramatic impact seen is that of mass bleaching in reef-forming corals. When water temperatures exceed a physiological threshold the coral start to eject symbiotic photosynthesising microorganisms called dinoflagellates that live in their tissues. As a result the corals bleach white, the colour of their calcareous skeletons. The corals depend on the dinoflagellates for their energy and nutrients and without them they will eventually die.

This mass coral bleaching was first detected in the late-1970s and in 1997/1998 a single mass bleaching event, driven by the combination of a major El Niño weather event with global warming killed 16 per cent of the world’s coral reefs. The incidence of these events has risen and through this year and the next coral reefs will suffer from another very large mass bleaching.

Increasing atmospheric CO2 has another effect on the ocean: acidification. When CO2 dissolves in seawater it forms carbonic acid which alters the carbonate chemistry of seawater, reducing the availability of calcium carbonate to marine life. Animals like corals, shellfish and many species of plankton use calcium carbonate to build their skeletons. A reduction of the concentration of this material in seawater slows their growth and weakens their skeletons. As a result of mass bleaching and acidification, coral reefs are now the most threatened ecosystems on Earth.

Acidification also affects other aspects of the biology of marine life. It interferes with the sense of smell of some marine animals so they cannot find their habitats, prey or detect predators. A warmer ocean influences the metabolism of marine life, so fish grow to smaller sizes.

These problems are daunting. However, modern science and technology have delivered the solutions to some. It is also worth considering that the oceans have not suffered the large-scale extinctions that many terrestrial ecosystems have undergone since human civilisation became industrialised, as it has taken longer for humans to dominate the sea than the land. We have a narrow window of time in which to act while the ocean is still in a resilient condition, with much of its biodiversity still present.

The causes of overfishing and the destructive effects of fishing are well understood. The United Nations Convention on the Law of the Sea (UNCLOS) clearly states that fisheries should be sustainable and that they should not cause long-term damage to the ecosystems in which they take place. The problem is one of implementation. There is a lack of transparency in how fisheries are managed and how they are financed. There is fragmentation in the institutional framework set up to manage the ocean.

Effective monitoring, control and surveillance are required to stamp out illegal fishing. More effective oversight of fisheries management bodies is needed, including a reform mechanism that targets organisations that are not improving marine sustainability with sufficient speed.

Protection of marine biodiversity requires that the most destructive fishing methods be eliminated. Marine protected areas are important as they maintain the natural structure of marine ecosystems, including the age and size structure of fish populations. This prevents the structural simplification of marine ecosystems, as well as their capacity to absorb shocks. It also protects fish populations from the genetic selection exerted by fishing which tends to drive fish to mature earlier, at a smaller size and thus for stocks to become less productive.

Enforcement is now being tackled through port inspections of vessels and market-based measures including the tracking of catches from capture to supermarket to make sure they are legitimate. There are also the more conventional fisheries patrols and onboard observers.

Problems of marine pollution originate on land. For many new chemicals that we use, there is a lack of scientific understanding of the effects on marine life. This demands further investment in scientific investigation of the toxicology of such materials, paid for by manufacturers with an independent party undertaking the testing.

States should be made responsible for monitoring and reporting the impacts of river runoff in their coastal waters, driving improvements required to reduce the occurrence of dead zones in coastal waters.

CO2 emissions are rising at a rate that is unprecedented for hundreds of millions of years. It is notable that almost all of the great extinctions of past oceans have been associated with a major disturbance in the global carbon cycle. The current and future effects on the ocean have been poorly considered at an international level but are extremely serious.

Even within the span of the rest of this century, whether humankind follows the current trajectory of CO2 emissions or whether emissions are cut drastically to limit climate change to a rise of 2 degrees Celsius in global temperatures will make a dramatic difference to the health of the oceans. If CO2 emissions continue unchecked, more marine ecosystems will be severely damaged or lost.

There is also likely to be a dangerous feedback mechanism, as the ocean’s capacity to absorb atmospheric CO2 decreases.

This is a global emergency. It requires an immediate response by the international community to reduce CO2 emissions to avoid warming above the 2 degrees Celsius level. If this is not achieved and if the international community fails to address the threat posed to the health of our seas, then sea life—and the health of all the planet—will suffer. The effects of doing nothing now may prove irreversible.