Main ocean stressors

Damage to the ocean is not as immediately apparent as terrestrial destruction, but it is just as serious. All of the stressors we have put on the ocean — from overfishing to pollution — have contributed to its ill-health. The situation is now so severe that we are altering the chemistry of the ocean, with significant impacts on marine life and the functioning of marine ecosystems.

Dr Alex Rogers, Scientific Director of IPSO and Professor of Conservation Biology at the Department of Zoology, University of Oxford, gives an overview of the main problems affecting the ocean — and some suggested solutions.
Professor Dan Laffoley, Senior Advisor at the International Union for the Conservation of Nature, emphasises the vital role of the ocean for humans and the deadly combination of stressors at play.

Extractive industries

One-third of oil is extracted from under the seabed and deep-sea rigs produce 5% of total hydrocarbons, with some wells deeper than 3km. Deep-sea mineral mining is technically feasible and is now in testing, with licences for full exploration to follow soon. Bottom trawl fishing extends below 2,200m across all oceans. The next logical steps for the fossil fuel industry are expansion into the Arctic, as summer sea ice disappears, and the exploitation of methane hydrates.


Fishing overcapacity

The global marine capture fish catch increased from 18 million tonnes in 1950 to nearly 90 million tonnes in 2010.

FAO estimates that about two-thirds of fish stocks are exploited to their maximum sustainable limit, and one-third beyond that limit. Two-thirds of high seas stocks are overexploited and/or depleted. Many stocks of the largest fish (such as tuna and swordfish) are below 10% of their historical level. The World Bank calculates that mismanagement of fisheries costs the world economy about US$83 billion per year.

The global fleet’s overall engine power grew 10-fold over the period. The catch is now falling due to declining stocks, but capacity continues to rise; ships use twice as much energy to catch a tonne of fish as 60 years ago. Too many vessels are competing for increasingly exploited stocks, increasing the imperative to fish illegally.

Climate change impacts

The ocean absorbs more than 90% of the energy trapped in the Earth System by greenhouse gas emissions and absorbs one-quarter of our carbon dioxide emissions. This has created a ‘deadly trio’ of impacts on the ocean – acidification, warming and deoxygenation.


Today’s rate of acidification is unparalleled over the last 300 million years. Biological impacts include displacement of organisms away from breeding and feeding grounds, reduced formation of shells and a reduction in the living space for large fish.

Professor Jason Hall-Spencer discusses ocean acidification.


The average temperature of the upper layers of the ocean has increased by 0.6°C over the last 100 years, with direct and well-documented physical and biogeochemical consequences. The biological implications encompass sea temperature-driven range shifts and species invasions, in particular a projected movement poleward of 30–130 km and 3.5m deeper by marine fish each decade and a loss of 60% of present biodiversity of exploited marine fish and invertebrates, including numerous local extinctions and increased disease prevalence as a result of pathogen range expansions.

The synergistic effect of acidification and warming are considered likely to lead to rapid and terminal decline of tropical coral reefs by 2050. All these changes will have massive economic and food security consequences, not least for the fishing industry and all those who depend on it. It is predicted that the redistribution of commercial fish species through range shifts will lead to a 40% decrease in catch potential in the tropics by 2050, and 30–70% increase in the high-latitude zones – where richer societies and more industrialised fisheries are located.

Professor Philip Chris Reid of Plymouth University discusses the latest science on ocean warming.

Deoxygenation – ocean dead zones

The number of areas in coastal waters with too little oxygen to support most marine life, otherwise known as dead zones, has greatly increased since the 1960s according to research. Four hundred systems worldwide, including 166 in US waters, now have documented dead zones. A review paper (Science, 15 August 2008) by Robert Diaz of the Virginia Institute of Marine Science, and Rutger Rosenberg of the University of Gothenburg, attributes this dramatic trend to increasing nutrient pollution and the burning of fossil fuels.

Professor Robert Diaz of the Virginia Institute of Marine Science explains dead zones.


More than 80% of marine pollution comes from land-based activities, including fertilisers, pesticides, sewage, garbage, plastics and oil. Disposal at sea of the most toxic materials was banned by the 1972 London Convention, with more restrictions added in 1996. However, the toxins already present, combined with the unregulated dumping of ‘emerging chemicals’ and ubiquitous plastic pollution, remain a major challenge. In 2014/5 IPSO scientists contributed to the discovery of microplastic articles and debris in the deep sea.

Craig Downs from the Heretics Environmental Laboratory discusses pollution research.