Discharges at Sea






In comparison to other means of transport, shipping remains by far the most energy efficient. However, shipping does come at a cost. It has an impact on the marine environment. Most harm caused to the marine environment stems from daily operational emissions of substances from land- and ship-based sources. Intentional and unintentional discharges at sea have ongoing negative effects globally on ocean life.  


Participants will:  

  • Identify how discharges at sea negatively affect the marine environment 
  • Determine ways to prevent discharge at sea  

Oil Pollution Act (OPA): Addresses the prevention, response, and costs for oil pollution incidents in navigable waters of the United States.  

Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA):  Liability to pay damages for the cleanup if sites contaminated with hazardous waste and the prevention of contamination of future sites.  

Eutrophication: Excess of nutrients that causes death to organisms from lack of oxygen.  

Bioaccumulation: Buildup of chemicals inside living organisms.  

Dive Deeper

Expand each section to identify and prevent discharges at sea.

While major oil spills receive most public attention, even a small spill can have large consequences. The combination of factors such as the size of the spill, where it took place, and when it took place determine the severity. 

Ecological Impacts 

Oil in the marine environment can kill various species. One of the most research effects of oil spills at sea is bird mortality. When oil sticks to the feathers of a seabird, they are no longer waterproof.  As a result, sea water reaches the body of the seabird and causes death from hypothermia due to the loss of body heat. An oil stain of 1 inch on a bird’s chest is enough to be lethal. Oil also impacts small organisms in the marine environment. Laboratory studies have demonstrated that dissolved oil compounds are toxic and lethal to zooplankton.  

Economic Impacts 

Industries that depend on the sea such as tourism and fishing, can suffer from oil pollution. The fishing and aquaculture industries are especially at risk when they operate in coastal areas. Shellfish such as mussels filter seawater and ingest oil compounds. This makes them dangerous for human consumption, ultimately impacting the shellfish industry.  

What Can You Do? 

In MARPOL Annex I, IMO has set limits to the discharge of oil into the ocean. As a seafarer or port employee, you are very important in minimizing these discharges. Following the regulations, using the technical means available and being pro-active can help prevent the discharge of oil.  Carry oil compounds onboard as much as you can and land them in port in well-organized reception facilities.  


Chemicals are transported by ships as bulk products (in chemical tankers, bulk carriers or gas tankers) or in packaged form (on container ships). Around a thousand different chemical substances are carried in liquid form in chemical tankers with methanol being the most common. 

Effects of Chemicals 

Chemical spills have the potential to harm important habitats for animals and plants. They impact fisheries, beaches, and degrade the quality of life for those who live close and work in the areas affected. 

The effects of a spilled chemical depend on several factors, such as the amount spilled, the toxicity of the chemical, where the spill takes place and whether the chemical remains at sea or comes ashore.  

The way a chemical affects an organism is largely determined by its toxicity, the duration of contact and the dose of the chemical. Acute toxic effects occur directly after a short exposure to high doses of a chemical. Chronic effects are caused by exposure over a longer time. Acute effects are relatively easy to identify and range from nausea, to coma and death. Long-term hazards of chemicals (also for humans) include cancer, changes in genetic material, and injuries to the nervous system. 

What Can You Do? 

Use the International Maritime Dangerous Goods (IMDG) code to determine if substances onboard are classified as marine pollutants. When transporting chemicals ensure marine pollutants are specially packed and labeled appropriately. Always confirm their storage is secured when onboard and kept in proper reception facilities.  

Like any place where people live, all ships produce sanitary waste or ‘sewage’. Some ships produce large amounts of sewage such as cruises, passenger vessels and cattle-carriers. Sewage discharge in the sea provides nutrients for marine organisms. In the open ocean, sewage is rarely a problem since nutrients are in short supply. However, coastal seas naturally contain many nutrients already. When sewage is discharged in coastal waters, these extra nutrients may increase the growth of algae and cause algal blooms. Algal blooms lead to hypoxic events where the amount of oxygen in the water decreases. This depletion can lead to the death of marine organisms. This process is called eutrophication.  

Sewage may also contain harmful chemicals that are detrimental to all living organisms such a pathogens.  

What Can You Do? 

According to MARPOL Annex IV, sewage can be discharged if it has been treated by an approved sewage treatment plant. Ensure sewage is properly treated before discharge and only when MARPOL conditions are met, such as maintaining a required distance from coastal waters.  

Solid waste in the marine environment (marine debris) is found worldwide. These items are found in ocean surface waters, coastal areas (e.g. beaches), and on the ocean floor. Of all marine debris items, plastics constitute around 60-80%. This is due to the inability of plastics to degrade. Instead, they break down into ever smaller fragments called microplastics and nanoplastics. As plastic begins to break down, chemicals that were used to create the plastic are released into the water, degrading water quality.  

Plastic Soup 

Marine litter became world news in 1997 when American sailor Captain Charles Moore discovered two continent-sized areas of floating debris in the Pacific Ocean. Now, there are five of these areas in our ocean referred to as plastic soup. Many recognize the Great Pacific Garbage Patch or the North Pacific Gyre which is estimated to contain an average of 334,000 plastic fragments per square kilometer.   

Ecological Impacts 

Marine debris impacts numerous species in the food web, including sea turtles, seabirds, dolphins, whales, seals, and fish. The most common impacts are entanglement and ingestion. When an animal becomes entangled, it means they are trapped by the debris. This ultimately leads to suffocation, drowning, and serious injuries. Animals often ingest litter because of its resemblance to their natural prey. This can lead to starvation or death caused by malnutrition.  


The sea contains a growing number of small pieces of plastics. These microplastics are the result of larger pieces of plastic breaking down. Microplastics contain chemicals. When small animals at the base of the food web consume microplastics, the chemicals enter the food web. These small animals are then eaten by larger animals which causes the chemicals to build up in higher concentrations, ultimately reaching the top of the food web. This process known as bioaccumulation causes hazards to large marine species and humans.  

Economic Impacts 

Marine litter ultimately effects industries that rely on the sea. Tourism in coastal areas may decrease when beaches are covered in debris, while incurring a cost to clean the beach. It can damage boats when ropes or nets become entangled and become entangled in fishing nets.  

What Can You Do? 

According to MARPOL Annex V, the discharge or throwing overboard of materials like plastic is not allowed. To prevent solid waste from entering the sea, be proactive to keep these materials onboard and land them in port. Use technical means available to effectively reduce your plastic usage abroad ships.   


MARPOL sets limits for the discharge of substances such as oil, chemicals, sewage and solid waste into the ocean:

  • Oil (Annex I);
  • Noxious liquid substances carried in bulk (Annex II);
  • Harmful substances carried in packaged form (Annex III); 
  • Sewage (Annex IV);
  • Garbage, solid waste (Annex V);

Worldwide several regions and states have supplementary regulations amplifying international conventions. In the US these regulations include the Oil Pollution Act (OPA) and the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA). 

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