Oceans, caught between devil and the deep blue sea… (Pun intended!) – IX

Impact of shipping on marine pollution:

With over 80 percent of global trade being carried on board ships and handled by seaports worldwide, the importance of maritime transport for business and development cannot be overemphasized. 10.3 billion tons (10,300,000,000 MT) of cargo was transported by sea in 2017.  The number of sea-going ships (cargo ships and all rest) is myriad – with over 52,000 being cargo ships, approximately 8,000 naval ships, 70 cruise ships and over 4 million fishing vessels.  And the list still does not include – high-speed crafts, off-shore oil vessels, supply vessels, harbor work crafts, tugboats, harbor ferries, luxury and cruising yachts, research vessels, survey vessels, ice-breakers, submersibles and so on.

cma-cgm-marco-polo2As seen in earlier articles, shipping industry imparts 10% contribution to overall marine pollution.  However, with above details of a countless number of sea-going vessels and vessel types, this data sounds fishy!  The damage, in my opinion, would be far more than just 10%.   There are various ways in which shipping industry imparts destruction to marine ecology.

  • Ballast water pollution
  • Atmospheric pollution
    • Conventional pollutants
    • GHG pollutants
  • Oil spills/bilge pollution
  • Greywater pollution
  • Blackwater pollution
  • Sound pollution
  • Wildlife collisions

What is ballast water pollution?

How does a vessel float? How is it supported by the water and why does a stable ship tend to return to its initial upright position? All of this can be explained by the two opposing forces acting on the vessel: its weight tending to push it down and its buoyancy tending to push it up. When the weight is equal to the buoyancy and is distributed evenly; the vessel floats and remains upright. 


Therefore, at a port when a ship discharges cargo loaded on it; her weight reduces and to compensate off-loaded weight, she takes water in her belly.  This water is discharged at port of destination, as she again loads more cargo and gains weight.  This water which is received in or released out is called as Ballast since it helps to add weight to the ship to keep it floating upright.  Ballast water is essential for the safe operation of the vessel. It is used to adjust the overall weight of the vessel and its internal distribution to keep her floating safely, upright and in a stable condition. See below figures which depict positions of ballast water tanks on board a vessel. 

Ballast tanks


Schema_Algemeen_zndr_logoWhen ships take in ballast water, plants and animals that live in the ocean are also picked up. Discharging this ballast water releases these organisms into new areas where they can become marine pests or invaders. Ballast water discharge typically contains a variety of biological materials, including plants, animals, viruses, and bacteria. These materials often include non-native, nuisance, exotic species that can cause extensive ecological and economic damage to aquatic ecosystems, along with human severe health issues including deaths. It is estimated that world shipping transfers about 10 billion tons ballast annually and contains thousands of species from coastal areas and having eutrophic water properties. Also, one another study suggests that ballast water carries about 7000 living species every hour every day from one port to the other. These species reproduce uncontrollably and occupy that area irreversibly.

Chapter1 6pp v3.indd

Scientific research has now proven that the discharge of ballast containing particular species can have a severe effect on the ecological balance of the environment in which ballast is being discharged. The introduction of foreign species can impact local biodiversity and environmental systems, human health and local economy. This issue has also been recognized by the IMO (International Maritime Organisation) and the WHO (World Health Organisation). The ballast water and sediments carried by ships have been identified as a significant pathway for the transport of harmful invasive aquatic organisms and pathogens. A few examples are as below.

  1. American comb jelly (Mnemiopsis leidyi) which ballast water carried to the Black Sea, created a calamity in that area. This species was brought from the North Sea to the Black Sea in the early 1980s. It rapidly took hold, and by 1989 an estimated 1 billion tons of the non-native species were consuming vast quantities of fish eggs and larvae, as well as the zooplankton, on which commercial fish was being fed on. By 1992, this species led to losses in the tune of 240 million dollars annually. In the Black Sea, as the population of the American comb jelly went on increasing, the zooplankton population decreased.
  2. In 1991, the microbe Vibrio Cholera (Fig. 5) was released and infected the drinking water in Peru. One million people were infected with cholera, and more than 10000 died.

There are hundreds of organisms carried in ballast water that cause problematic ecological effects outside of their natural range. The International Maritime Organization (IMO) lists the ten most unwanted species as:

  • Cholera Vibrio cholerae (various strains)
  • Cladoceran Water Flea Cercopagis pengoi
  • Mitten Crab Eriocheir sinensis
  • Toxic algae (red/brown/green tides) (various species)
  • Round Goby Neogobius melanostomus
  • North American Comb Jelly Mnemiopsis leidyi
  • North Pacific Seastar Asterias amurensis
  • Zebra Mussel Dreissena polymorpha
  • Asian Kelp Undaria pinnatifida
  • European Green Crab Carcinus maenas

Ballast waterI will end this article here, and in next articles, we will see other impacts of shipping industry on the marine environment. 


  1. Effect of Ballast Water on Marine and Coastal Ecology (PDF Download Available). Available from: Huseyin ELÇİÇEK; Mehmet Cakmakcı; Adnan Parlak
  2. Ballast Water Impacts – Hans Buch
  3. OTEN Maritime Studies tutorial site
  4. Marine Insight
  5. The UK Marine SACs Project

Milind Joshi


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