Shipping problems: Alien invaders | WWF

It's a surprising fact that one of the biggest polluters of the marine environment is ... water.

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Ships load water for balance and stability when cargo levels are low, and then release the water when they pick up new cargo at another port. Some 10 billion tonnes of this ballast water is transferred around the world each year.

Ballast water is loaded with thousands of marine species, including plankton, algae, fish, jellyfish, and other invertebrates.

Most of the stowaways are not able to survive in their new environment when released from the ship. But some can - and only too well. These aliens can become invasive, rapidly out-competing local fauna or flora. They can alter the entire local ecology, leading to the collapse of fisheries and threatening endangered species. Exotic algal species can also pose a risk to human health by contaminating seafood.

Indeed, alien invasive species can be as damaging as oil spills, and their effects much more persistent.
© IMO/Globallast
Ballast water discharge in port.
© IMO/Globallast

Ballast water discharge in port.

Invasive jellies

The invasion of the Black Sea by a voracious comb jellyfish from North America (Mnemiopsis leidyi) is one of the best-documented examples of a marine alien invasive species introduced through ballast water.

It might look harmless, but the North American comb jellyfish (Mnemiopsis leidyi) has had a devasting affect on the Black Sea. © WWF
The comb jellyfish arrived on ships from the American Atlantic coast in 1982. It eats both zooplankton, the food of commercially important fish in the Black Sea, and the eggs and larvae of the same fish species.

With no enemies in their new home, the jellies propagated at an alarming rate. By the mid-1990s, they accounted for 90% of the total biomass in the Black Sea - a biomass more than the total annual fish catch around the world. The species quickly spread into the neighbouring Azov Sea too.

The invasion contributed to the near collapse of Black Sea commercial fisheries within a few years. The once quite prosperous seafood industry has lost about US$1 billion since the jellies were released. Anchovy fisheries in the Azov Sea, already under stress from pollution and overfishing, have completely collapsed.

Dolphin numbers in the Black and Azov Seas also dropped dramatically, as the fish they used to feed on disappeared. The entire ecosystem has been disrupted - the jellies have even reduced the amount of oxygen in the Black Sea.

They've now entered the Caspian Sea, where they are similarly wreaking havoc, and have been found in the Baltic Sea and along the Atlantic coast of Norway too.
Silent Invasion - The spread of marine invasive
species via ships’ ballast water
© Report: Silent Invasion - The spread of marine invasive species via ships’ ballast water © WWF