The Ecological Impacts of Ballast Water Discharge

"We are on the verge of the sixth extinction" Stuart Pimm, Professor of Conservation Ecology at Duke University concludes.  Earlier this year a study, of which Pimm was lead author, calculated current human-induced rates of extinction to be 1000 times greater than the background.  The human-induced causes for extinction are myriad but can be broadly categorised as habitat loss, over-exploitation, pollution, climate change, and non-native invasive species.  Non-native species are any species that has been brought to an area outside its natural past or present day distribution by human activity.  When a non-native spreads altering the environment to which it has been introduced, and/or impacting on human health or the economy, the species is classed as invasive. 

Some invasive species are those species deliberately introduced, like the cane toad in Australia which was deliberately released in Queensland in 1935 to control sugar cane pests.  Most invasives however, are introduced accidentally.  The pathways for accidental invasion are inextricably tied with human movement. 

Be it the actual organism itself, seeds, eggs, or gametes, “the spread of problematic non-native species” Dr Mark Spencer, senior curator of the British and Irish herbarium at the Natural History Museum London told the UK Parliamentary Environmental Audit Committee just this year, “has been closely correlated with the development of international trade over the last 150 years”.  With over 90% of international trade involving ocean travel, sea freight is a major invasive pathway, and the prime culprit is ballast water. Ballast water, which is essential to maintain the effective and safe running of freight vessels when they are not fully loaded, is typically taken from sea water at or near one port, and discharged at another. Alongside the sea water itself, marine organisms – small animals and larvae, viruses, bacteria, and algae are also taken in – and discharged.  As shipping has increased and extended across the globe, so too has the amount of ballast water.  Today an estimated 3.1 billion tonnes of ballast water is discharged annually.  

Not all organisms taken on in ballast water will become invasive.  Depending on the time between taking in ballast water and discharging it again, organisms may simply die in transit, either due to short life spans or simply lack of sufficient resources needed to stay alive.  Those which survive may not necessarily become invasive, with unfavourable environmental conditions at the discharge point hampering population growth.  Never-the-less the marine invasive problem has fast become a global issue.  A 2008 study lead by Jennifer Molnar, director of science for The Nature Conservancy found that invasions had been reported in 84% of the 232 World’s marine ecoregions.

When a marine species does become invasive, the impacts can be devastating.  The Chinese mitten crab (Eriocheir sinensis), a native of Korea and China was first reported in Europe in 1912 in Germany, but has rapidly spread.  Their penchant of burrowing has altered habitats and eroded estuarine and river banks, with an estimated economic cost to Germany alone at €80 million.   Alexandrium minutum, the dinoflagellate famed for causing it’s toxic ‘red-tide’ has spread from its native European waters to the South China Sea, New Zealand, south-east Australia, and New York.  With blooms of the dinoflagellate presenting a high risk of causing paralytic shellfish poisoning (PSP), red-tides effectively halt fishing and aquaculture in their vicinity, as well as presenting toxic impacts on fish and invertebrates.  In the Gulf of Mexico the Australian spotted jellyfish (Phyllorhiza punctata) has become both an ecological and an economic threat to the region due to its propensity to consume vast quantities of eggs and larvae - including those of commercially important species.  Once established, it is virtually impossible to eradicate marine invasives from an area.  This is not simply a case of ‘prevention is better than cure’, more that cure is unlikely to even be an option.  Prevention, the United Nations and IMO hope, could lie in the 2004 International Convention for the Control and Management of Ships' Ballast Water and Sediments...

The full article was published in - and can be read in - The Marine Professional, a publication of the Institute of Marine Engineering, Science & Technology (IMarEST)

Image: An invader in Mexico - the Australian spotted jellyfish. Credit Cloudtail the Snow Leopard/Flickr (CC BY-NC-ND 2.0)