Ballast tank arrangements on board different types of vessels are to meet the requirement of ballast water on board. The figures below give the structural arrangements and ballast capacities on different classes of vessels. Ballast Tank Arrangements for different types of ships Cross section of an ore carrier Cross section of a gas carrier Cross section of a container ship Cross section of double hull oil tanker   Ballast water capacities for different types of ships VESSEL TYPE DWT BALLAST CONDITION NORMAL (tonnes) % of DWT HEAVY (tonnes) % of DWT   Bulk carrier 250,000 75,000 30 113,000 45 Bulk carrier 150,000 45,000 30 67,000 45 Bulk carrier 70,000 25,000 36 40,000 57 Bulk carrier 35,000 10,000 30 17,000 49     Tanker 100,000 40,000 40 45,000 45 Tanker 40,000 12,000 30 15,000 38     Container 40,000 12,000 30 15,000 38 Container 15,000 5,000 30 n/a     General cargo 17,000 6,000 35 n/a General cargo 8,000 3,000 38 n/a     Passenger/RORO 3,000 1,000 33 n/a     The distribution of ballast within a vessel will depend on the design criteria, size and strength of the vessel. Source: Australian Quarantine & Inspection Service 1993. Ballast Water Management. Ballast Water Research Series Report No. 4 AGPS Canberra. Ballast needs Vessel types Typical pumping rates (m3/h) Ballast replaces cargo Ballast required in large quantities, primarily for return voyage. Dry bulk carriers Ore carriers Tankers Liquefied-gas carriers Oil bulk ore carriers 5,000–10,000 10,000 5,000–20,000 5,000–10,000 10,000–15,000 Ballast for vessel control Ballast required in almost all loading conditions to control stability, trim, and heel. Container ships Ferries General cargo vessels Passenger vessels Roll-on, roll-off vessels Fishing vessels Fish factory vessels Military vessels 1,000–2,000 200–500 1,000–2,000 200–500 1,000–2,000 50 500 50–100 Ballast for loading and unloading operations Ballast taken on locally in large volumes and discharged in same location. Float-on, float-off vessels Heavy lift vessels Military amphibious assault vessels Barge-carrying cargo vessels 10,000–15,000 5,000 5,000 1,000–2,000 a The three categories of ballast needs are not mutually exclusive. For example, a vessel in which ballast replaces cargo may also require ballast to control stability.   A potentially serious environmental problem arises when this ballast water contains marine life. There are thousands of marine species that may be carried in ships’ ballast water; basically anything that is small enough to pass through a ships’ ballast water intake ports and pumps.
These include bacteria and other microbes, small invertebrates and the eggs, cysts and larvae of various species.  The problem is compounded by the fact that virtually all marine species have life cycles that include a planktonic stage or stages.  Even species in which the adults are unlikely to be taken on in ballast water, for example because they are too large or live attached to the seabed, may be transferred in ballast during their planktonic phase. Clam life cycle
Prawn life cycle
Over the past millennia, marine species have dispersed throughout the oceans by natural means, carried on currents and attached to floating logs and debris. Natural barriers, such as temperature and land masses, have prevented many species from dispersing into certain areas. This has resulted in the natural patterns of biogeography observed in the oceans today.  In particular, the pan-global tropical zone has separated the northern and southern temperate and cold water zones. This has allowed many species to evolve quite independently in these latter zones, resulting in quite different marine biodiversity between the north and the south.  In tropical areas species have not faced the same barriers. This is exemplified by the relatively homogenous marine biodiversity spanning the huge area of the Indo-Pacific, from the east coast of Africa to the west coast of South America.

An example of marine biogeography/biodiversity contours.

Humans have of course aided this process for as long as they have sailed, mainly by dispersing marine species that have attached to the hulls of vessels. The commencement of the use water as ballast, and the development of larger, faster ships completing their voyages in ever shorter times, combined with rapidly increasing world trade, means that the natural barriers to the dispersal of species across the oceans are being reduced. In particular, ships provide a way for temperate marine species to pierce the tropical zones, and some of the most spectacular introductions have involved northern temperate species invading southern temperate waters, and vice versa.

It is estimated that at least 7,000 different species are being carried in ships’ ballast tanks around the world. The vast majority of marine species carried in ballast water do not survive the journey, as the ballasting and deballasting cycle and the environment inside ballast tanks can be quite hostile to organism survival. Even for those that do survive a voyage and are discharged, the chances of surviving in the new environmental conditions, including predation by and/or competition from native species, are further reduced. However, when all factors are favourable, an introduced species by survive to establish a reproductive population in the host environment, it may even become invasive, out-competing native species and multiplying into pest proportions.

As a result, whole ecosystems are being changed. In the USA, the European Zebra Mussel Dreissena polymorpha has infested over 40% of internal waterways and may have required between US$750 million and US$1 billion in expenditure on control measures between 1989 and 2000. In southern Australia, the Asian kelp Undaria pinnatifida is invading new areas rapidly, displacing the native seabed communities. In the Black Sea, the filter-feeding North American jellyfish Mnemiopsis leidyi has on occasion reached densities of 1kg of biomass per m2. It has depleted native plankton stocks to such an extent that it has contributed to the collapse of entire Black Sea commercial fisheries. In several countries, introduced, microscopic, ‘red-tide’ algae (toxic dinoflagellates) have been absorbed by filter-feeding shellfish, such as oysters. When eaten by humans, these contaminated shellfish can cause paralysis and even death. The list goes on, hundreds of examples of major ecological, economic and human health impacts across the globe. It is even feared that diseases such as cholera might be able to be transported in ballast water. 

There are hundreds of other examples of catastrophic introductions around the world, causing severe human health, economic and/or ecological impacts in their host environments.

Invasive marine species are one of the four greatest threats to the world’s oceans! Unlike other forms of marine pollution, such as oil spills, where ameliorative action can be taken and from which the environment will eventually recover, the impacts of invasive marine species are most often irreversible!