Different species of plants and animals have their respective places on Earth where they are native, or endemic. They live and interact in a mutually beneficial way with other species in their environment. Taken away from their “home”, they create havoc in their adopted places.
An example is the Maryland Blue Crab (Callinectes sapidus Rathbun). It is the most valuable commercial catch in Maryland, USA. It is widely distributed from Nova Scotia to northern Argentina. Along the coasts of North America, it is most abundant from Texas to Massachusetts.
While a prized catch in Maryland, they are a problem in another part of the world. In Italy’s norther coast, the crabs are decimating clams, mussels, eggs, other fish and mollusks. Excellent swimmers and weighing up to one kilogram, they eat almost everything. Their sharp, blue-tinted claws are particularly adept at prying open clam shells. Fishermen can’t even lower their nets because the crabs swim into the nets and break them. They have no natural predators in Italian waters so their numbers are unchecked.
The blue crab moves quickly and can travel 15 kilometers a day. Even at this speed however, they can’t possibly travel the 7,000-kilometer distance from Maryland to Italy, unless someone or something took them there. The most probable route is the ballast water from ships. Large cargo ships routinely take in and discharge seawater using internal storage containers called ballast tanks. The purpose is to balance their weight and keep them stable during a voyage.
Although it is essential for the safety of the ship, ballast water can be harmful to the marine environment as its discharge can release potentially invasive species into a new marine environment. When ballast water is pumped into a ship, sediment and microscopic organisms are also transferred into the ballast tanks. These organisms include bacteria, microbes, small invertebrates, eggs, cysts, and larvae of various species. Many of these organisms are able to survive for extended periods inside the ballast tanks.
When the ballast water is discharged, the organisms are released into the local marine environment where they become invasive species threatening and even eliminating native populations. Approximately ten billion tonnes of ballast water are transported worldwide every year. An estimated 7,000 aquatic species are transferred in ballast water every hour of every day. One new invasion occurs every nine weeks.
The zebra mussels’ invasion of the Great Lakes is one example of the devastation a species can cause when introduced to a new marine environment. Since its arrival in the late 1980s due to ship ballast water discharge, this freshwater mussel, native to the Black and Caspian Seas in Europe, has caused significant environmental damage, including disrupting the local food chain.
The European Green Crab, one of the world’s most unwanted invasive species, was first introduced by ship to eastern Canada in the 1950s and, more recently, to the west coast of Vancouver Island in British Columbia. It is a formidable predator that out-competes native crabs for food and disrupts essential eelgrass beds and the crustaceans, molluscs, and fish they shelter.
The ‘International Convention for the Control and Management of Ships' Ballast Water and Sediments’ which came into force last September 08, 2017, aims to address this environmental concern.