The largest container ship in the world set sail for its maiden voyage from Guangzhou Port in China in April 2023. MSC Irina measures 399.9 metres in length, and 61.3 metres wide, with the tallest container layer equivalent to the height of a 22-story building. The first of six such behemoth ships being built by the Chinese Jiangsu Yangzijiang Shipbuilding Group, the ocean giant is able to carry up to 235,341 tonnes of cargo. Royal Caribbean International also recently announced that their Icon of the Seas, the the world’s biggest cruise ship, measuring 365 metres long and able to hold 5,610 passengers and 2,350 crew, set sail on 27 January 2024.

As the world’s oceans grow busier, and ships grow ever-larger, the risk of stowaways also increases. But amongst conservation scientists, it’s non-human ‘stowaways’ and ‘hitchers’ that are causing the greatest concern. “Unwanted passengers are a vastly underrated issue,” says James Nikitine, founder of Blue Cradle Foundation, a New Zealand-based ocean conservation organisation, who believes marine invasive species pose an ecological and economic threat as they travel around the world on or in ships and enter local ecosystems.

There are two ways ‘unwanted passengers’ are travelling: by attaching to a vessel’s hull or getting entrained or taken on in a ship’s ballast waters. “Any living marine organism can voluntarily or accidentally penetrate ballast waters, or hook onto a vessel’s hull, whether it’s a tanker, bulk carrier, recreational cruiser, fishing or passenger ship, and then travel long distances,” says Nikitine. “These include small plankton, fish and coral larvae, molluscs, worms, and seaweed. Virtually any marine fauna or flora found in the ocean and capable of surviving short or long-distance travel can be a ‘hitchhiker’, depending on the availability of nutrients (food), light, and other necessary survival conditions.”

11 billion tonnes of cargo are transported by ship each year. “There are over 100,000 large commercial ships globally with thousands of containers coming and going, representing 90% of all goods exchanged around the world,” Nikitine explains. “This figure excludes smaller fishing vessels and recreational boats. With an increasing number of boats and different policies in ports, modelling suggests that, if left uncontrolled, we’re likely to see an increase in the spread of marine invasive species, especially as marine debris, such as plastics, have also been proven to carry ‘hitchhikers’ around the globe.”

Though invasive species can attach to or ‘board’ large and small vessels, massive ships have vast external surface areas and internal spaces for unwanted passengers to go undetected. A single ship can carry 100,000 cubic metres of ballast water, which is often taken up from coastal waters in one global location and transferred to bays or ports in another location, launching invasions that can spread. “Marine invasive species can modify the balance of an existing marine ecosystem,” explains Nikitine. “The presence of a foreign species has potential consequences on the predation of other species, and the availability of resources for native and endemic species. The worst-case scenario is where an outbreak leads to a depletion of resources or an imbalance that catastrophically impacts other species to the point of collapse or extinction.”

“As island ecosystems are strongly dependent on healthy oceans for their livelihoods, including for fisheries, tourism, and aquaculture, one can easily see how the wrong ‘hitcher’ entering an ecosystem can have severe negative ecological impacts, causing rapid economic harm,” he adds.