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  • Writer's pictureTravis Turgeon

The Lesser-Known Conservation Implications of Dark Shipping

Updated: Feb 21, 2023

"On some of the islands in the desolate Indian Ocean, invasive rat populations have made their way from ships to shore."

It should come as no surprise that the shipping industry imposes many environmental and marine ecosystem conservation challenges.

We know that the fuels used by large cargo and trade vessels directly impact global CO2 emissions (~3% of the global totals), and we are well aware that fishing fleets are slowly but surely diminishing fish populations.

But where does the impact end? Are we on course to regulate some of the more severe consequences the industry has on our world's natural resources and marine animals, or are we headed down a path that is spiraling out of control?

Are we even fully aware of the damages caused by the industry, or are we overlooking more minor, yet still essential impacts on local and global levels?

In this article, we look at a surprising and lesser-known challenge the shipping industry causes and talk about how dark shipping fleets further the issue to unknown degrees.

We also outline how many of these challenges seem negligible without context and explain why we must continue to shed light on every aspect of marine conservation to make progress toward ocean sustainability.

Surprising Conservation Challenges Imposed by the Shipping Industry

It seems like every day, we hear about new consequences attached to shipping and trade, be it human trafficking, overfishing, or a myriad of other issues. Still, it’s not every day that you hear how rat infestations on ships have relatively significant impacts on ocean habitats and natural resources.

Invasive Rats: From Ships to Shore to Coral Reefs

Rats are quite literally everywhere, even in some of the most unexpected places. And while rats on ships may not be entirely surprising, the consequences the infestations are causing almost certainly are.

In February, an article was published about how rat infestations on ships directly lead to the destruction of coral reefs and the fish populations that live on them.


On some of the islands in the desolate Indian Ocean, invasive rat populations have made their way from ships to shore. Once on these remote islands, the rat populations begin booming, leading to cascading effects on nearby coral reefs.

This happens by way of birds and the (relatively annoying) damselfish species.

If you've ever been swimming over coral reefs while snorkeling in parts of Asia and the Indian Ocean, you may have noticed small, pale, off-white fish guarding their territory with their lives. These fish dart toward you (and other fish) with a ferocity you wouldn’t expect, seeing as though you’re about 100 times their size. Their charging attempt may seem humorous, but their bite is anything but pleasant.

While annoying, damselfish play an important role in coral ecosystems. They are a staple species on reefs, and they help regulate nutrients and provide some protection from predators that could otherwise alter the habitat.

Anyways, now that we have that picture - back to the rats.

When rat populations overtake these small islands, the flow of nutrients to the reefs from shore is significantly altered, and because of it, damselfish are changing their behavior patterns.

The rats don’t directly alter the flow of nutrients, though. Instead, it’s the birds - but everything ties together.

On islands without rats, birds feed openly in the water over the nearby coral reefs. They fly from shore, swoop up something to eat, and head back to the beach, where they ultimately deposit their guano near the shore. This is where the biggest flow of nutrients comes from, and seabird guano is highly beneficial for the health and productivity of coral reefs.

However, birds don’t tend to stick around long when islands become infested with invasive rat populations. This leads to diminishing seabird populations and, in turn, reduces nutrient flows to the reefs.

It’s not that the birds have a fear of rats. Rather, the rats eat the eggs and offspring of the birds, leading to a severe decline in populations. It is estimated that islands without invasive rat populations have more than 750 times the density of seabirds on them than islands with infestations.

Throughout studies, researchers have found a clear correlation between the growth rate, size, and population density of fish when looking at rat-infested islands.

On islands without rat populations and, in turn, more birds, the fish tend to grow much faster and much larger than islands infested with rats. This is likely a direct result of fewer nutrients in the water, which results in damselfish expanding their territory, making them more sparse and becoming less aggressive. Without the normal ferocity of these fish, the reef sees more predators and completely alters the habitat of the reef.

The added pressure on the coral reefs compounds alongside rising ocean temperatures, acidification, and pollution. While the alteration of nutrients on the reef alone may not justify raising the alarms, it certainly makes it harder to cope with larger, more significant threats.

How Dark Shipping Fleets May Contribute to Island Infestations

As we know, dark ships and dark fleets operate in secret, most often to avoid detection while carrying out illicit activity such as illegal fishing. This may spell problems for coral reefs surrounding small and desolate islands around the globe, and not for the typical reasons.

Dark ships are notorious for being dirty, unregulated, and unprepared for sea journeys, and while each ship has a different story to tell, rat infestations are far from unlikely. In any case, if an undocumented ship were to come ashore on an island for any reason, an infestation could very well spread, leading to compounding problems for coral reefs, as discussed above.

While island authorities themselves have some stake in stopping these events from happening, the bigger responsibility lies with national and international maritime leaders to prevent the spread of dark shipping in the first place.

AIS monitoring is simply not enough. Now, more than ever, we must actively seek new methods of dark ship monitoring and management, which have known and unknown consequences for people, wildlife, and the planet.

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