Meet the World’s Largest Freshwater Crayfish

This story is part of a series designed to introduce the perspectives of alumni from the National Geographic Society and The Nature Conservancy’s global youth externship program. Each guest author is an emerging leader in conservation and storytelling.

Would you believe me if I told you there are crayfish out there that can weigh up to 6 kilograms? Well, it’s true! They swim in the cool waters of Tasmania. Unfortunately, there are processes at play much larger than crayfish, which means the crayfish face an uncertain future.

It is mid-summer in Tasmania, arguably the best time to visit. Incredible expanses of mountains, forests, and crystal-clear rivers cover vast areas of the island. These areas are made up of lush greenery and towering trees which create an invigorating and serene atmosphere. It is warm and dry with temperatures ranging from approximately 20 to 24 degrees Celsius. A perfect paradise for many unique species.

When I arrived in Tasmania, it was also summer, sometime around February, and I began a bachelor’s course in zoology. I’ve always had a fascination with the natural world, whether that was the bugs in my backyard or the sea life that would float down the canal at my Nana’s place.

These creatures always captivated me. And what better way to learn about them than with a degree in zoology! But one creature in particular caught my attention, and it was from the research I had done through university. So when the opportunity arose to take part in a freshwater externship with Paragon One, The Nature Conservancy and National Geographic Society, I was extraordinarily excited.

It was through this opportunity that I got to undertake my own research into freshwater issues. This research focused on an area three to four hours north of where I was based: in the cool dark streams and rivers of the northern parts of Tasmania.

Here, dense canopies of vegetation provide a layer of shade which ultimately means cooler water temperatures. Within the water are logs and branches that fall through the canopy and start to decay. Occupying this decaying vegetation are Tasmanian giant freshwater crayfish.

They’re odd-looking creatures that make up a group of organisms called crustaceans. These crayfish are found nowhere else in the world. I certainly never saw them in the backyard canal at Nana’s!

Growing Up Crayfish

As they mature, the young crayfish are trying to find areas with lots of moss and boulder coverage, the perfect conditions for shelter and protection. Here they are away from the prying eyes of fish or platypus, predators that feed upon the young crayfish if the opportunity presents itself.

The young crayfish use cavities between the boulders as something of a dining room to consume a diet preferably high in animal products. They use their claws to crush the food so it can be consumed and use the energy the food provides to slowly grow bigger. They eventually grow to weights of up to 6 kilograms.

Growing up is a slow process, but once grown, the adults range in colour from blue to brown. They now consume a much more variable diet including decaying wood, insects, leaves and some small animals and fish.

This provides a service to the wider ecosystem. The crayfish help control the populations of the animals they consume. The crayfish are also helping the wider ecosystem by shredding leaves, which helps cycle nutrients and organic matter.

Smaller Size, Disturbed Waters: What Does the Future Hold?

The crayfish live in streams and rivers on Tasmania, an island that is 320 kilometres across. This island is home to many other species all of which have their own lives to live. As one of many individuals sharing the island, my research was now focused on why the crayfish population numbers had declined so drastically.

Literature suggested the crayfish were either endangered or vulnerable. But why? The answer is not a simple one. But can broadly be answered with two words: Fishing and disturbance.

When thinking of places to explore in nature, I know I prefer areas in pristine condition. The invigorating and serene atmosphere is still intact and there is little to no rubbish or noise. Well, apart from the rustling of the tree or the cheerful melody of birds chirping.

The same goes for the crayfish; it too prefers sites that are undisturbed. There is no silt filling their homes in the boulders, trees and vegetation are there to create shade and shelter, and the water is calm and peaceful for the young crayfish to grow up in.

I had always thought of disturbance in the ecosystem as bad. But this is not necessarily the case. In some cases, disturbance does some good.

Elephants will open the canopy and disturb the soil which in turn creates opportunities for new species to be recruited. Beavers drastically alter forest habitats by building dams. These you could say are disturbances that ultimately do good for the ecosystem.

Unfortunately, disturbances occurring near crayfish habitats are less beneficial. In fact, increased runoff and sedimentation from processes such as agriculture and farming fill the cavities the young crayfish use with sediments and silt. And the crayfish have nowhere to hide.

In addition to this, the crayfish were also being fished. Now fishing isn’t a bad thing, in fact I encourage you to fish. But it must be done sustainably.

Sustainable fishing is done at a rate that enables the crayfish to regrow their population. As the crayfish have a slow growth rate this is easier said than done. A slow growth rate unfortunately means the effects of potential threats such as fishing are exacerbated. Unsustainable fishing has also contributed to the crayfish becoming smaller in size. You are unlikely today to see a crayfish larger than 2 to 3 kilograms.

Unfortunately, these threats combined meant the crayfish population drastically decreased. And whilst there may be a long way to go in terms of preventing further decline of the species, there is some hope.

The most obvious is that externships like the one that I was completing exist! Their aim is to educate young people like me about these issues and guide us in researching and finding solutions to freshwater issues. Which is fantastic!

Freshwater issues need more research and awareness. There are organisations out there researching and restoring habitat for the species and putting in place measures that protect the species from further decline.

There are also plenty of ways you can help, including spreading awareness of the Tasmanian giant freshwater crayfish. I encourage you, the reader, to tell your friends and family about this species. I would also encourage you, if you are ever in Tasmania (a great place to visit for nature lovers), to look at ways to help these creatures. Volunteering is a great place to start. Or to go and research the species for yourself and learn more about them! Or perhaps you will be inspired to complete an externship for yourself.

I have now completed the externship and had the amazing opportunity to present my research to a panel of experts, as well as share this information with my fellow externs and now you, the reader. This sharing of information may be the key to protecting the species for future generations.

Discover the conservation work of other externs from Colombia, the Maldives, and the United States in Nature Conservancy Magazine.