top of page

Cray-zy Invaders

The Game:

Oh no - your river has been invaded by signal crayfish! It's your job to protect your local fish using a combination of big barriers, small barriers, and traps. You only have $150 to spend, so make sure you use it wisely...

Play the game below, and make sure to add your score to the leaderboard when you're done!

If you want to start thinking like a researcher, try to consider these questions while you're playing:

1. What is present in our rivers that might act as a barrier to crayfish?

2. If barriers can block crayfish, what effects might they have on the local species?

3. Why doesn't trapping capture all of the crayfish?

4. What methods might be better than trapping crayfish?

You can find the answers to these questions by clicking here, and some more information about the science behind the game here.

Please note - this game is an early version. If there are any problems just refresh the page and everything should work fine.

Enjoy the game? I would love to hear your feedback!

Top 10:

Baylor Elkins


1. What is present in our rivers that might act as a barrier to crayfish?

Believe it or not, there are a lot of things in our rivers that might act as barrier to crayfish! Broadly speaking, then can be categorised into natural and non-natural barriers.

Natural barriers include things like waterfalls or areas where the river flows very quickly.

Non-natural barriers include a wide variety of structures that humans have built in rivers - things such as dams, weirs, and culverts.


2. If barriers can block crayfish, what effects might they have on the local species?

This is a really important question for researchers to consider because barriers are known to have some very severe side effects on native species.

For species that migrate long distances (e.g. salmon and eels), barriers in rivers can stop them reaching the areas where they breed. This has led to significant declines in the populations of migratory species, and many conservationists are campaigning to get all these barriers removed from rivers.

Barriers can also lead to big changes in the local habitat. Constructing a barrier that limits the flow of water (e.g. a dam or weir) creates an area of still water upstream (i.e. a reservoir). Converting a flowing river into a pond or lake has severe consequences on species that depend on fast-flowing water.



3. Why doesn't trapping capture all of the crayfish?

The first reason why trapping doesn't capture all crayfish is simply a result of the trap design. Crayfish traps are legally required to have large gaps in the mesh (see picture below) to allow fish and other species to escape, but this also means that the smallest crayfish can escape!

The second reason relates to the personality of the crayfish (I know that sounds crazy but bear with me!). Behavioural experiments have shown that some crayfish are consistently bolder, more active, more exploratory and more aggressive than others, and scientists refer to these differences as 'personality'. ​Bolder individuals that explore the environment thoroughly are much more likely to find and enter the traps, whereas shy individuals are likely to stay away.




4. What methods might be better than trapping crayfish?

This is the golden question for crayfish researchers! Unfortunately, we still do not have a perfect answer...

Lots of different methods have been tried, including electrocution, poisoning, introducing fish that eat crayfish, and even draining the whole river and removing crayfish by hand! All of these can have big impacts on local wildlife, which stops them from being widely used.

If you have any good ideas please feel free to let me know!


Natural (left) and unnatural (right) barriers in UK rivers.


Migrating salmon can be blocked by barriers (photo credit: Derrick Mercer).


Example of a common crayfish trap.

The Science:

Invasive species (those that are transported around the world by humans and have negative ecological impacts) are one of the biggest problems for our lakes and rivers. One of the worst of these species in the UK is the American signal crayfish, which was introduced in the 1960s for food. They are bigger and more aggressive than our native crayfish, eat the young of native fish, and carry a disease that native crayfish are highly susceptible to. They also burrow into river banks, which increases bank erosion and has a negative impact on water quality.

Trapping is one of the most common methods of controlling invasive crayfish. In some areas, intensive trapping programs have removed thousands of signal crayfish from relatively small areas, and the biggest programs have removed over a million from a single lake. However, crayfish move very rapidly into areas where trapping has occurred, meaning trapping is often not very effective.

Barriers such as dams and weirs might be able to slow down crayfish as they try to move through river systems. There is still some uncertainty about how effective these barriers are, and researchers are currently trying to identify the types of barriers that are most effective, and what characteristics make crayfish more likely to pass these barriers. Recent studies have had mixed results, but these barriers have been used successfully for other invasive species (mostly fish).

For more information, see the "My Research" section of this website...

Give Me Your Feedback!
Rate UsPoorFairGoodVery goodExcellentRate Us

Thanks for submitting!

bottom of page