Attempted eradication of red swamp crayfish in Malta did not deliver – The Applied Ecologist


Feature image: Invasive Red swamp crayfish (Procambarus clarkii) © Alex Caruana

Author Alex Caruana shares learnings from their failed attempts to eradicate invasive crayfish from the Fiddien Valley in Malta.

Invasive alien crayfish, like the Red Swamp Crayfish (Procambarus clarkii), are a major threat to freshwater ecosystems all over the world. They have a significant impact on freshwater food webs, can carry diseases such as the Crayfish plague (Aphanomyces astaci) and Chytrid fungus (Batrachochytrium dendrobatidis), and can also alter the properties of freshwater bodies.

There have been attempts to eradicate crayfish, but such projects often suffer from lack of post project monitoring. In our recent paper in Ecological Solutions and Evidence, we return to a project two years after a major EU-funded eradication attempt to document its success.

Malta’s Largest Ecological Restoration Project

In 2016, invasive crayfish were first discovered in the Maltese Islands. Following this, the Maltese Government, funded by the European Regional Development Fund, carried out an ecological restoration project in the Fiddien Valley System, which is also known as Chadwick Lakes. The total cost of this restoration project was over €4.2 million.

In an effort to eliminate the invasive Red Swamp crayfish (Procambarus clarkii), the restoration project used mechanical excavators to remove sediment ranging from 0.5m to 3m deep from the stream bed. The sediment was then taken away from the stream to an abandoned and secluded quarry.

This work was done during the summer months when the Fiddien valley system’s ephemeral stream is dry.

Mechanical excavation in progress © Omar Camilleri

The aim was to not only remove individual live crayfish but also their burrow networks and any remaining eggs within the sediment. Over the course of the restoration project, a total of 53,000m3 of sediment and debris were excavated from the valley system.

Before and after mechanical excavation © Chadwick Lakes Trail & Alex Caruana, respectively

What we did

We conducted a survey to determine the distribution and relative abundance of crayfish in the Fiddien valley system three years after the eradication attempt. We placed three nylon crayfish traps per site, each baited with 50g of fresh mackerel to explore the relative abundance at 33 sites throughout the valley system.

Crayfish Traps - Alex Caruana
Nylon crayfish traps filled with Red Swamp Crayfish (Procambarus clarkii) © Alex Caruana

What we found

Despite the eradication attempt, crayfish were abundant throughout the area which had been supposedly restored and up to 4 km downstream.

invasive crayfish trapping locations
Invasive crayfish distribution and relative abundance (CPUE) across the Fiddien valley system recorded in 2023. The circles indicate trapping locations with the size of the circle and colour indicating the Catch Per Unit Effort (crayfish per trap night). The arrow indicates a small area of the restoration site which was inaccessible and where mechanical excavation did not occur

Our findings clearly show that mechanical excavation did not eradicate the invasive crayfish found within the Fiddien valley system. We speculate that there are two main reasons why this method might have failed:

1. It is highly likely that some live crayfish may not have been removed from the site during excavation by hiding within the rubble walls or stream banks and subsequently, which formed the basis of the post-eradication population.

2. Even if mechanical excavation was successful within the area where the mechanical excavation occurred, re-invasion could have occurred from outside the restored area.

Recommendations & Conclusion

Eradicating established populations of crayfish is challenging. Crayfish grow rapidly and reproduce quickly, and individuals can hide in hide in refuges. We do not recommend using mechanical excavation as an eradication protocol as it is very expensive and there is no evidence to suggest that it had any impact on the crayfish population.

Read the full article: “Mechanical excavation of wetland habitat failed to eradicate invasive American red swamp crayfish (Procambarus clarkii) in Malta” in Ecological Solutions and Evidence.

Plus, read the project report: “Assessment Of Red Swamp Crayfish Presence, Distribution And Abundance Within The Fidden Valley System” in Applied Ecology Resources and find out how you can publish your reports in AER as an ESE author.

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