Charting The Spread Of Invasive Crayfish

Image Credit: Nature.Catcher, CC BY-SA 2.0, Image Cropped

Distribution and establishment of the alien Australian redclaw crayfish, Cherax quadricarinatus, in the Zambezi Basin (2021) Madzivanzira, South et al., Aquatic Conservation, https://doi.org/10.1002/aqc.3703

The Crux

While some of us may love certain seafood, and are willing to carry that seafood all over the globe, often the local species are none to happy about it. Such is the case with the Australian redclaw crayfish, a rare example of Australia finally delivering back to the world that which it has received so many of – an invasive species. The redclaw is actually one of nine crayfish that has been introduced to mainland Africa, and if their record (and the records of other crayfish species) is anything to go by, it could mean everything from the spread of parasites and complete ecosystem upheaval to severe damage to the local fishing industry.

It’s crucial to figure out exactly where invasives have spread to, and how quickly they’ve done it. It allows managers and conservation experts in other areas to prepare, and to keep an eye out. This week’s team tried to determine how quickly the crayfish are spreading from their introduction point in the Zembezi River Basin.

What They Did

Over two years the team set out almost 1,200 crayfish traps at 68 different sites. At every site they measured physical and chemical characteristics of the local environment, like water temperature, visibility, and pH. All the crayfish that were captured were measured and weighed, and their sex was determined (where possible).

For each site, detection probability was calculated using the proportion of the traps that contained at least one crayfish. Catch per unit effort (CPUE) was used to determine the abundance of crayfish at a site, and was expressed as number of crayfish per trap per night. The team also calculated rate of spread, based on the distance from the initial introduction points that sites containing crayfish were found.

Did You Know: CPUE

Catch Per Unit Effort is a metric often used when studying aquatic species. The idea here is that the number of individuals that turn up in the net is a good indication of the number of species there are in the nearby area. It’s obviously not a precise conversion, so scientists always have to take into account different aspects of the species (is it smart enough to avoid traps, does it usually occur at low densities) or the environment (how easy is it to avoid the trap/net) when using CPUE. For more on gauging abundance from a snapshot of the population check out Sara Stoudt’s recent Stats Corner post.

What They Found

In two of the five larger regions surveyed (including the Okavango Floodplains, an ecologically sensitive region), no crayfish turned up at all. Yet along the Kariba Reservoir, the Kafue River, and the Barotse Floodplains, the crayfish were found in relatively similar abundances. Intersex crayfish (possessing both female and male genitalia) were found in every population.

With the distance from the closest introduction points, the team calculated that the rate of spread of the crayfish downstream was around 49 kilometres a year, and about 12 kilometres a year upstream.

The Okavango Floodplains region could be threatened by the crayfish within the next decade (Image Credit: Pavel Špindler, CC BY 3.0)

Problems?

I’ve studied similar problems myself in Norway, and find charting the spread of invasives fascinating. Which is probably why I was a bit disappointed at the lack of detail about the rate of spread. The paper mentions that due to the total inundation of the floodplains, the crayfish spread was calculated as the crow flies, but I would have loved to see different spreading scenarios based on the potential fluctuations in climate that could be seen over the coming decade, as well as the likely impact of immediate action to eradicate the crayfish. Having said that, it’s A LOT to fit into a paper.

So What?

While it may not seem important, the presence of intersex crayfish at each site is worrying. As the paper lays out, their behaviour and offspring tend to increase the reproductive potential of the population, and could lead to much quicker establishments and population growth in the region.

Studies like this are proactive, and essential in early stages of invasion. It’s much harder to get rid of a species than to stop it entering somewhere, so prevention is key, and knowing when species are likely to hit and where they’re likely to enter can make all the difference.


Dr. Sam Perrin is a freshwater ecologist who completed his PhD at the Norwegian University of Science and Technology and still has an odd fear of getting nipped by crayfishes (childhood trauma DON’T ASK). You can read more about his research and the rest of the Ecology for the Masses writers here, see more of his work at Ecology for the Masses here, or follow him on Twitter here.

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