De Levende Natuur nummer 4 van 2021 (English summary)

Dutch crayfish: a tasty history 

Bram Koese  

The history of subsequent introductions of crayfish to The Netherlands is described. A total of ten crayfish species have been observed: one is the native species (the noble crayfish); nine are non-native species. Of these nine species, seven have become established in the Netherlands. 

Introducing: the Dutch crayfish 

Bram Koese  

All crayfish from the Netherlands, one native species and seven established non-native species, are described. It concerns the following species: noble crayfish Astacus astacus (native), narrow clawed crayfish Pontastacus leptodactylus (recorded since 1977), the signal crayfish Pacifastacus leniusculus (since 2004), spiny-cheek crayfish Faxonius limosus (since 1968), virile crayfish Faxonius virilis (since 2004), white river crayfish Procambarus acutus (since 2002), red swamp crayfish Procambarus clarkii (since 1985) and marbled crayfish Procambarus virginalis (since 2004). 

11 years of crayfish monitoring around Vinkeveen 

Piet Heemskerk & Bram Koese  

Changes in presence and abundance of crayfish species have been studied during an 11-year monitoring from 2010 to 2020 at the Vinkeveense Noordplas (Plas; daily monitored) and Demmerikse polder (weekly monitored) in the Dutch province of Utrecht. In the Demmerikse polder, the virile crayfish was the only crayfish present at the start of the monitoring. The first red swamp crayfish appeared in 2011 and became the dominant species from 2015 onwards. The virile crayfish declined gradually and almost disappeared from the catches by 2017 but showed a slight recovery in 2020. In the Plas, the red swamp crayfish was abundant in 2010 and remained dominant in the catches throughout the whole period, although numbers varied greatly between years. Two other species, the virile crayfish and spiny-cheek crayfish were caught only occasionally here. Because of differences in trapping intensities between the two areas, a statistical comparison between them was not performed. We assume that densities are higher in the Demmerikse polder. A test near the Plas site for 55 weeks with one trap weekly monitored, compared with the regular daily monitored trap showed an 8.5 times higher yield when daily monitored. At the same time, the yield per trap in the Demmerikse polder was roughly 2 times higher. Based on mortality data (higher for the virile crayfish), loss of chelae (higher for the red swamp crayfish) and differences in life history parameters, we cannot explain the decline of the virile crayfish in the Demmerikse polder by competitive exclusion with the red swamp crayfish. 

The expansion of the marbled crayfish in the Netherlands 

Pim Lemmers, Frank Spikmans & Bram Koese  

The first marbled crayfish in the Netherlands was recorded in 2004. The next was observed a decade later, in 2014. Since then the number of observations has increased significantly up to 14 sites in 2020, spread all over the country. The marbled crayfish became established in at least four sites of which one has been eliminated in 2020. Animals are mainly found in vegetation- rich, shallow parts of isolated waters. Other crayfish species were not present in these sites. The main introduction route of marbled crayfish is release by pet keepers. The EU Regulation 1143/2014 on Invasive Alien Species seems to have limited effect on the ongoing invasion of this species yet, as both trade and new introductions are still taking place. Swift actions, considered both cost effective and mandatory under EU legislation, are urgently needed to eliminate the species while this is still feasible. 

Determining the density of the red swamp crayfish: capture or eDNA? 

Annelies van de Craats, Kees van Bochove, Casper Cusell & Ernst Raaphorst  

For management purposes it is crucial to know the density of red swap crayfish in water systems. Currently the most popular method to obtain these numbers is with baited traps, but this method is labor intensive, requires permits, and is therefore costly. This study explores the use of eDNA as a substitute that is cheaper and requires no permits. Results show the method has potential but needs finetuning and some extra research to be a useful substitute for the bait-trap method. 

Management of the red swamp crayfish based on control factors 

Michiel Tangerman, Ellen Weerman, Casper Cusell, Annelies van de Craats, Lucas Doef, Mitzi Jans, Daan Lammers, Bob Brederveld, Luke Moth, Jouke Kampen, Rob van de Haterd & Annemieke Kooijman 

 The American red swamp crayfish has invaded waterbodies throughout the lower-Netherlands, although they are not evenly dispersed. Here we describe a comparative study on potential control factors for this invasive species, in order to develop management tools. Crayfish were caught and an estimation of the relative abundance was made in 138 different locations between the end of April and the end of May 2019. Simultaneously, for each location, a total of 153 different habitat variables were measured or provided by the manager of the area. A total of 2,617 red swamp crayfish were caught, with a great variety in density between locations. Population density was positively correlated with the external phosphorus load, possibly due to better water quality and more diverse fish communities. A similar effect was found for the width of the reed bank, where a wider reed bank resulted in fewer crayfish, probably because the extensive root systems reduced digging holes for protection. Lastly, crayfish abundance showed a positive correlation with the presence of invasive water plants, which are less sensitive to crayfish than native species. Therefore, reducing phosphorous input and enlarging reed banks could be beneficial in the reduction of crayfish populations. 

Water soldier cutting by the red American crayfish 

Suzanne Kanters, Maarten Schrama & Casper Cusell  

Water soldier is an ecosystem engineer that facilitates the mesotrophic succession towards highly diverse floating fens. The red swamp crayfish can play a key role in the disappearance of dense fields of water soldier by destruction of the leaves, both of submerged and floating individuals, as our experiment demonstrated. This indicates that this invasive crayfish might prevent the formation of new floating fens in The Netherlands. The red swamp crayfish destroys the leaves as well as the roots of both floating and submerged individuals of water soldier. 

Water quality affects the critical lobster density for crab shear survival 

Suzanne Kanters, Bob Brederveld & Casper Cusell  

The red swamp crayfish destroys the leaves of the ecosystem engineer water soldier, and thereby affects the survival of the species. Our study shows that the extent to which water soldier can survive in presence of the red swamp crayfish is dependent on the abiotic conditions of the water body. Under conditions that are optimal for water soldier, the critical crayfish density under which water soldier can survive is circa 1 individual per square meter. Increasing nutrient loads decrease the maximum crayfish density under which water soldier can persist. Crayfish removal leads to the regrowth of water soldier when trapping is performed with high intensity. However, when trapping is stopped, the crayfish population recovers quickly because of its high reproduction rate. 

Reduction of red swamp crayfish: first pilot in a peaty lake 

Winnie Rip, Jouke Kampen, Gerard ter Heerdt, Yannick Janssen, André Roeffen, Ron Beenen, Bert van Dijk & Hans Kampf  

After reduction of the phosphorous load of the Molenpolder, the aquatic ecosystem did not spontaneously recover. We hypothesized that the red swamp crayfish frustrated the recovery. In 2018 and 2019 the population of red swamp crayfish in a small lake in the Molenpolder was successfully reduced from, on average, 1.5 to 0.6 crayfish per m2 . Chlorophyll-a and suspender matter levels decreased and light climate improved. Submerged vegetation recovered significantly in comparison to the untreated areas of the Molenpolder. This suggests that crayfish indeed frustrated nature recovery and motivates for a sequel on a larger scale. 

Socio-economic costs and benefits of reducing red swamp crayfish 

Rob Nieuwkamer, Casper Cusell, Winnie Rip, Marit Meier, Bas van der Wal, Floortje Cieraad, Suzanne Kanters, Jouke Kampen & Saskia Guldemond 

A socio-economic cost-benefit analysis (SCBA) has been carried out for two areas to investigate whether the social benefits of reducing the population of the red swamp crayfish outweigh the costs. The SCBA shows that in two case study areas the social benefits appear to be considerably greater than the costs, despite remaining uncertainties in the prediction of the ecosystem condition. The various benefits and costs are explained, including the reliability of these results. 

By-catches of crayfish fisheries 

Ivo Roessink, Piet Heemskerk, Fabrice Ottburg, Jouke Kampen & Bram Koese  

The by-catches of crayfish fisheries in several individual projects are described. Fishing method and duration had an impact on the number of by-catches and their survival in the devices used. Although some by-catches seem inevitable, trap mesh size, the use of grids before the trap entrance and short fishing periods can reduce them considerably. 

Risks and Potential Management of Invasive Crayfish 

Pim Lemmers, Frank Collas, Ronald Gylstra, Ben Crombaghs, Gerard van der Velde & Rob Leuven  

Since the 1950s, nine alien crayfish species (7 from North America, 1 from Southeast Europe, 1 from East Europe/Asia) have been introduced in the Netherlands. Seven of these species have well-established populations. Their impact on biodiversity and ecosystem functioning mainly results from disease transmission, predation on native fauna, and fragmentation and consumption of aquatic plants. Burrowing activities of some crayfish species cause bank instability, increase risk of dike breaches in peatlands and enhance sedimentation in ditches and canals. First-line risk assessments show that all North American crayfish species have a high risk of impact on biodiversity, water safety and ecological status of water bodies. The risk of spread via interconnected rivers, canals and small watercourses is high for these species. Eradication of invasive crayfish populations in extensive and open networks of interconnected watercourses is not feasible. Six management strategies for control of alien crayfish species were assessed using criteria for cost-effectivity and societal support, and subsequently prioritized using an unweighted Multi Criteria Analysis. Feasible strategies for population control combine measures for enhancing robustness and resilience of ecosystems with crayfish trapping by commercial fishermen, water authorities and well-informed citizens.