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All issues No 426 (2025) Knowl. Manag. Aquat. Ecosyst., 426 (2025) 22 Full HTML
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Knowl. Manag. Aquat. Ecosyst.
Number 426, 2025
Management of habitats and populations/communities
Article Number 22
Number of page(s) 12
DOI https://doi.org/10.1051/kmae/2025017
Published online 09 September 2025

© M. Harwood et al., Published by EDP Sciences 2025

Licence Creative CommonsThis is an Open Access article distributed under the terms of the Creative Commons Attribution License CC-BY-ND (https://creativecommons.org/licenses/by-nd/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. If you remix, transform, or build upon the material, you may not distribute the modified material.

1 Introduction

The ongoing spread of invasive non-native species (sensu Soto et al., 2024) is a major driver of biodiversity loss and economic strain globally (Roy et al., 2023). Invasive non-native species have negative impacts on their new environments, and it is estimated that they cost the British economy £4 billion each year (Eschen et al., 2023). Of these, invasive crayfish, can cause significant damage to irrigation structures and banks of rivers and lakes through burrowing activity and may alter community composition through predation, competition and ecosystem engineering (Holdich, 1999; Bubb et al., 2004; O'Hea Miller et al., 2024). In the United Kingdom (UK), the signal crayfish (Pacifastacus leniusculus) alone, was attributed to have caused US$15.3 million between 2000–2020 (Kouba et al., 2022). There is less information on other invasive crayfish, and their environmental and economic impacts, including the focal species of this study, the narrow-clawed crayfish (Pontastacus leptodactylus). The narrow-clawed crayfish is a data deficient invasive non-native species considered native to the Ponto-Caspian region (Skurdal and Taugbøl, 2002). They are a large freshwater crayfish species found in both lentic and lotic environments but considered to be still-water specialists (Bök et al., 2013). Invasive populations are widespread throughout Europe and are predicted to spread through central-western Europe under future climate scenarios (Hodson et al., 2024).

There are 16 main water and sewage companies supplying the UK with water resource management, including 273 major reservoirs which account for 90% of total water storage capacity (Williams et al., 2010; Durant and Counsell, 2018). These reservoirs can indirectly act as sources, sinks and stepping stones for the spread of invasive non-native species (Havel et al., 2005). Reservoirs can be easily invaded due to regulated flow, low biotic diversity, and high anthropogenic influence (Moyle and Light, 1996; Havel et al., 2005; Clavero et al., 2013). Some reservoir sites also have high numbers of anthropogenic pathways, such as recreational activities including boating, kayaking and angling. Where they are required, compensation flows from reservoirs allows the release of water from a reservoir to the connected waterbodies downstream. In addition to compensation flows, the Reservoir Safety Act 1975 requirements include operating scour valves to maintain the safety at reservoirs. These are key biosecurity issues as a large volume of water is released into connecting waterways. The Wildlife and Countryside Act 1981 outlines responsibilities and liabilities for water companies around managing invasive non-native species at their sites. The Invasive Alien Species (Enforcement and Permitting) Order 2019 also introduced penalties for the spread of ‘species of special concern’.

The General Invasion Curve illustrates the process of invasion in a given area to categorise threat and determine possible interventions (Harris et al., 2018). The key aim of the graph is to establish the threat of an invasive population on a national scale, here we apply it on a more local scale, looking at a single site. When a waterbody is invaded, management actions are often working on a time lag between invasion arrival, detection, and impact. Assessing the stage of invasion must be prioritised and determined rapidly to inform appropriate decisions following the General Invasion Curve (Fig. 1; Harris et al., 2018) which guides mitigation measures along stages related to the progression of the invasion: 1) prevention, 2) eradication, 3) containment and 4) asset protection (Harris et al., 2018). Further frameworks provide summarises of management options relative to the stage of invasion, for example Robertson et al. (2020). Failure to enact appropriate management may result in further invasive non-native species spread and increased cost of management interventions (Cuthbert et al., 2022). Appropriate management is relative to the stage of invasion of the population, and can include different key forms of management, including pathway management and population suppression (Robertson et al., 2020). It is important to note that these frameworks and curve can be applied at different spatial scales, thus whilst assessing one site within this study, when considering management options, it is imperative to understand the overall spread and impact of the species, especially at a catchment/regional scale, with connected waterbodies potentially facilitating further spread in the absence of anthropogenic pathways.

Boshaw Whams, a Yorkshire Water owned reservoir in Holmfirth, West Yorkshire, United Kingdom, contains the only known population of invasive narrow-clawed crayfish in Yorkshire. This population was illegally introduced around 2014, and local anglers started to report them as a nuisance in 2019 (pers. comm Huddersfield Angling Club). Although the impact that other invasive non-native crayfish have on narrow-clawed crayfish within their native range has been studied (Hudina et al., 2016; Lele and Pârvulescu, 2017), Pontastacus leptodactylus is a data deficient species regarding its invasion dynamics and ecological impacts. Thus, given the data deficiency of the species' invasion ecology and the likelihood of further spread (Hodson et al., 2024) our overarching aim was to rapidly assess the stage of the invasion on the Generalised Invasion Curve to advise management at this study site. In doing so, we tested two bait types and compared traditional (trapping) monitoring methods with Remote Underwater Video (RUV) and baited RUV (BRUV) for efficiency in detecting presence/absence and abundance dynamics of narrow-clawed crayfish throughout the year at Boshaw Whams reservoir. We then used mark-recapture to estimate population size and characterise reproductive ecology over a year. Finally, we provide comments on the economic resources needed to assess a crayfish invasion in a small reservoir and how to optimise invasion related costs.

thumbnail Fig. 1

General Invasion Curve for invasive non-native species, there are four stages on the curve; 1) Prevention, when the species is absent from the asset, 2) Eradication, when populations are small and isolated, 3) Containment, when populations are rapidly increasing, and 4) Asset protection, when there is a widespread population.

2 Methods

2.1 Study site

Boshaw Whams (53°32′N; 001°46′W) (Fig. 2) is a freshwater reservoir, and is used for angling and boating recreational activity, located near to Holmfirth in Kirklees, West Yorkshire, United Kingdom. The reservoir is situated at an altitude of 300 m and has a circumference of ∼ 900 m and a maximum depth of ca. 6 m. The reservoir is stocked with triploid rainbow trout (Oncorhynchus mykiss) and brown trout (Salmo trutta) monthly from March to July with a final stocking in September, on an annual basis, with approximately 200 fish stocked per event (Huddersfield Angling Club, pers. comm.). There is a wooden jetty along one side of the reservoir with the remaining sides consisting of a grass bank with a gradual incline above the water and cobbles below. A population of native white-clawed crayfish has been reported at Armitage Bridge (Fig. 2) WoC ID: WK9180 (for A. pallipes, observation date: 2022) (Ion et al., 2024) approximately 10 km downstream from Boshaw Whams reservoir.

Twice a year a scour valve test is conducted to ensure that the reservoir is managed in compliance with the Reservoir Safety Act (1975). During a scour valve test, water is released from the reservoir into Upper House Dike, which is 250 m long and then flows directly into Dean Dike. Dean Dike is a small stream that runs through woodland, Strahler order 2 from Boshaw Whams reservoir. Dean Dike is 1.5 m wide at the widest point and flows for 1 km before entering Lower Mill Pond. The banks consist of woody vegetation and are bordered by farmlands.

thumbnail Fig. 2

Map showing Boshaw Whams Reservoir location and downstream waterbodies as well as closest population of native white-claw crayfish (Austropotamobius pallipes) species.

2.2 Bait comparison

Method comparison between bait types and equipment types were completed to determine best operating procedures for detecting crayfish and method efficiency. This data was also used to describe the population dynamics in relative abundance over the course of a year. To compare bait types, crayfish trapping was conducted between October 2022 and February 2024 using collapsible fladen crayfish traps (570 mm × 290 mm, 25 mm mesh size). Bait comparison tests were conducted within a subsection of these surveys between October 2022 and June 2023, when both dry meat flavoured dog food and wet poultry flavoured cat food were used in the traps. Up to 49 traps were deployed per survey, all traps were deployed for 18 h overnight. Half of the traps were baited with dry food and deployed on one side of the jetty and the other half were baited with wet food and deployed on the other side of the jetty. The side of the jetty the bait type was deployed on was alternated monthly. The jetty split the two bait types by approximately ten meters, and the olfactory dispersal of bait plumes was not enough to cause overlaps. The catch per unit effort (CPUE) i.e., − total catch per trap per night was determined for both bait types. All crayfish caught were removed from the waterbody and either euthanised with an overdose of MS-222 or brought to the facilities at the University of Leeds. A paired t-test was used on square root transformed CPUE to assess differences between in CPUE wet and dry bait types. Olfaction plays a key role in crayfish foraging (Willman et al., 1994) thus we predicated that wet bait would have a higher CPUE due to the scent diffusing more rapidly.

2.3 Gear comparison

We test the potential of Remote Underwater Video (RUV) and Baited Remote Underwater Video (BRUV) surveys as an alternative to trapping (Fig. 3).

B/RUV surveys were conducted once a month between March 2023 and February 2024. The BRUV was baited using wet poultry flavoured cat food. Both cameras were deployed at separate locations within the reservoir, not simultaneously, for 1 hour, using a GoPro HERO10 Black (GoPro, Inc., USA) recording at 30 frames per second with 1080p resolution. Cameras were deployed between 12:00 and 14:00 before traps as narrow clawed are active during both day and night (Skurdal and Taugbøl, 2002) and this allowed for suitable light levels for video analysis. They were also deployed at different times to avoid interference between gear types. All footage was manually reviewed by the same observer noting the maximum number of individuals present in frame throughout the deployment (MaxN).

Monthly trapping was conducted as describe in Section 2.2. The CPUE for trapping was calculated by taking the total number of crayfish caught in traps and dividing it by the total number of traps (number of crayfish per trap). To account for the different time of deployment taken for each gear we also compared the detection probability of each gear to accurately compare gear. CPUE was calculated and compared between each of the gears to assess the effectiveness of B/RUVs at detecting crayfish to traditional (trapping) methods. Detection probability of each gear was calculated Eq. (1). Due to high number of zero observations delta-X corrections were applied to normalise the dataset (Madzivanzira et al., 2021; Nawa et al., 2024), we therefore used ANOVA to compare CPUE and detection probability between gear. A chi-squared test was used to establish if there was a relationship between the number of crayfish caught in traps and the detection probability of either the RUV or BRUV.

Detection  Probability=Deployments  observing  crayfishTotal  Deployments(1)

thumbnail Fig. 3

A) Photograph of the Remote Underwater Video (left) and Baited Remote Underwater Video (right) rigs used in the surveys and B) a standard crayfish trap used throughout the surveys (570 mm × 290 mm, 25 mm mesh size).

2.4 Mark-recapture survey

A mark-recapture experiment was conducted in March 2023 following the methods of Guan (1997) and using trapping protocols described previously to obtain an estimate of the total population size of the crayfish present within the reservoir. An important assumption for mark-recapture surveys is that there is no immigration or emigration (Rabeni et al., 1997). Boshaw Whams is an isolated reservoir with little external influence and no other populations of narrow-clawed crayfish locally. Forty-nine traps were deployed daily over a three-day period. Each trap was checked at 24 hours intervals and all crayfish present within the traps were removed, and morphometric measurements taken for all crayfish, measuring the carapace length (mm), carapace width (mm), total length (mm), claw length (mm) and mass (g). After measuring, a triangular segment was cut from the tail, changing the segment of tail that had a segment removed each day so that the date of capture could be identified on any recaptures. Each marked crayfish was returned to the waterbody in a similar location as to where it was captured. Tail marks on crayfish are distinct and have durability for between two and three moults (Nowicki et al., 2008).

All crayfish captured during the monthly trapping events (total 10 months) conducted after the mark-recapture survey were also checked for marked tails. Chapman's corrected mark-recapture formula (Eq. 2) with a normal approximation for 95% confidence interval was then used to estimate the population size to help mitigate bias under low recapture numbers.

N=((M+1)*(C+1))(R+1)−1(2)

where N is the estimated population size, M is the number of marked individuals, C is the total captured after marking and R is the number recaptured.

2.5 Population dynamics

Monthly trapping was used to gain an understanding of the population dynamics at the reservoir. Traps were deployed as described in Section 2.2. After June 2023 all traps were baited solely with wet cat food. All traps were deployed in the afternoon and left overnight for 18 hours. Traps were retrieved, noting the number of individual crayfish in each trap, the sex of each crayfish and if females were berried. Morphometric measurements were taken for all crayfish as described in Section 2.4. The air temperature was recorded during each survey to compare how season and temperature effected the population dynamics and CPUE.

A chi-squared test was conducted to assess the male/female sex ratio in the trapped individuals. Two linear regressions F-tests with were conducted to establish whether number of crayfish removed over time and air temperature affected the trapping CPUE, all model assumptions were checked via QQ plots.

2.6 Economic costs

In order to compare the economic costs of the different survey methods, the economic costs were calculated using the initial equipment and startup costs in combination with the annual cost to conduct monthly surveys (S3). Economic costs of assessing the population dynamics of narrow-clawed crayfish at one reservoir site were calculated first by identifying the equipment needed for surveys using; 1) traps, 2) RUV, 3) BRUV and the initial startup costs. The monthly cost of each survey was then calculated, accounting for travel to and from the site, car hire, fuel costs (one journey required for camera surveys but two for trapping surveys), bait, and surveyor time (at a minimum rate of £12/ hour). The values presented here were typical for the survey undertaken by the University of Leeds and fuel and distances are calculated appropriately, distance from the University of Leeds to Boshaw Whams is 48 km. The initial costs and monthly costs were then scaled up to establish the total costs for 12 months' worth of surveys for each of the three different gears. The costs for each method are assessed and presented as either initial one-off costs or continuous monthly costs.

3 Results

3.1 Bait comparison

For baited traps, there was no significant difference between the catch per unit effort (CPUE) obtained by the two bait types, dry dog food and wet cat food (t = 1.23, df = 8, p = 0.25).

3.2 Gear comparison

There was no significant difference between the CPUE of any of the three gears (trapping, RUV, BRUV) (ANOVA, F = 0.979, p = 0.388; Fig. 4, Tabs. 1 and 2). There was also no significant difference between the detection probability of any of the three gears (trapping, RUV, BRUV) (ANOVA, F = 0.066, p = 0.936; Fig. 5, Tabs. 1 and 2).

There was no relationship between the number of crayfish caught in traps and the detection probability of either camera method (χ2 = 1.550, df = 1, p = 0.2131). The cumulative removal of crayfish from the waterbody after each trapping survey did not bias the results.

Table 1

The average Catch Per Unit Effort (CPUE) and detection probability of each crayfish surveying methods.

thumbnail Fig. 4

Monthly Catch Per Unit Effort (CPUE) of narrow-clawed crayfish (Pontastacus leptodactylus) for Baited Remote Underwater Video (BRUV), Remote Underwater Video (RUV) and trapping surveys. CPUE for BRUV and RUV is the sum of MaxN (maximum number of individuals in frame at a single moment in deployment) divided by the number of deployments for each survey. CPUE for trapping is the total number of crayfish caught divided by the number of traps deployed for each survey.
Table 2

Monthly Catch Per Unit Effort and Detection Probability and water quality parameters for each surveying method. Trapping CPUE is the total number of crayfish caught in traps, divided by the total number of traps deployed. Both RUV and BRUV CPUE is the sum of MaxN for all B/RUV deployments made during a survey (maximum number of individuals on screen throughout a deployment) divided by the total number of camera deployments for each gear.

thumbnail Fig. 5

Mean detection probability for Baited Remote Underwater Video, Remote Underwater Video and trapping of narrow-clawed crayfish (Pontastacus leptodactylus). Error bars are the standard error of detection probability for each method.

3.3 Mark-recapture experiment

A total of 286 crayfish were marked over a three-day trapping period (Tab. 3). Over the six-month period following the initial marking survey 384 crayfish were trapped, ten of these were recaptured marked individuals (3.5% of those marked). The current population size of narrow-clawed crayfish at Boshaw Whams reservoir is estimated at 10,045 ± 5602 (95% CI) individuals in a 50,000 m2 reservoir, equating to approximately 0.20 crayfish per meter squared.

The distinctiveness of marks on crayfish varied over time. Markings on the tails of individuals recaptured closer to the initial experiments were clear, but after a six-month period the markings on the tails of some individuals became fainter and less distinct (S1) after potential healing and moulting. All captured individuals were thoroughly checked to identify any potential markings, and we are confident that no marked individuals recaptured were missed.

Table 3

Mark-recapture experiment summary of narrow-clawed crayfish (Pontastacus leptodactylus) at Boshaw Whams reservoir. Three-day trapping and marking period highlighted in green.

3.4 Population dynamics

We deployed a total of 682 traps at Boshaw Whams reservoir during this survey. In which, 756 crayfish were captured, of these 490 were males, 266 females and 30 of which were berried. Carapace length ranged from male 22.5–99.0 mm and females: 23.8–79.1 mm (Fig. 6). Traps caught significantly more males that females (χ2 = 66.370, df = 1, p < 0.05), and these trends are visible when comparing the monthly CPUE (Fig. 6).

Females carrying young were recorded between February and April (Tab. 4). The range of carapace lengths for berried females was 44.8–79.1 mm (Fig. 6).

There was no relationship between cumulative number crayfish removed and trapping CPUE (F(1,13) = 0.2356, p = 0.6355) and air temperature did not have a relationship with CPUE (F(1,13) = 1.273, p = 0.2796).

thumbnail Fig. 6

Histograms comparing the monthly sex ratios and size distributions with catch per unit effort.
Table 4

Monthly trapping and crayfish morphometrics on the Narrow Claw Crayfish (Pontastacus leptodactylus) at Boshaw Whams Reservoir.

3.5 Economic costs

Trapping had the lowest initial set-up cost, but the highest survey costs and the highest 12-month survey cost (Tab. 5) compared to RUV and BRUV.

Table 5

Total cost for each method for monthly surveys over 12 months of surveying, where method is deployed once a month.

4 Discussion

This study aimed to rapidly assess the population ecology of narrow-clawed crayfish (Pontastacus leptodactylus) at Boshaw Whams reservoir. Here we provide the first year-round monitoring data of population dynamics of a narrow-clawed crayfish invasion in the UK and report a large and established population at Boshaw Whams reservoir. All tested monitoring methods (traps and B/RUVs) had comparable efficacy in detecting narrow-clawed crayfish in this reservoir. Narrow-clawed crayfish are highly abundant in this reservoir which may have masked the differences in surveying methods. We recommend a multi-method approach to monitoring as well and suffest that biodiversity managers assess their capacity for monitoring (cost, people time, accessibility) and specific information needs to inform the best approach to designing and monitoring campaign for narrow-clawed crayfish.

4.1 Bait comparison

We predicted that due to wet food solubility that it would have a higher efficacy in attracting crayfish (Willman et al., 1994) but in this instance there was no difference in CPUE between bait type. There is a high degree of variation in bait efficiency with some reporting no difference (Somers and Stechey, 1986), whilst other studies have concluded both that wet bait achieved higher CPUE (Beecher and Romaire, 2010) or that dry bait achieved a higher CPUE (Rach and Bills, 1987). We opted to use wet cat food for the remainder of the surveys due to the ease of use in distributing into mesh pouched to place in the traps. However, it is likely that the amino acid content of the bait, scent plume diffusion and physical properties of the water body all mediate efficacy (Westerberg and Westerberg, 2011).

4.2 Method comparison

All trialled methods had comparable detection probability therefore in relatively clear, still-waters we would recommend any of the methods for detecting presence/absence of invasive crayfish populations. To assess relative abundance, we recommend a multi-method approach as traditional trapping as the MaxN value from the B/RUV setups provides a conservative estimate of abundance whereas trapping can provide more nuanced information regarding population demographics. Our results are similar to other attempts to compare novel monitoring methods to traditional approaches in freshwaters in that there is not a one size fits all monitoring method, but rather, specific methods may be better suited dependent on the question. For example, Castañeda et al. (2020) found that underwater cameras were more time consuming than snorkel surveys but that cameras were better for detecting rare freshwater fish species. Thus, choice of monitoring methods depends on what information the surveyor is aiming to achieve.

Trapping can be a time-consuming method which may or may not require entering waterbodies. Furthermore, it requires a minimum of two days of staff time to conduct a single survey, which can limit the total number of surveys that can be conducted in a fixed timeframe. Cameras therefore provide an alternative survey method which, although initially costly, can reduce the amount of labour needed to complete the surveys and allow video data to be assessed at any time in the future, thus removing immediate time constraints − especially in remote locations (Ebner et al., 2014; Castañeda et al., 2020; Broom et al., 2023). Video data still requires human time to assess but developments in artificial intelligence image recognition may remove this barrier (Siri et al., 2024).

Lighting and water turbidity can limit the effectiveness of RUV as this hampers the capacity to obtain usable video (Ebner et al., 2014; King et al., 2018), this was not an issue for our surveys at Boshaw Whams as the nature of the reservoir meant that underwater visibility averaged >1 m for all surveys. Crayfish species are largely nocturnal thus the need for overnight sampling has been a barrier to developing rapid surveying methods, besides eDNA approaches, due to light level limitations (Fanjul-Moles and Prieto-Sagredo, 2003; Mallet and Pelletier, 2014). Narrow-clawed crayfish show high feeding intensity during both day and night (Skurdal and Taugbøl, 2002) and were regularly observed on the cameras. Studies must account for these limitations when designing and deploying RUVs for crayfish monitoring to avoid wasted effort and costs.

4.3 Mark-recapture experiment

The mark-recapture experiment indicated that there were over ten-thousand narrow-clawed crayfish individuals in the reservoir (one per 5 metres squared (0.20 crayfish/m2)). Considering only 10 individuals were recaptured, representing 3.5% of all trapped individuals after the mark-recapture event, this data is likely unreliable but offers an indication of the extremely high density of crayfish. This is reinforced by the large confidence intervals post Chapman correction implementation. There is limited information about the densities of narrow-clawed crayfish found in waterbodies in their native range. However, in comparison to other invasive crayfish populations in lotic waterbodies (signal crayfish; Pacifastacus leniusculus) in the UK, they are much lower (3-20/m2 (Guan and Wiles, 1997), 20/m2 (Bubb et al., 2004), 21-110/m2 (Chadwick et al., 2021)).

Trapping and marking can have a short-term negative effect on crayfish recapture chances (Nowicki et al., 2008), with marked individuals potentially having a negative association with traps and avoiding them. Individuals found over six months after the marking event only had faint markings (S1). This is supported by other studies which found that markings from mark-recapture events tend to last two to three moult cycles (Nowicki et al., 2008).

4.4 Population dynamics

The narrow-clawed crayfish population in Boshaw Whams is established, detectable throughout the year and reproducing at small sizes. The number of male narrow-clawed crayfish caught was 1.84 times higher than females. This sex bias may be a result of the aggressive behaviour of larger males resulting in females and smaller males avoiding the traps (Momot and Gowing, 1977; Holdich, 2002; Hein et al., 2007; Hilber et al., 2020). Except for January 2023, crayfish were caught each month throughout the year with an increase in CPUE between March and May (2023), which was also when the first observations of berried females occurred. In their native range berried females can be found between December and May (Cìlbìz, 2020) and reproduction generally occurs in cold water between 7–12 °C (Farhadi and Harlıoglu, 2018). In our study most berried females were caught during these months and a small number of berried individuals were observed in subsequent months. In their native range females reach sexual maturity at 64.3 mm total length but spawning only occurred for crayfish with a total length ≥ 82 mm (Berber and Mazlum, 2009). This is smaller than the total length of the smallest berried individual found in the present study (carapace length: 44.8 mm, total length of 90.8 mm), suggesting that reproduction happens at smaller sizes in the invasive range.

Our monitoring campaign did not appear to have had an impact on the population size at Boshaw Whams reservoir. Despite removing over 700 crayfish there was no negative trend in the monthly CPUE which indicates that removal of crayfish did not bias our results. This is due to the relatively low monthly trapping effort (Peay, 2001; Green et al., 2018).

4.5 Economic costs

The largest cost that goes into crayfish sampling is staff-time (Peay, 2004). We recommend that surveyors first determine their data needs such as whether rapid detection is the aim or whether population level information is needed at each stage of assessment. Beyond this, accessibility issues, economic and time constraints can also influence choice of method. In this scenario, i.e., a high-density invasion in clear water, all three methods were able to detect crayfish, however, more granular information on breeding period, length frequency, sex ratio were only available with trapping. Thus, a staggered approach to monitoring where cameras are first deployed and this data used to inform trapping may be the most effective.

5 Conclusion

Boshaw Whams reservoir should be considered in the ‘Containment’ or ‘Asset Protection’ stage of the General Invasive Curve, which means there are large populations within the waterbody, but we cannot confirm that they have colonised the entire area of it. The population of narrow-clawed crayfish at Boshaw Whams reservoir is estimated to be 10,045, and breeding activity was seen between February and April. Potentially this could mean that the opportunity to eradicate this population has passed and eradication is no longer viable, however the feasibility of eradication would need to be assessed on a site and catchment specific scale, beyond the scope of this study. Narrow clawed crayfish ranges are expected to expand and shift with predicted climate change therefore mitigation of further spread is required urgently (Hodson et al., 2024).

Acknowledgements

We thank Yorkshire Water for funding this research. MH acknowledges support from the John Henry Garner Scholarship and APEM. JS acknowledges funding from UKRI Future Leaders Fellowship [Grant/Award Number: MR/X035662/1].

Supplementary Material

Sup. 1. Triangular notch marking used in mark-recapture survey and marks remaining on individuals captured on 13/10/2023, six months after the initial marking experiment.

Sup. 2. Boxplot showing the median and upper and lower quartiles for the Catch Per Unit Effort of surveys using cat food and dog food as bait. CPUE was calculated by dividing the total number of crayfish caught by the number of traps deployed for each survey and each bait type.

Sup. 3. Breakdown of initial set up costs for each method, per unit of method. Initial costs were calculated per trap or per camera unit.

Sup. 4. Boxplot showing the monthly variations in sizes between males and females.

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Cite this article as: Harwood M, Stebbing PD, Dunn AM, Cole ZK, Bradbeer SJ, Aston B, South J. 2025. Rapid assessment of population dynamics and monitoring methods for invasive narrow clawed crayfish Pontastacus leptodactylus in a freshwater reservoir. Knowl. Manag. Aquat. Ecosyst., 426, 22. https://doi.org/10.1051/kmae/2025017

All Tables

Table 1

The average Catch Per Unit Effort (CPUE) and detection probability of each crayfish surveying methods.

In the text
Table 2

Monthly Catch Per Unit Effort and Detection Probability and water quality parameters for each surveying method. Trapping CPUE is the total number of crayfish caught in traps, divided by the total number of traps deployed. Both RUV and BRUV CPUE is the sum of MaxN for all B/RUV deployments made during a survey (maximum number of individuals on screen throughout a deployment) divided by the total number of camera deployments for each gear.

In the text
Table 3

Mark-recapture experiment summary of narrow-clawed crayfish (Pontastacus leptodactylus) at Boshaw Whams reservoir. Three-day trapping and marking period highlighted in green.

In the text
Table 4

Monthly trapping and crayfish morphometrics on the Narrow Claw Crayfish (Pontastacus leptodactylus) at Boshaw Whams Reservoir.

In the text
Table 5

Total cost for each method for monthly surveys over 12 months of surveying, where method is deployed once a month.

In the text

All Figures

thumbnail Fig. 1

General Invasion Curve for invasive non-native species, there are four stages on the curve; 1) Prevention, when the species is absent from the asset, 2) Eradication, when populations are small and isolated, 3) Containment, when populations are rapidly increasing, and 4) Asset protection, when there is a widespread population.
In the text
thumbnail Fig. 2

Map showing Boshaw Whams Reservoir location and downstream waterbodies as well as closest population of native white-claw crayfish (Austropotamobius pallipes) species.
In the text
thumbnail Fig. 3

A) Photograph of the Remote Underwater Video (left) and Baited Remote Underwater Video (right) rigs used in the surveys and B) a standard crayfish trap used throughout the surveys (570 mm × 290 mm, 25 mm mesh size).
In the text
thumbnail Fig. 4

Monthly Catch Per Unit Effort (CPUE) of narrow-clawed crayfish (Pontastacus leptodactylus) for Baited Remote Underwater Video (BRUV), Remote Underwater Video (RUV) and trapping surveys. CPUE for BRUV and RUV is the sum of MaxN (maximum number of individuals in frame at a single moment in deployment) divided by the number of deployments for each survey. CPUE for trapping is the total number of crayfish caught divided by the number of traps deployed for each survey.
In the text
thumbnail Fig. 5

Mean detection probability for Baited Remote Underwater Video, Remote Underwater Video and trapping of narrow-clawed crayfish (Pontastacus leptodactylus). Error bars are the standard error of detection probability for each method.
In the text
thumbnail Fig. 6

Histograms comparing the monthly sex ratios and size distributions with catch per unit effort.
In the text

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