A review of the use of ark sites and associated conservation measures to secure the long-term survival of White-clawed crayfish Austropotamobius pallipes in the United Kingdom and Ireland

eDNA monitoring as a tool for evaluating the reintroduction of Austropotamobius pallipes after a crayfish plague outbreak

The crayfish plague, a severe disease caused by the oomycete Aphanomyces astaci, is responsible for most population declines of susceptible crayfish in Europe. This pathogen has been devastating native populations of Austropotamobius pallipes since the 1970s in the Iberian Peninsula. In this study, we report a massive mortality event in one of the most important Spanish populations of A. pallipes. We aimed to: (i) identify the cause of the mortality, and (ii) evaluate the reintroduction viability of the species. Over the course of six months, we used environmental DNA (eDNA) and traditional trap-based methods to detect the presence of A. astaci or of native or invasive crayfish in order to evaluate the reintroduction viability of A. pallipes to the affected population. We did not capture any live crayfish or detect the presence of A. astaci in the reservoir water during the six months following the mass mortality event. Our analyses indicated that it was feasible to initiate a reintroduction program at the site, which will continue to be monitored for three to five years and will help improve the conservation status of A. pallipes.

Using eDNA to simultaneously detect the distribution of native and invasive crayfish within an entire country

The introduction of invasive crayfish has led to a decline of many European native species of crayfish across their range. In this study, novel duplex assays for all crayfish occurring in Switzerland were developed. We aimed to identify the distribution of the seven species using a traditional trap surveillance method as well by collecting water samples to detect eDNA by species-specific quantitative real-time PCR. We reveal our overall experience in finding optimal field and laboratory techniques to discover the distribution and abundance of native and invasive species in order to enhance knowledge of early invasive species invasion and highlight important pockets of populations where native species remain, for implementation of conservation strategies. Using eDNA, important populations of native noble and white-clawed crayfish were revealed in multiple waters across various cantons. The successful identification of native and invasive crayfish species in Switzerland using eDNA can be applied to future nationwide projects. This method which has the ability to detect all species simultaneously across an entire country, will allow an improvement in freshwater crayfish conservation management.

Seasonality, DNA degradation and spatial heterogeneity as drivers of eDNA detection dynamics

In recent years, eDNA-based assessments have evolved as valuable tools for research and conservation. Most eDNA-based applications rely on comparisons across time or space. However, temporal, and spatial dynamics of eDNA concentrations are shaped by various drivers that can affect the reliability of such comparative approaches. Here, we assessed (i) seasonal variability, (ii) degradation rates and (iii) micro-habitat heterogeneity of eDNA concentrations as key factors likely to inflict uncertainty in across site and time comparisons. In a controlled mesocosm experiment, using the white-clawed crayfish as a model organism, we found detection probabilities of technical replicates to vary substantially and range from as little as 20 to upwards of 80% between seasons. Further, degradation rates of crayfish eDNA were low and target eDNA was still detectable 14-21 days after the removal of crayfish. Finally, we recorded substantial small-scale in-situ heterogeneity and large variability among sampling sites in a single pond of merely 1000m² in size. Consequently, all three tested drivers of spatial and temporal variation have the potential to severely impact the reliability of eDNA-based site comparisons and need to be accounted for in sampling design and data analysis of field-based applications.

Effects of Different Diet Types on Growth and Survival of White-Clawed Crayfish Austropotamobius pallipes in Hatcheries

Developing an optimal diet for rearing endangered white-clawed crayfish Austropotamobius pallipes is important for captive breeding success prior to wild release. Four ex situ, 40-day experiments assessed survival and growth of crayfish fed different treatment diets. Two experiments (A and B) were undertaken with hatchlings, to determine if live food was an essential dietary component during the first few weeks after hatching. The second set of experiments (C and D) were undertaken with juvenile (60-day-old) A. pallipes, to determine an optimal diet after the initial critical feeding stage. In experiment A, we fed hatchlings: i) live Artemia nauplii + plankton (Live + P); ii) decapsulated Artemia cysts + plankton (Cyst + P) or iii) decapsulated Artemia cysts + plankton encapsulated in agar gel (Gel + CP). Survival and growth was significantly greater with Live + P than with the other two diets. In experiment B we compared Live + P with commercially available feeds by feeding hatchlings: i) live Artemia nauplii + Australian pellet (Live + Aus); ii) live Artemia nauplii plus New Zealand pellet (Live + NZ); iii) live Artemia nauplii + plankton (Live + P); or (iv) practical Spanish crayfish pellet diet (Spain). Under these experimental conditions crayfish survival was significantly higher with Live + P diet than with Live + Aus or Spain. Growth was also significantly greater with Live + P than with the Live + NZ or Spanish treatment diets. In experiment C, 60-day-old juvenile A. pallipes were fed: i) defrosted plankton plus vegetables (Standard) or (ii) defrosted plankton plus vegetables encapsulated in agar gel (Gel + PV). Survival was not significantly different between the diets; however, growth was significantly greater with the Standard diet rather than Gel + PV. In experiment D, juveniles were fed four different diets: i) Australian pellet (Australia); ii) New Zealand pellet (New Zealand); iii) plankton and vegetables (Standard); or iv) practical Spanish diet (Spain). Survival was significantly lower in crayfish fed the New Zealand diet. Crayfish growth was significantly greater with the Standard diet of plankton and vegetables than all three pellet diets. Our results showed that live food is optimal for high survival and growth in A. pallipes hatchlings and a plankton, plus vegetable, diet produces higher growth in juveniles compared to pellet diets.

Selective fish passage: Restoring habitat connectivity without facilitating the spread of a non-native species

River managers are challenged to address two key threats to freshwater biodiversity. The first is the effects of habitat fragmentation by instream structures, such as dams and weirs, that disrupt migrations and impact species distributions. The second is the impact of non-native species on native species and ecological processes. However, mitigating anthropogenic habitat fragmentation through the installation of passage facilities can facilitate the invasion and spread of non-native species. This study compared the potential of two existing low-cost fish passage technologies designed for sloping weirs, a cylindrical bristle cluster (CBC) array and horizontally oriented studded tiles, to facilitate upstream movement of native European fish while preventing dispersal of non-native American signal crayfish (Pacifastacus leniusculus); thus providing a selective fish passage solution. Crayfish movement and passage was experimentally quantified at a Crump weir installed in a recirculating flume under two velocity regimes (low and high), without (control) and with the addition of either a CBC array or studded tiles. Results were compared to passage efficacy (PE) data for native fish species for both technologies (existing data). Most (84.4%) crayfish were active during the trials, exhibiting frequent up and downstream movements below the weir. During control conditions under the high velocity regime, high velocities (ca. 2.39 m s^-1) prevented crayfish reaching the foot of the weir (PE: 0%). Under the low velocity regime, relatively low velocities (ca. 0.74 m s^-1) at the weir crest prevented most crayfish from passing (PE: 10-16%). Crayfish movement speed and total distance moved were lower under the high than the low velocity regime. Neither fish pass technology improved crayfish maximum distance of ascent on the downstream weir face or PE under either velocity regime. Under comparable conditions to the high velocity regime tested here, previous studies have shown both technologies improve PE for native fish. Hence, both CBC arrays and studded tiles would likely function as suitable selective fish passes where the conservation objective is not to aid the spread of non-native crayfish. Additional passage inhibiting technologies will be required at sites where complete blockage of crayfish movement is required.