Reintroducing the white-clawed crayfish Austropotamobius pallipes

Summary

Study 1

In Britain, the native white-clawed crayfish Austropotamobius pallipes is becoming increasingly scarce since the introduction of the larger North American signal crayfish Pacifastacus leniusculus, which kill and eat or displace native crayfish. Furthermore they carry a fungal disease which has wiped out several populations of white-clawed crayfish.

In 1993, the River Lathkill in western England lost its entire population of white-clawed crayfish in a mass mortality (this was not due to signal crayfish – instead it was thought to be due to an outbreak of crayfish plague). The UK LIFE project “Safeguarding Natura 2000 rivers in the UK” aims to expand the range or increase populations of white-clawed crayfish where suitable habitat is available but not presently occupied due to past historical impacts that had resulted in their extirpation. As part of this project a pilot crayfish reintroduction in 1999/2000 demonstrated that conditions in the River Lathkill were such that it could once again support white-clawed crayfish.

English Nature and David Rogers Associates conducted an experimental reintroduction and looked at methods of rearing large numbers of white-clawed crayfish from relatively small numbers of imported stock (of UK origin). A particular aim was to experimentally develop methods for rearing captive crayfish for reintroduction. A summary of preliminary attempts to raise juveniles in captivity during 2000, is outlined.

Study site: In April 2000, ten ovigerous (egg-carrying) female white-clawed crayfish Austropotamobius pallipes (transferred via a quarantine facility from a donor population at Bestwood ponds, Nottinghamshire) were held for re-introduction purposes within a large holding tank adjacent to the River Lathkill in Derbyshire, western England.

Crayfish cages Individual crayfish cages within the holding tank measured 15 cm x 10 cm x 10 cm (rigid 8 mm square mesh. These cages were surrounded by a one cubic metre fine meshed cage (mesh dimensions: 2 mm diameter circular). This mesh was fine enough to retain hatchling juveniles which were able to disperse through the 8 mm mesh of the brood cages, thus reducing the potential for their mother to predate them. The individual cages were also supported above the bottom of the holding tank to reduce the potential for bacterial and fungal infections.

The ten females and resulting eggs / juveniles were monitored for the remainder of the year.

Summary of results:

1) On 1 April 2000, the average number of eggs held by each female was estimated at 40.

2) On 20 June 2000, the eggs were observed hatching and there did not appear to be any losses.

3) On 10 July 2000, the number of Stage 2/3 juveniles was estimated at 40 per adult.

4) On 31 July 2000, it was observed that the juveniles had left their parent.

The adults in their individual cages were removed, leaving the juveniles in the fine mesh cage. Only 13 juvenile crayfish could be found.

The fate of the majority of potentially free-living juvenile crayfish (400) was not known.

Possibilities included: heavy predation by the adult females; and escape of juveniles through the 2 mm mesh of the 1m³ fine mesh cage into the main holding tank.

The juvenile crayfish were monitored on four further occasions until 20 May 2001. Full details of the number of surviving juveniles are given in Table 1 (attached). No juveniles survived beyond 10 months.

Conclusions: Initially, this method yielded similar numbers of eggs and Stage 2/3 juveniles to wild populations. However, the rearing of juveniles hatched in 2000 was not successful so in 2001 several new methods were trialled in an attempt to increase productivity (for a summary please see below)
 

Study 2

In Britain, the native white-clawed crayfish Austropotamobius pallipes is becoming increasingly scarce since the introduction of the larger North American signal crayfish Pacifastacus leniusculus, which kill and eat or displace native crayfish. Furthermore they carry a fungal disease which has wiped out several populations of white-clawed crayfish.

In 1993, the River Lathkill in western England lost its entire population of white-clawed crayfish in a mass mortality (this was not due to signal crayfish – instead it was thought to be due to an outbreak of crayfish plague). The UK LIFE project “Safeguarding Natura 2000 rivers in the UK” aims to expand the range or increase populations of white-clawed crayfish where suitable habitat is available but not presently occupied due to past historical impacts that had resulted in their extirpation. As part of this project a pilot crayfish reintroduction in 1999/2000 demonstrated that conditions in the River Lathkill were such that it could once again support white-clawed crayfish.

English Nature and David Rogers Associates conducted an experimental reintroduction and looked at methods of rearing large numbers of white-clawed crayfish from relatively small numbers of imported stock (of UK origin). A particular aim was to experimentally develop methods for rearing captive crayfish for reintroduction. A summary of attempts to raise juveniles in captivity and to try to increase juvenile survival rate during 2001, is outlined.

Study site: In July 2000, 210 white-clawed crayfish were taken from Bestwood Ponds, Nottinghamshire to a quarantine facility in Castle Donnington, Derbyshire. From this stock a breeding population of 62 individuals (30 males and 32 females) was established in a holding tank adjacent to the River Lathkill to use for experimentally rearing juveniles in 2001.

Captive rearing: By May 2001 only 24 (75%) females had survived and of these only 13 (41%) had four or more eggs. On 15 May 2001 six ovigerous (egg-carrying) female white-clawed crayfish Austropotamobius pallipes were transferred to individual aquaria for daily observation, while the seven remaining ovigerous females were returned to individual cages within the holding tank (the method used for rearing juveniles in 2000 – see, Case 328).

During 2000, ovigerous female crayfish had been held individually in cages within a 1m³ fine mesh cage suspended within their holding tank. This was designed to provide juveniles with a refuge where they could avoid predation by their mother. However, this method proved unsuccessful (see Case 328) so in 2001 four new methods were trialled:

1) Remove juveniles as they leave the female and rear in a separate aquarium

2) Remove juveniles as they leave the female, hold in a separate aquarium for four weeks then transfer them to a fine mesh cage (1 m³, – 2 mm diameter circular mesh) in the holding tank

3) Remove juveniles as they leave the female and hold in a titanium mesh tank (selected due to inert properties of titanium – thought to reduce risk of infection)

4) Remove juveniles as they leave the female and hold in a small, partially covered fine mesh flexible net (approximately 25 cm square, made from net curtains) suspended in a large holding tank

The juveniles reared by these four methods came from the six females held in individual aquaria, while the seven ovigerous females that were returned to cages within the holding tank acted as controls.

Observation of females in 2001 revealed that many eggs were dislodged by the behaviour of the mother and some juveniles were certainly eaten by the parent, but the fate of the majority is unknown. By June 2001, 87 surviving juveniles were independent of the females and were transferred to one of the four rearing facilities described above, to monitor survival and growth.

Most juveniles survived approximately four weeks after being transferred but subsequently died before or during the first free-living moult (Stage 3) away from the mother. The exceptions were the juveniles held in a folded flexible net in a large tank (method 4). Of 17 Stage 3 juveniles in the flexible net in June 2001, 11 survived until November 2001 undergoing several moults and reaching an average carapace length of 10 mm. It was thought that the refugia provided by the folds of the net contributed to enhanced survival (see Table 1, attached for full details of juvenile progress in 2001).

Conclusions: Further attempts will be made to improve juvenile survival rates including removing them at different stages, i.e. at egg stage before hatching to when they leave their parent naturally. From this study so far, it is recommended that before the juveniles leave the mother they are removed and housed in an environment where they can find secure places to hide during moulting, such as those provided by rearing method 4 i.e. a secure small mesh cage with sufficient bunched net curtain material included.

 

Study 3

In Britain, the native white-clawed crayfish Austropotamobius pallipes is becoming increasingly scarce since the introduction of the larger North American signal crayfish Pacifastacus leniusculus, which kill and eat or displace native crayfish. Furthermore they carry a fungal disease which has wiped out several populations of white-clawed crayfish in the UK.

In 1993, the River Lathkill in western England, lost its entire population of white-clawed crayfish in a mass mortality event (thought to be due to an outbreak of crayfish plague). The UK LIFE project “Safeguarding Natura 2000 rivers in the UK” aims to expand the range or increase populations of white-clawed crayfish where suitable habitat is available but not presently occupied due to past historical impacts that had resulted in their extirpation. As part of this project, a pilot crayfish reintroduction in 1999/2000 demonstrated that conditions in the River Lathkill were such that it could once again support white-clawed crayfish.

English Nature and David Rogers Associates conducted an experimental reintroduction and looked at methods of rearing large numbers of white-clawed crayfish from relatively small numbers of imported stock (of UK origin). This case study outlines this reintroduction of white-clawed crayfish to the River Lathkill.

Source population: By September 2000, 79 crayfish had been trapped from Bestwood Ponds, Nottinghamshire, and quarantined in facilities at David Rogers Associates (Castle Donington, Leicestershire). On 26 September 2000 these 79 crayfish were introduced directly from the quarantine facility into the River Lathkill.

The release site was selected based on the habitat requirements of the white-clawed crayfish (see Holdich & Rogers 2000) but also taking into consideration that a site with reasonable access for monitoring and not interfered with by the general public, was needed.

The site selected was downstream of a small waterfall with moderately fast flowing water and and a riverbed of stones and boulders, which provided ample cover and refugia for the released crayfish. It is within the Derbyshire Dales National Nature Reserve, a Site of Special Scientific Interest and a Special Area of Conservation under the European Habitats Directive.

Monitoring: In order to monitor the movement and population size of the crayfish introduced into the River Lathkill, each crayfish was marked individually using pleural and uropodal clipping (see Chien & Avault, 1979 for details). Searches were undertaken in May, July, August and October 2001.

Following the release, monitoring was undertaken as follows:

1) In May 2001 English Nature staff searched the vicinity of the crayfish introduction site for crayfish. No crayfish were found.

2) On 4 July 2001 English Nature staff again searched the area and found one crayfish 200 m downstream of the introduction site.

3) On 25 August 2001 David Rogers Associates searched a 300 m stretch of the River Lathkill downstream of the introduction site. No crayfish were found.

4) In October 2001 English Nature staff observed crayfish whilst undertaking remedial works 300 m downstream of the introduction site.

Sightings of crayfish in the vicinity of the release site indicate that some of the introduced crayfish did survive at least one year and it is possible that breeding may have occurred although no evidence could be found.

Conclusions: The reintroduction of white-clawed crayfish into the River Lathkill appears to have been at least been partially successful. A clearer understanding of how many individuals have survived since reintroduction, and whether breeding has occurred, is needed. If the reintroduced population is to become self-sustaining in the long-term, additional releases of crayfish may be required.

Detection of low-density crayfish populations is known to be difficult. As the habitat into which the crayfish were introduced consists of very large numbers of rocks and stones, even extended periods of searching may not expose areas that could be occupied by the introduced crayfish. Therefore, it may be worthwhile to attempt live-trapping for monitoring purposes in the future.


References:

Chien Y.H. & Avault J.W.Jr. (1979). Double cropping rice, Oryza sativa, and red swamp crawfish, Procambarus clarkii. Freshwater Crayfish, 4, 263-271.

Holdich D.M. & Rogers W.D. (2000). Habitat requirements of the white-clawed crayfish, Austropotamobius pallipes. Proceedings of Crayfish Conference, Leeds, April 2000.

Note: If using or referring to this published study, please read and quote the original report, this is avaialable at: http://publications.naturalengland.org.uk/publication/70044