Individual study: Captive rearing of juvenile white-clawed crayfish Austropotamobius pallipes during 2001 for reintroduction into the River Lathkill, Derbyshire, England
(2003) Reintroducing the white-clawed crayfish into the River Lathkill. Conserving Natura 2000 Rivers. Conservation Techniques Series No. 8. English Nature, Peterborough (added by: Bird J.P. 2006). Reintroducing the white-clawed crayfish into the River Lathkill
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.
Note: If using or referring to this published study, please read and quote the original report, this is avaialable at: http://www.english-nature.gov.uk/LIFEinUKRivers/publications/lathkill-crayfish.pdf