Burrowing behaviour and movements of the signal crayfish Pacifastacus leniusculus (Dana)
The major burrowing characteristics leading to, during and after burrow construction are described. Burrow initiation was significantly correlated to crayfish size; smaller individuals beginning construction more quickly. Field burrow morphologies, examined using an optic cable video camera, showed 92% to be simple with only a single opening (Length range 3.5 - 79.0 cm). Significant associations were found between the clay/sand content of stream bank sediments and crayfish burrow densities. Substrate selection experiments indicated a significant preference for artificial shelter over burrowing in clay for adult crayfish, and a significant preference for clay and artificial shelter over mud or gravel in juveniles. The rate of range extension of P. leniusculus along the Gaddesby Brook shows polynomial expansion i.e. the speed of new habitat colonisation is increasing each year. Juveniles, on the basis of burrow sizes and movements made by adult crayfish, are mainly responsible for this colonisation. Measurements of burrow water O[subscript 2], CO[subscript 2], ammonia and pH were made and showed lightly hypoxic conditions and elevated levels of ammonia in occupied burrows. Burrow irrigation rates were examined with average turnover rates being 14.8 1 h[superscript -1] for adults (mass range = 31.7 - 117 g). Crayfish movements were monitored by means of radio tracking. Results indicated that activity was greatest during and immediately following dusk and that crayfish activity was significantly less in winter than summer. Most individuals were position-fixed at the same burrow/shelter for the duration of radio tracking, a few made occasional large movements between stationary phases of between 2-8 days. The maximum distance recorded by any individual in one night was 89.6 m. During two flood events, all tagged animals maintained their pre-flood positions. Abdominal tags used to measure longer-term movements (over 2 years) gave an overall recapture rate of 19.0% (51 from 268) and generally showed that adult P. leniusculus remained in the same vicinity for in excess of 2 years. Hopefully this study will help in the control and management of P. leniusculus. For example, forecasts on preferred sites for population expansion and identification of sites vulnerable to bank damage can be deduced from substrate preferences. Furthermore, information on burrow construction, behaviour and irrigation rates may be relevant in biocide application and assessing the effects on bank stability. A knowledge of crayfish movement and activity is important for predicting time scale of spread and colonisation of new habitats.