Detecting North American signal crayfish (Pacifastacus leniusculus) in riffles

1. The spread of the invasive signal crayfish (Pacifastacus leniusculus) outside its natural range is of widespread concern due to the threats posed to native biodiversity. To date, there is no standard protocol for determining signal crayfish presence or absence in a watercourse. 2. For the purposes of this investigation, the crayfish detection ability of active sampling methods - hand-netting, electrofishing (one, two and three runs), kick sampling and Surber sampling - was tested at 30 sites along the River Clyde, southern central Scotland. 3. No single technique was successful in detecting crayfish in 100% of the sites known to contain crayfish and so the application of combinations of techniques was considered. The combination of techniques that resulted in a 100% detection rate was electrofishing (three runs) together with kick sampling. These results suggest that three-run electrofishing and kick sampling are the best candidates for incorporation into a crayfish detection protocol. 4. The mean time taken to apply electrofishing (three runs) was significantly greater than the mean time to apply kick sampling. Given the lower effort required for its application, kick sampling is recommended as the preliminary technique: if kick sampling yields a negative result, the application of electrofishing will decrease the chance of recording a false negative presence. If both kick sampling and electrofishing fail to detect crayfish, trapping may further decrease the risk of a false negative result. 5. These findings have assisted in the development of a crayfish detection protocol, which will be applied across Scotland to determine the current distribution of signal crayfish