Evaluating the population control of invasive crayfish using removals and male sterilisation.

Abstract

This research tests the control of invasive American signal crayfish Pacifastacus leniusculus using a combination of intensive trapping (baited traps and artificial refuge traps: ARTs) and the mechanical sterilisation and release of large males (Sterile Male Release Technique: SMRT) using both field studies (on the River Barle, Devon, Southern England) and laboratory-based experiments in controlled conditions. In the laboratory experiments, no differences were found in the ability of sterilised males to win dominance contests and compete for mates. Sterilised and non-sterilised males were equally likely to guard their mates post copulation, and females had low levels of promiscuity. When testing the functionality of the sterilisation technique, sterilised males were found to deposit significantly less spermatophore on females and it was placed less accurately. Full gonopod regeneration took at least two years and the re-trimmed gonopods were frequently deformed. However, the resultant brood sizes from these males did not decrease significantly from non-sterilised males in either the laboratory experiments or in the field study. Sterilised male recapture rates in the field were low and its low effectiveness was potentially influenced negatively by male age and migration. The field studies on the River Barle compared the efficacy of the two trap types and the population responses of male sterilisation and intensive trapping. Artificial refuge traps (ARTs) had a significantly higher catch per unit effort (CPUE) and were an effective way of capturing females and small individuals when compared with baited funnel traps. However, after six years of the application of a management programme involving the combined use of removals with ARTs and SMRT, the overall CPUE of the crayfish population was not significantly reduced. However, by year 6, the CPUE of the smallest crayfish size class (≤ 24 mm CL) had decreased significantly, along with some shifts in the size and sex structure of the population and with some evidence of reduced reproductive efficiency. A number of potential reasons for the lack of more substantive population responses were identified, including sterilised male survival and low trapping effort, and will be the subject of ongoing studies. Overall, these results highlight the difficulty of managing invasive crayfish in open lotic systems.