Evaluation of visual encounter surveys of the noble crayfish, Astacus astacus, and the spiny-cheek crayfish, Orconectes limosus
Several methods of sampling are commonly used to detect freshwater crayfish (Decapoda: Malacostraca). Many of them are laborious, time-consuming, and require dedicated equipment. The aims of this contribution are i) to compare visual encounter surveys and baited trap success in the detection of the noble crayfish, Astacus astacus (L.), which is endangered in Poland, and ii) to assess the time needed to detect the invasive spiny-cheek crayfish, Orconectes limosus (Raf.). The study is based on data collected between 2016–2018 in various habitats of Astacus astacus and Orconectes limosus in Poland. Visual encounter surveys are at least as effective in assessing the presence of A. astacus as the trapping method. The modal value for the detection time of O. limosus at all sites and all surveys was two minutes. Sample rarefaction showed that one survey covered 11.33 (SD = 0.43) of a maximum of 12 detections per survey. This suggests that, despite some limitations, visual detection might be an efficient method for determining crayfish presence/absence for a wide range of applications.